U.S. patent application number 09/783966 was filed with the patent office on 2001-07-12 for filter device and method of acquring filter coefficients.
Invention is credited to Abe, Ryoji.
Application Number | 20010007433 09/783966 |
Document ID | / |
Family ID | 11803896 |
Filed Date | 2001-07-12 |
United States Patent
Application |
20010007433 |
Kind Code |
A1 |
Abe, Ryoji |
July 12, 2001 |
Filter device and method of acquring filter coefficients
Abstract
A filter device includes a first section for receiving data of a
desired cutoff frequency. A second section operates for deciding
whether the desired cutoff frequency is in a predetermined low
frequency band or a predetermined high frequency band. The
predetermined low frequency band is lower in frequency than the
predetermined high frequency band. A third section stores data of
filter coefficients corresponding to different cutoff frequencies
in the predetermined low frequency band. A fourth section stores
data of precalculated basic coefficients corresponding to different
cutoff frequencies in the predetermined high frequency band. The
precalculated basic coefficients are equal to values resulting from
a part of calculation to provide final filter coefficients. A fifth
section operates for calculating final filter coefficients from
precalculated basic coefficients through a coefficient expanding
process. In cases where the second section decides that the desired
cutoff frequency is in the predetermined low frequency band, data
of filter coefficients corresponding to the desired cutoff
frequency are read out from the third section, and are set in a
filtering section. In cases where the second section decides that
the desired cutoff frequency is in the predetermined high frequency
band, data of precalculated basic coefficients corresponding to the
desired cutoff frequency are read out from the fourth section.
Then, the fifth section is caused to calculate final filter
coefficients from the read-out precalculated basic coefficients,
and data of the calculated final filter coefficients are set in the
filtering section.
Inventors: |
Abe, Ryoji; (Yokohama-shi,
JP) |
Correspondence
Address: |
Law Offices of Louis Woo
1901 North Fort Myer Drive
Suite 501
Arlington
VA
22209
US
|
Family ID: |
11803896 |
Appl. No.: |
09/783966 |
Filed: |
February 16, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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09783966 |
Feb 16, 2001 |
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09482080 |
Jan 13, 2000 |
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6219392 |
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Current U.S.
Class: |
327/552 |
Current CPC
Class: |
H03H 17/0294
20130101 |
Class at
Publication: |
327/552 |
International
Class: |
H03D 013/00 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 20, 1999 |
JP |
11-12388 |
Jan 20, 1999 |
JP |
11-12388 |
Claims
What is claimed is:
1. A filter device comprising: first means for receiving data of a
desired cutoff frequency; second means for deciding whether the
desired cutoff frequency is in a predetermined low frequency band
or a predetermined high frequency band, the predetermined low
frequency band being lower in frequency than the predetermined high
frequency band; third means for storing data of filter coefficients
corresponding to different cutoff frequencies in the predetermined
low frequency band; fourth means for storing data of precalculated
basic coefficients corresponding to different cutoff frequencies in
the predetermined high frequency band, the precalculated basic
coefficients being equal to values resulting from a part of
calculation to provide final filter coefficients; fifth means for
calculating final filter coefficients from precalculated basic
coefficients through a coefficient expanding process; a filtering
section; sixth means for, in cases where the second means decides
that the desired cutoff frequency is in the predetermined low
frequency band, reading out data of filter coefficients
corresponding to the desired cutoff frequency from the third means,
and setting the data of the filter coefficients corresponding to
the desired cutoff frequency in the filtering section; and seventh
means for, in cases where the second means decides that the desired
cutoff frequency is in the predetermined high frequency band,
reading out data of precalculated basic coefficients corresponding
to the desired cutoff frequency from the fourth means, causing the
fifth means to calculate final filter coefficients from the
read-out precalculated basic coefficients, and setting data of the
calculated final filter coefficients in the filtering section.
2. A filter device as recited in claim 1, further comprising first
interpolating means for implementing interpolation with respect to
the filter coefficients represented by the data read out from the
third means, and second interpolating means for implementing
interpolation with respect to the data read out from the fourth
means or the final filter coefficients calculated by the fifth
means.
3. A filter device as recited in claim 1, further comprising first
interpolating means for implementing logarithmic interpolation with
respect to the filter coefficients represented by the data read out
from the third means, and second interpolating means for
implementing logarithmic interpolation with respect to the data
read out from the fourth means or the final filter coefficients
calculated by the fifth means.
4. A filter device comprising: first means for receiving data of a
desired cutoff frequency; second means for deciding which of a
predetermined low frequency band, a predetermined intermediate
frequency band, and a predetermined high frequency band the desired
cutoff frequency exists in, the predetermined low frequency band
being lower in frequency than the predetermined intermediate
frequency band, the predetermined intermediate frequency band being
lower in frequency than the predetermined high frequency band;
third means for storing data of filter coefficients corresponding
to different cutoff frequencies in the predetermined low frequency
band; fourth means for storing data of precalculated basic
coefficients corresponding to different cutoff frequencies in the
predetermined intermediate frequency band, the precalculated basic
coefficients being equal to values resulting from a part of
calculation to provide final filter coefficients; fifth means for
calculating final filter coefficients from precalculated basic
coefficients through a coefficient expanding process; sixth means
for calculating filter coefficients from a cutoff frequency in the
predetermined high frequency band; a filtering section; seventh
means for, in cases where the second means decides that the desired
cutoff frequency is in the predetermined low frequency band,
reading out data of filter coefficients corresponding to the
desired cutoff frequency from the third means, and setting the data
of the filter coefficients corresponding to the desired cutoff
frequency in the filtering section; eighth means for, in cases
where the second means decides that the desired cutoff frequency is
in the predetermined intermediate frequency band, reading out data
of precalculated basic coefficients corresponding to the desired
cutoff frequency from the fourth means, causing the fifth means to
calculate final filter coefficients from the read-out precalculated
basic coefficients, and setting data of the calculated final filter
coefficients in the filtering section; and ninth means for, in
cases where the second means decides that the desired cutoff
frequency is in the predetermined high frequency band, causing the
sixth means to calculate filter coefficients from the desired
cutoff frequency, and setting data of the calculated filter
coefficients in the filtering section.
5. A filter device as recited in claim 4, further comprising first
interpolating means for implementing interpolation with respect to
the filter coefficients represented by the data read out from the
third means, and second interpolating means for implementing
interpolation with respect to the data read out from the fourth
means or the final filter coefficients calculated by the fifth
means.
6. A filter device as recited in claim 4, further comprising first
interpolating means for implementing logarithmic interpolation with
respect to the filter coefficients represented by the data read out
from the third means, and second interpolating means for
implementing logarithmic interpolation with respect to the data
read out from the fourth means or the final filter coefficients
calculated by the fifth means.
7. A filter device as recited in claim 2, wherein each of the first
and second interpolating means comprises N partial interpolating
means for implementing double interpolation, and N denotes a
predetermined natural number equal to or greater than 2.
8. A filter device as recited-in claim 3, wherein each of the first
and second logarithmically interpolating means comprises N partial
logarithmically interpolating means for implementing double
logarithmic interpolation, and N denotes a predetermined natural
number equal to or greater than 2.
9. A filter device as recited in claim 5, wherein each of the first
and second interpolating means comprises N partial interpolating
means for implementing double interpolation, and N denotes a
predetermined natural number equal to or greater than 2.
10. A filter device as recited in claim 6, wherein each of the
first and second logarithmically interpolating means comprises N
partial logarithmically interpolating means for implementing double
logarithmic interpolation, and N denotes a predetermined natural
number equal to or greater than 2.
11. A method of acquiring filter coefficients, comprising the steps
of: 1) receiving data of a desired cutoff frequency; 2) deciding
whether the desired cutoff frequency is in a predetermined low
frequency band or a predetermined high frequency band, the
predetermined low frequency band being lower in frequency than the
predetermined high frequency band; 3) in cases where the step 2)
decides that the desired cutoff frequency is in the predetermined
low frequency band, reading out data of filter coefficients
corresponding to the desired cutoff frequency from a first memory
section, and setting the data of the filter coefficients
corresponding to the desired cutoff frequency in a filtering
section; and 4) in cases where the step 2) decides that the desired
cutoff frequency is in the predetermined high frequency band,
reading out data of precalculated basic coefficients corresponding
to the desired cutoff frequency from a second memory section,
calculating final filter coefficients from the read-out
precalculated basic coefficients through a coefficient expanding
process, and setting data of the calculated final filter
coefficients in the filtering section.
12. A method of acquiring filter coefficients, comprising the steps
of: 1) receiving data of a desired cutoff frequency; 2) deciding
which of a predetermined low frequency band, a predetermined
intermediate frequency band, and a predetermined high frequency
band the desired cutoff frequency exists in, the predetermined low
frequency band being lower in frequency than the predetermined
intermediate frequency band, the predetermined intermediate
frequency band being lower in frequency than the predetermined high
frequency band; 3) in cases where the step 2) decides that the
desired cutoff frequency is in the predetermined low frequency
band, reading out data of filter coefficients corresponding to the
desired cutoff frequency from a first memory section, and setting
the data of the filter coefficients corresponding to the desired
cutoff frequency in a filtering section; 4) in cases where the step
2) decides that the desired cutoff frequency is in the
predetermined intermediate frequency band, reading out data of
precalculated basic coefficients corresponding to the desired
cutoff frequency from a second memory section, calculating final
filter coefficients from the read-out precalculated basic
coefficients through a coefficient expanding process, and setting
data of the calculated final filter coefficients in the filtering
section; and 5) in cases where the step 2 decides that the desired
cutoff frequency is in the predetermined high frequency band,
calculating filter coefficients from the desired cutoff frequency,
and setting data of the calculated filter coefficients in the
filtering section.
13. A filter device comprising: first means for deciding whether a
desired cutoff frequency is in a predetermined low frequency band
or a predetermined high frequency band, the predetermined low
frequency band being lower in frequency than the predetermined high
frequency band; second means for storing data of filter
coefficients corresponding to different cutoff frequencies in the
predetermined low frequency band; third means for storing data of
precalculated basic coefficients corresponding to different cutoff
frequencies in the predetermined high frequency band, the
precalculated basic coefficients being equal to values resulting
from a part of calculation to provide final filter coefficients; a
filtering section; fourth means for, in cases where the first means
decides that the desired cutoff frequency is in the predetermined
low frequency band, reading out data of filter coefficients
corresponding to the desired cutoff frequency from the second
means, and setting the data of the filter coefficients
corresponding to the desired cutoff frequency in the filtering
section; and fifth means for, in cases where the first means
decides that the desired cutoff frequency is in the predetermined
high frequency band, reading out data of precalculated basic
coefficients corresponding to the desired cutoff frequency from the
third means, calculating final filter coefficients from the
read-out precalculated basic coefficients through a coefficient
expanding process, and setting data of the calculated final filter
coefficients in the filtering section.
14. A filter device comprising: first means for deciding which of a
predetermined low frequency band, a predetermined intermediate
frequency band, and a predetermined high frequency band the desired
cutoff frequency exists in, the predetermined low frequency band
being lower in frequency than the predetermined intermediate
frequency band, the predetermined intermediate frequency band being
lower in frequency than the predetermined high frequency band;
second means for storing data of filter coefficients corresponding
to different cutoff frequencies in the predetermined low frequency
band; third means for storing data of precalculated basic
coefficients corresponding to different cutoff frequencies in the
predetermined intermediate frequency band, the precalculated basic
coefficients being equal to values resulting from a part of
calculation to provide final filter coefficients; a filtering
section; fourth means for, in cases where the first means decides
that the desired cutoff frequency is in the predetermined low
frequency band, reading out data of filter coefficients
corresponding to the desired cutoff frequency from the second
means, and setting the data of the filter coefficients
corresponding to the desired cutoff frequency in the filtering
section; fifth means for, in cases where the first means decides
that the desired cutoff frequency is in the predetermined
intermediate frequency band, reading out data of precalculated
basic coefficients corresponding to the desired cutoff frequency
from the third means, calculating final filter coefficients from
the read-out precalculated basic coefficients, and setting data of
the calculated final filter coefficients in the filtering section;
and sixth means for, in cases where the first means decides that
the desired cutoff frequency is in the predetermined high frequency
band, calculating filter coefficients from the desired cutoff
frequency, and setting data of the calculated filter coefficients
in the filtering section.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates to a filter device such as a digital
filter device having a variable cutoff frequency. Also, this
invention relates to a method of acquiring filter coefficients to
be set in a filter device.
[0003] 2. Description of the Related Art
[0004] It is known to use a digital signal processor (DSP) to form
a filtering section of a digital filter device having a variable
cutoff frequency. The filtering section of such a digital filter
device contains a memory loaded with a signal of a set of filter
coefficients. The actual cutoff frequency of the digital filter
device is determined by the set of the filter coefficients.
[0005] A first known digital filter device includes a DSP-based
filtering section having a variable cutoff frequency, and a ROM
storing signals of sets of filter coefficients which correspond to
different cutoff frequencies respectively. The ROM is accessed in
response to an input command signal representative of a desired
cutoff frequency so that a signal of a set of filter coefficients
corresponding to the desired cutoff frequency will be read out from
the ROM. The read-out signal of the filter coefficient set is
written into a memory within the filtering section to equalize the
actual cutoff frequency of the filtering section to the desired
cutoff frequency. When many signals of sets of filter coefficients
are required to be stored in the ROM, the capacity of the ROM needs
to be great.
[0006] A second known digital filter device includes a DSP-based
filtering section having a variable cutoff frequency, and a CPU
programmed to calculate a set of filter coefficients. Specifically,
the CPU calculates a set of filter coefficients from a desired
cutoff frequency represented by an input command signal. The CPU
writes a signal of the set of the calculated filter coefficients
into a memory within the filtering section to equalize the actual
cutoff frequency of the filtering section to the desired cutoff
frequency. When accurate and fast calculation of filter
coefficients is required, a high-grade expensive CPU is needed.
[0007] A background-art digital filter device which is not prior
art to this invention includes a DSP-based filtering section having
a variable cutoff frequency, a ROM storing signals of sets of first
filter coefficients which correspond to different cutoff
frequencies respectively, and a CPU programmed to calculate a set
of second filter coefficients. The ROM is accessed in response to
an input command signal representative of a desired cutoff
frequency so that a signal of a set of first filter coefficients
corresponding to the desired cutoff frequency will be read out from
the ROM. The CPU receives the read-out signal of the set of the
first filter coefficients.
[0008] The CPU calculates a set of second filter coefficients from
the set of the first filter coefficients. The CPU writes a signal
of the set of the calculated second filter coefficients into a
memory within the filtering section to equalize the actual cutoff
frequency of the filtering section to the desired cutoff frequency.
When many signals of sets of first filter coefficients are required
to be stored in the ROM, the capacity of the ROM needs to be great.
When accurate and fast calculation of second filter coefficients is
required, a high-grade expensive CPU is needed.
SUMMARY OF THE INVENTION
[0009] It is a first object of this invention to provide an
improved filter device.
[0010] It is a second object of this invention to provide an
improved method of acquiring filter coefficients.
[0011] A first aspect of this invention provides a filter device
comprising first means for receiving data of a desired cutoff
frequency; second means for deciding whether the desired cutoff
frequency is in a predetermined low frequency band or a
predetermined high frequency band, the predetermined low frequency
band being lower in frequency than the predetermined high frequency
band; third means for storing data of filter coefficients
corresponding to different cutoff frequencies in the predetermined
low frequency band; fourth means for storing data of precalculated
basic coefficients corresponding to different cutoff frequencies in
the predetermined high frequency band, the precalculated basic
coefficients being equal to values resulting from a part of
calculation to provide final filter coefficients; fifth means for
calculating final filter coefficients from precalculated basic
coefficients through a coefficient expanding process; a filtering
section; sixth means for, in cases where the second means decides
that the desired cutoff frequency is in the predetermined low
frequency band, reading out data of filter coefficients
corresponding to the desired cutoff frequency from the third means,
and setting the data of the filter coefficients corresponding to
the desired cutoff frequency in the filtering section; and seventh
means for, in cases where the second means decides that the desired
cutoff frequency is in the predetermined high frequency band,
reading out data of precalculated basic coefficients corresponding
to the desired cutoff frequency from the fourth means, causing the
fifth means to calculate final filter coefficients from the
read-out precalculated basic coefficients, and setting data of the
calculated final filter coefficients in the filtering section.
[0012] A second aspect of this invention is based on the first
aspect thereof, and provides a filter device further comprising
first interpolating means for implementing interpolation with
respect to the filter coefficients represented by the data read out
from the third means, and second interpolating means for
implementing interpolation with respect to the data read out from
the fourth means or the final filter coefficients calculated by the
fifth means.
[0013] A third aspect of this invention is based on the first
aspect thereof, and provides a filter device further comprising
first interpolating means for implementing logarithmic
interpolation with respect to the filter coefficients represented
by the data read out from the third means, and second interpolating
means for implementing logarithmic interpolation with respect to
the data read out from the fourth means or the final filter
coefficients calculated by the fifth means.
[0014] A fourth aspect of this invention provides a filter device
comprising first means for receiving data of a desired cutoff
frequency; second means for deciding which of a predetermined low
frequency band, a predetermined intermediate frequency band, and a
predetermined high frequency band the desired cutoff frequency
exists in, the predetermined low frequency band being lower in
frequency than the predetermined intermediate frequency band, the
predetermined intermediate frequency band being lower in frequency
than the predetermined high frequency band; third means for storing
data of filter coefficients corresponding to different cutoff
frequencies in the predetermined low frequency band; fourth means
for storing data of precalculated basic coefficients corresponding
to different cutoff frequencies in the predetermined intermediate
frequency band, the precalculated basic coefficients being equal to
values resulting from a part of calculation to provide final filter
coefficients; fifth means for calculating final filter coefficients
from precalculated basic coefficients through a coefficient
expanding process; sixth means for calculating filter coefficients
from a cutoff frequency in the predetermined high frequency band; a
filtering section; seventh means for, in cases where the second
means decides that the desired cutoff frequency is in the
predetermined low frequency band, reading out data of filter
coefficients corresponding to the desired cutoff frequency from the
third means, and setting the data of the filter coefficients
corresponding to the desired cutoff frequency in the filtering
section; eighth means for, in cases where the second means decides
that the desired cutoff frequency is in the predetermined
intermediate frequency band, reading out data of precalculated
basic coefficients corresponding to the desired cutoff frequency
from the fourth means, causing the fifth means to calculate final
filter coefficients from the read-out precalculated basic
coefficients, and setting data of the calculated final filter
coefficients in the filtering section; and ninth means for, in
cases where the second means decides that the desired cutoff
frequency is in the predetermined high frequency band, causing the
sixth means to calculate filter coefficients from the desired
cutoff frequency, and setting data of the calculated filter
coefficients in the filtering section.
[0015] A fifth aspect of this invention is based on the fourth
aspect thereof, and provides a filter device further comprising
first interpolating means for implementing interpolation with
respect to the filter coefficients represented by the data read out
from the third means, and second interpolating means for
implementing interpolation with respect to the data read out from
the fourth means or the final filter coefficients calculated by the
fifth means.
[0016] A sixth aspect of this invention is based on the fourth
aspect thereof, and provides a filter device further comprising
first interpolating means for implementing logarithmic
interpolation with respect to the filter coefficients represented
by the data read out from the third means, and second interpolating
means for implementing logarithmic interpolation with respect to
the data read out from the fourth means or the final filter
coefficients calculated by the fifth means.
[0017] A seventh aspect of this invention is based on the second
aspect thereof, and provides a filter device wherein each of the
first and second interpolating means comprises N partial
interpolating means for implementing double interpolation, and N
denotes a predetermined natural number equal to or greater than
2.
[0018] An eighth aspect of this invention is based on the third
aspect thereof, and provides a filter device wherein each of the
first and second logarithmically interpolating means comprises N
partial logarithmically interpolating means for implementing double
logarithmic interpolation, and N denotes a predetermined natural
number equal to or greater than 2.
[0019] A ninth aspect of this invention is based on the fifth
aspect thereof, and provides a filter device wherein each of the
first and second interpolating means comprises N partial
interpolating means for implementing double interpolation, and N
denotes a predetermined natural number equal to or greater than
2.
[0020] A tenth aspect of this invention is based on the sixth
aspect thereof, and provides a filter device wherein each of the
first and second logarithmically interpolating means comprises N
partial logarithmically interpolating means for implementing double
logarithmic interpolation, and N denotes a predetermined natural
number equal to or greater than 2.
[0021] An eleventh aspect of this invention provides a method of
acquiring filter coefficients. The method comprises the steps of 1)
receiving data of a desired cutoff frequency; 2) deciding whether
the desired cutoff frequency is in a predetermined low frequency
band or a predetermined high frequency band, the predetermined low
frequency band being lower in frequency than the predetermined high
frequency band; 3) in cases where the step 2) decides that the
desired cutoff frequency is in the predetermined low frequency
band, reading out data of filter coefficients corresponding to the
desired cutoff frequency from a first memory section, and setting
the data of the filter coefficients corresponding to the desired
cutoff frequency in a filtering section; and 4) in cases where the
step 2) decides that the desired cutoff frequency is in the
predetermined high frequency band, reading out data of
precalculated basic coefficients corresponding to the desired
cutoff frequency from a second memory section, calculating final
filter coefficients from the read-out precalculated basic
coefficients through a coefficient expanding process, and setting
data of the calculated final filter coefficients in the filtering
section.
[0022] A twelfth aspect of this invention provides a method of
acquiring filter coefficients. The method comprises the steps of 1)
receiving data of a desired cutoff frequency; 2) deciding which of
a predetermined low frequency band, a predetermined intermediate
frequency band, and a predetermined high frequency band the desired
cutoff frequency exists in, the predetermined low frequency band
being lower in frequency than the predetermined intermediate
frequency band, the predetermined intermediate frequency band being
lower in frequency than the predetermined high frequency band; 3)
in cases where the step 2) decides that the desired cutoff
frequency is in the predetermined low frequency band, reading out
data of filter coefficients corresponding to the desired cutoff
frequency from a first memory section, and setting the data of the
filter coefficients corresponding to the desired cutoff frequency
in a filtering section; 4) in cases where the step 2) decides that
the desired cutoff frequency is in the predetermined intermediate
frequency band, reading out data of precalculated basic
coefficients corresponding to the desired cutoff frequency from a
second memory section, calculating final filter coefficients from
the read-out precalculated basic coefficients through a coefficient
expanding process, and setting data of the calculated final filter
coefficients in the filtering section; and 5) in cases where the
step 2 decides that the desired cutoff frequency is in the
predetermined high frequency band, calculating filter coefficients
from the desired cutoff frequency, and setting data of the
calculated filter coefficients in the filtering section.
[0023] A thirteenth aspect of this invention provides a filter
device comprising first means for deciding whether a desired cutoff
frequency is in a predetermined low frequency band or a
predetermined high frequency band, the predetermined low frequency
band being lower in frequency than the predetermined high frequency
band; second means for storing data of filter coefficients
corresponding to different cutoff frequencies in the predetermined
low frequency band; third means for storing data of precalculated
basic coefficients corresponding to different cutoff frequencies in
the predetermined high frequency band, the precalculated basic
coefficients being equal to values resulting from a part of
calculation to provide final filter coefficients; a filtering
section; fourth means for, in cases where the first means decides
that the desired cutoff frequency is in the predetermined low
frequency band, reading out data of filter coefficients
corresponding to the desired cutoff frequency from the second
means, and setting the data of the filter coefficients
corresponding to the desired cutoff frequency in the filtering
section; and fifth means for, in cases where the first means
decides that the desired cutoff frequency is in the predetermined
high frequency band, reading out data of precalculated basic
coefficients corresponding to the desired cutoff frequency from the
third means, calculating final filter coefficients from the
read-out precalculated basic coefficients through a coefficient
expanding process, and setting data of the calculated final filter
coefficients in the filtering section.
[0024] A fourteenth aspect of this invention provides a filter
device comprising first means for deciding which of a predetermined
low frequency band, a predetermined intermediate frequency band,
and a predetermined high frequency band the desired cutoff
frequency exists in, the predetermined low frequency band being
lower in frequency than the predetermined intermediate frequency
band, the predetermined intermediate frequency band being lower in
frequency than the predetermined high frequency band; second means
for storing data of filter coefficients corresponding to different
cutoff frequencies in the predetermined low frequency band; third
means for storing data of precalculated basic coefficients
corresponding to different cutoff frequencies in the predetermined
intermediate frequency band, the precalculated basic coefficients
being equal to values resulting from a part of calculation to
provide final filter coefficients; a filtering section; fourth
means for, in cases where the first means decides that the desired
cutoff frequency is in the predetermined low frequency band,
reading out data of filter coefficients corresponding to the
desired cutoff frequency from the second means, and setting the
data of the filter coefficients corresponding to the desired cutoff
frequency in the filtering section; fifth means for, in cases where
the first means decides that the desired cutoff frequency is in the
predetermined intermediate frequency band, reading out data of
precalculated basic coefficients corresponding to the desired
cutoff frequency from the third means, calculating final filter
coefficients from the read-out precalculated basic coefficients,
and setting data of the calculated final filter coefficients in the
filtering section; and sixth means for, in cases where the first
means decides that the desired cutoff frequency is in the
predetermined high frequency band, calculating filter coefficients
from the desired cutoff frequency, and setting data of the
calculated filter coefficients in the filtering section.
BRIEF DESCRIPTION OF THE DRAWINGS
[0025] FIG. 1 is a diagram of a first prior-art digital filter
device.
[0026] FIG. 2 is a diagram of a second prior-art digital filter
device.
[0027] FIG. 3 is a diagram of a background-art digital filter
device.
[0028] FIG. 4 is a block diagram of a digital filter device
according to a first embodiment of this invention.
[0029] FIG. 5 is a flowchart of a segment of a program for
controlling a CPU in FIG. 4.
[0030] FIG. 6 is an operation flow diagram of the digital filter
device in the first embodiment of this invention.
[0031] FIG. 7 is a diagram of a filter coefficient table provided
in a first memory section in FIG. 6.
[0032] FIG. 8 is a diagram of a filter coefficient table provided
in a second memory section in FIG. 6.
[0033] FIG. 9 is an operation flow diagram of a digital filter
device according to a second embodiment of this invention.
[0034] FIG. 10 is a diagram of a table of sets of filter
coefficients which include sets of filter coefficients provided by
a first averaging section in FIG. 9.
[0035] FIG. 11 is a diagram of a table of sets of basic filter
coefficients which include sets of basic filter coefficients
provided by a second averaging section in FIG. 9.
[0036] FIG. 12 is an operation flow diagram of a digital filter
device according to a third embodiment of this invention.
[0037] FIG. 13 is an operation flow diagram of a digital filter
device according to a fourth embodiment of this invention.
[0038] FIG. 14 is an operation flow diagram of a digital filter
device according to a fifth embodiment of this invention.
[0039] FIG. 15 is an operation flow diagram of a digital filter
device according to a sixth embodiment of this invention.
[0040] FIG. 16 is an operation flow diagram of a digital filter
device according to a seventh embodiment of this invention.
[0041] FIG. 17 is an operation flow diagram of a digital filter
device according to an eighth embodiment of this invention.
[0042] FIG. 18 is an operation flow diagram of a digital filter
device according to a ninth embodiment of this invention.
[0043] FIG. 19 is an operation flow diagram of a digital filter
device according to a tenth embodiment of this invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0044] Prior-art filter devices and a background-art filter device
will be explained below for a better understanding of this
invention.
[0045] FIG. 1 shows a first prior-art digital filter device. The
prior-art digital filter device of FIG. 1 includes an input section
901, a memory section 902, and a filtering section 903. The input
section 901 receives an input command signal representative of a
desired cutoff frequency. The memory section 902 includes a ROM
storing signals of sets of filter coefficients which correspond to
different cutoff frequencies respectively. The filtering section
903 is formed by a digital signal processor (DSP). The filtering
section 903 has a variable cutoff frequency. Specifically, the
filtering section 903 includes a memory loaded with a signal of a
set of filter coefficients which determines the actual cutoff
frequency of the filtering section 903.
[0046] The prior-art digital filter device of FIG. 1 operates as
follows. When an input command signal is received by the input
section 901, the memory section 902 is accessed in response to the
input command signal so that a signal of a set of filter
coefficients corresponding to a desired cutoff frequency
represented by the input command signal is read out from the memory
section 902.
[0047] The read-out signal of the filter coefficient set is written
into the memory within the filtering section 903 to equalize the
actual cutoff frequency of the filtering section 903 to the desired
cutoff frequency.
[0048] In the prior-art digital filter device of FIG. 1, when many
signals of sets of filter coefficients are required to be stored in
the ROM within the memory section 902, the capacity of the ROM
needs to be great.
[0049] FIG. 2 shows a second prior-art digital filter device. The
prior-art digital filter device of FIG. 2 includes an input section
911, a calculating section 912, and a filtering section 913. The
input section 911 receives an input command signal representative
of a desired cutoff frequency. The calculating section 912 includes
a CPU programmed to calculate a set of filter coefficients. The
filtering section 913 is formed by a DSP. The filtering section 913
has a variable cutoff frequency. Specifically, the filtering
section 913 includes a memory loaded with a signal of a set of
filter coefficients which determines the actual cutoff frequency of
the filtering section 913.
[0050] The prior-art digital filter device of FIG. 2 operates as
follows.
[0051] An input command signal is transmitted via the input section
911 to the calculating section 912. The CPU within the calculating
section 912 calculates a set of filter coefficients from a desired
cutoff frequency represented by the input command signal. The
calculating section 912 writes a signal of the set of the
calculated filter coefficients into the memory within the filtering
section 913 to equalize the actual cutoff frequency of the
filtering section 913 to the desired cutoff frequency.
[0052] In the prior-art digital filter device of FIG. 2, when
accurate and fast calculation of filter coefficients is required,
the calculating section 912 needs a high-grade expensive CPU.
[0053] FIG. 3 shows a background-art digital filter device which is
not prior art to this invention. The background-art digital filter
device of FIG. 3 includes an input section 921, a memory section
922, a calculating section 923, and a filtering section 924. The
input section 921 receives an input command signal representative
of a desired cutoff frequency. The memory section 922 includes a
ROM storing signals of sets of precalculated basic coefficients
(basic filter coefficients) which correspond to different cutoff
frequencies respectively. The calculating section 923 includes a
CPU programmed to calculate a set of final filter coefficients. The
filtering section 924 is formed by a DSP. The filtering section 924
has a variable cutoff frequency. Specifically, the filtering
section 924 includes a memory loaded with a signal of a set of
filter coefficients which determines the actual cutoff frequency of
the filtering section 924.
[0054] The background-art digital filter device of FIG. 3 operates
as follows. When an input command signal is received by the input
section 921, the memory section 922 is accessed in response to the
input command signal so that a signal of a set of precalculated
basic coefficients corresponding to a desired cutoff frequency
represented by the input command signal is read out from the memory
section 922. The read-out signal of the set of the precalculated
basic coefficients is fed to the calculating section 923. The CPU
within the calculating section 923 calculates a set of final filter
coefficients from the set of the precalculated basic coefficients
according to a coefficient expansion algorithm. The set of the
calculated final filter coefficients corresponds to the desired
cutoff frequency represented by the input command signal. The
calculating section 923 writes a signal of the set of the
calculated final filter coefficients into the memory within the
filtering section 924 to equalize the actual cutoff frequency of
the filtering section 924 to the desired cutoff frequency.
[0055] In the digital filter device of FIG. 3, when many signals of
sets of precalculated basic coefficients (basic filter
coefficients) are required to be stored in the ROM within the
memory section 922, the capacity of the ROM needs to be great. When
accurate and fast calculation of final filter coefficients is
required, the calculating section 923 needs a high-grade expensive
CPU.
First Embodiment
[0056] The theoretical base of a first embodiment of this invention
will be explained. In genera.sub.1, the transfer function H(Z) of a
second-order IIR (infinite impulse response) filter is expressed as
follows.
H(Z)=(b.sub.0+b.sub.1Z.sup.-1+b.sub.2Z.sup.-2)/(1--a.sub.1Z.sup.-1-a.sub.2-
Z.sup.-2) (1)
[0057] A parametric equalizer is a filter referred to as a PKG
(peaking filter). Parameters related to the parametric equalizer
include a cutoff frequency (or a center frequency) "fc", a gain
(first gain) "k", and a band width "fB". The transfer function H(S)
of the parametric equalizer is defined as follows.
H(S)=1+k.cndot.HB(S) (2)
[0058] where HB(S) denotes the transfer function of a second-order
BPF (band pass filter), and "k" denotes a second gain related to
the first gain "k" as "k=10.sup.k/20-1". The transfer function
HB(S) of the BPF is given by the following equation.
HB(S)=(.omega.c/Q)S/{S.sup.2+(.omega.c/Q)S+.omega.c.sup.2} (3)
[0059] where .omega.c=2.pi.fc, and Q denotes a Q factor given as
"Q=fB/fc".
[0060] Thus, the transfer function H(S) of the PKG is expressed as
follows.
H(S)=1+k(.omega.c/Q)S/{S.sup.2+(.omega.c/Q)S+.omega.c.sup.2}
(4)
[0061] When bilinear z-transform is executed, the equation (4) is
changed into the following equation.
H(Z)=1+k.cndot.(1-Z.sup.-2)/(1-a.sub.1Z.sup.-1-a.sub.2Z.sup.-2)
(5)
[0062] The equation (5) is rewritten as follows.
H(Z)=(b.sub.0+b.sub.1Z.sup.-1+b.sub.2Z.sup.-2)/(1-a.sub.1Z.sup.-1-a.sub.2Z-
.sup.-2) (6)
[0063] where filter coefficients a.sub.1, a.sub.2, b.sub.0,
b.sub.1, and b.sub.2 are given as follows.
a.sub.1=A.cndot.2.cndot.(1-.omega.c.sup.2)/{1+(.omega.c/Q)+.omega.c.sup.2}
(7)
a.sub.2=A.cndot.{(.omega.c/Q)-1-.omega.c.sup.2}/{1+(.omega.c/Q)+.omega.c.s-
up.2} (8)
b.sub.0=A.cndot.{1+(1+k).cndot.(.omega.c/Q)+.omega.c.sup.2}/{1+(.omega.c/Q-
)+.omega.c.sup.2} (9)
b.sub.1=-A.cndot.2.cndot.(1-.omega.c.sup.2)/{1+(.omega.c/Q)+.omega.c.sup.2-
} (10)
b.sub.2=A.cndot.{1+(1+k).cndot.(.omega.c/Q)+.omega.c.sup.2}/{1+(.omega.c/Q-
)+(.omega.c.sup.2} (11)
[0064] where "A" denotes a constant. Variables ".alpha." and
".beta." are used to indicate common terms in the equations
(7)-(11). Specifically, the values ".alpha." and ".beta." represent
the common terms as follows.
.alpha.=(.omega.c/Q)/{1+(.omega.c/Q)+.omega.c.sup.2} (12)
.beta.=(1-.omega.c.sup.2)/{1+(.omega.c/Q)+.omega.c.sup.2} (13)
[0065] Sets of the values ".alpha." and ".beta." which correspond
to different cutoff frequencies respectively are previously
calculated. The previously-calculated values ".alpha." and ".beta."
are used as precalculated basic coefficients (basic filter
coefficients). Signals of the sets of the precalculated basic
values (basic filter coefficients) ".alpha." and ".beta." are
stored in a memory such as a ROM. Specifically, the memory (ROM)
stores a table in which the sets of the precalculated basic values
(basic filter coefficients) ".alpha." and ".beta." are assigned to
different cutoff frequencies respectively. The filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 are called "final
filter coefficients". Expansion equations for calculating final
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2
from precalculated basic coefficients ".alpha." and ".beta.", a Q
factor, and a gain "k" are previously determined on the basis of
the equations (7)-(13).
[0066] The memory (ROM) is accessed in response to an input command
signal so that a signal of a set of precalculated basic
coefficients (basic filter coefficients) ".alpha." and ".beta."
corresponding to a desired cutoff frequency represented by the
input command signal is read out from the memory (ROM) through a
table look-up process.
[0067] A set of final filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 is calculated from the set of the
precalculated basic coefficients ".alpha." and ".beta.", a g
factor, and a gain "k" according to the expansion equations. In
this case, the total number of the bits of signals stored in the
memory (ROM) is suppressed so that a smaller capacity of the memory
(ROM) suffices. In addition, the use of the expansion equations
reduces a load on the calculation of a set of final filter
coefficients.
[0068] FIG. 4 shows a digital filter device according to the first
embodiment of this invention. The digital filter device of FIG. 4
includes a CPU 801, a ROM 802, and a DSP (digital signal processor)
803. The CPU 801 is connected to the ROM 802 and the DSP 803.
[0069] A command signal representing a desired cutoff frequency in
either a low frequency band or-a high frequency band is inputted
into the CPU 801. The CPU 801 includes a combination of an
input/output port (an interface), a signal processing section, a
RAM, and a ROM. The CPU 801 operates in accordance with a program
stored in its internal ROM.
[0070] The DSP 803 forms a filtering section corresponding to a PKG
(peaking filter) having variable lower-side and higher-side cutoff
frequencies. The DSP 803 includes a memory divided into a first
section and a second section. The first section of the memory is
loaded with a signal of a set of filter coefficients which
determines the lower-side cutoff frequency of the filtering
section. The second section of the memory is loaded with a signal
of a set of filter coefficients which determines the upper-side
cutoff frequency of the filtering section.
[0071] A digital signal to be filtered is inputted into the DSP
803. The DSP 803 filters the input digital signal according to
filtering characteristics corresponding to the PKG. The DSP 803
outputs the filtering-resultant digital signal. The input digital
signal is, for example, a digital audio signal. The PKG formed by
the DSP 803 has variable cutoff frequencies, that is, a variable
lower-side cutoff frequency and a variable higher-side cutoff
frequency. The frequency range in which the cutoff frequencies of
the PKG are variable extends between 20 Hz and 20 kHz. This
frequency range is separated into the low frequency band and the
high frequency band. For example, the low frequency band extends
between 20 Hz and 100 Hz while the high frequency band extends
between 100 Hz and 20 kHz. In genera.sub.1, the lower-side cutoff
frequency of the PKG exists in the low frequency band, and the
higher-side cutoff frequency of the PKG exists in the high
frequency band.
[0072] The ROM 802 is divided into a first section and a second
section. The first section of the ROM 802 stores signals of sets of
previously-calculated filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 (see the equations (7)-(11)) which
correspond to different cutoff frequencies in the low frequency
band, respectively. Specifically, the first section of the ROM 802
stores a table in which the sets of the filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 are assigned to
different cutoff frequencies respectively. The second section of
the ROM 802 stores signals of sets of precalculated basic
coefficients ".alpha." and ".beta." (see the equations (12) and
(13)) which correspond to different cutoff frequencies in the high
frequency band, respectively.
[0073] Specifically, the second section of the ROM 802 stores a
table in which the sets of the precalculated basic coefficients
".alpha." and ".beta." are assigned to different cutoff frequencies
respectively. The precalculated basic coefficients are also
referred to as the basic filter coefficients.
[0074] FIG. 5 is a flowchart of a segment of the program for
controlling the CPU 801. The program segment in FIG. 5 is started
each time a command signal representative of a desired cutoff
frequency is inputted into the CPU 801.
[0075] As shown in FIG. 5, a first step 811 of the program segment
decides whether the desired cutoff frequency represented by the
present command signal is in the low frequency band or the high
frequency band. When the desired cutoff frequency is in the low
frequency band, the program advances from the step 811 to a step
812. When the desired cutoff frequency is in the high frequency
band, the program advances from the step 811 to a step 813.
[0076] The step 812 accesses the first section of the ROM 802 in
response to the desired cutoff frequency represented by the present
command signal. The step 812 implements a table look-up process,
and thereby reads out, from the first section of the ROM 802, a
signal of a set of filter coefficients a.sub.1, a.sub.2, b.sub.0,
b.sub.1, and b.sub.2 which corresponds to the desired cutoff
frequency. A step 814 following the step 812 writes the signal of
the set of the filter coefficients a.sub.1, a.sub.2, b.sub.0,
b.sub.1, and b.sub.2, which is provided by the step 812, into the
first section of the memory within the DSP 803. As a result, the
actual lower-side cutoff frequency of the filtering section formed
by the DSP 803 is set or equalized to the desired cutoff frequency
represented by the present command signal. After the step 814, the
current execution cycle of the program segment ends.
[0077] The step 813 accesses the second section of the ROM 802 in
response to the desired cutoff frequency represented by the present
command signal. The step 813 implements a table look-up process,
and thereby reads out, from the second section of the ROM 802, a
signal of a set of precalculated basic coefficients ".alpha." and
".beta." which corresponds to the desired cutoff frequency.
[0078] A step 815 following the step 813 calculates a set of final
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2
from the set of the precalculated basic coefficients ".alpha." and
".beta." provided by the step 813, a Q factor, and a gain "k"
according to a predetermined coefficient expansion algorithm. The
predetermined coefficient expansion algorithm uses the
previously-mentioned expansion equations for calculating final
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2
from precalculated basic coefficients ".alpha." and ".beta.", a Q
factor, and a gain "k". The Q factor and the gain "k" may be preset
or variable. Information of the Q factor and the gain "K" may be
fed from an external device (not shown).
[0079] A step 816 subsequent to the step 815 generates a signal of
the set of the final filter coefficients a.sub.1, a.sub.2, b.sub.0,
b.sub.1, and b.sub.2 which is provided by the step 815. The step
816 writes the signal of the set of the final filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 into the second
section of the memory within the DSP 803. As a result, the actual
higher-side cutoff frequency of the filtering section formed by the
DSP 803 is set or equalized to the desired cutoff frequency
represented by the present command signal. After the step 816, the
current execution cycle of the program segment ends.
[0080] FIG. 6 is an operation flow diagram of the digital filter
device in the first embodiment of this invention. With reference to
FIG. 6, a command signal representative of a desired cutoff
frequency is fed via an input section 1 to a judgement section 2.
The input section 1 and the judgment section 2 correspond to the
CPU 801 in FIG. 4.
[0081] The judgment section 2 decides whether the desired cutoff
frequency represented by the command signal is in a predetermined
low frequency band or a predetermined high frequency band. When the
desired cutoff frequency is in the low frequency band, a memory
section 3 is used. On the other hand, when the desired cutoff
frequency is in the high frequency band, a memory section 4 is
used.
[0082] The memory sections 3 and 4 correspond to the ROM 802 in
FIG. 4. The memory section 3 stores signals of sets of
previously-calculated filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 (see the equations (7)-(11)) which
correspond to different cutoff frequencies in the low frequency
band, respectively. The memory section 4 stores signals of sets of
precalculated basic coefficients ".alpha." and ".beta." (see the
equations (12) and (13)) which correspond to different cutoff
frequencies in the high frequency band, respectively.
[0083] In the case where the judgment section 2 decides that the
desired cutoff frequency is in the low frequency band, the memory
section 3 is accessed in response to the desired cutoff frequency
so that a signal of a set of filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 which corresponds to the desired
cutoff frequency is read out therefrom.
[0084] Then, the signal of the set of the filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 is written into a
first section of a memory within a filtering section 6 which is
assigned to a lower-side cutoff frequency. As a result, the actual
lower-side cutoff frequency of the filtering section 6 is set or
equalized to the desired cutoff frequency represented by the
command signal. The filtering section 6 corresponds to the DSP 803
in FIG. 4. The filtering section 6 provides a PKG (peaking
filter).
[0085] In the case where the judgment section 2 decides that the
desired cutoff frequency is in the high frequency band, the memory
section 4 is accessed in response to the desired cutoff frequency
so that a signal of a set of precalculated basic coefficients
".alpha." and ".beta." which corresponds to the desired cutoff
frequency is read out therefrom. Then, the read-out signal of the
set of the precalculated basic coefficients ".alpha." and ".beta."
is fed to a coefficient expanding section 5. The coefficient
expanding section 5 corresponds to the CPU 801 in FIG. 4. The
coefficient expanding section 5 calculates a set of final filter
coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 from
the set of the precalculated basic coefficients ".alpha." and
".beta.", a Q factor, and a gain "k" according to a predetermined
coefficient expansion algorithm. The predetermined coefficient
expansion algorithm uses the previously-mentioned expansion
equations for calculating final filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2 from precalculated basic
coefficients ".alpha." and ".beta.", a Q factor, and a gain "k".
The Q factor and the gain "k" may be preset or variable.
Information of the Q factor and the gain "k" may be fed from an
external device (not shown). The coefficient expanding section 5
generates a signal of the calculated set of the final filter
coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2.
[0086] Then, the signal of the set of the final filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 is written into a
second section of the memory within the filtering section 6 which
is assigned to a higher-side cutoff frequency. As a result, the
actual higher-side cutoff frequency of the filtering section 6 is
set or equalized to the desired cutoff frequency represented by the
command signal.
[0087] For example, an input digital signal to be filtered by the
filtering section 6 is a digital audio signal. As previously
indicated, the filtering section 6 provides a PKG (peaking filter).
The PKG has variable cutoff frequencies, that is, a variable
lower-side cutoff frequency and a variable higher-side cutoff
frequency. The frequency range in which the cutoff frequencies of
the PKG are variable extends between 20 Hz and 20 kHz. This
frequency range is separated into the low frequency band and the
high frequency band. For example, the low frequency band extends
between 20 Hz and 100 Hz while the high frequency band extends
between 100 Hz and 20 kHz. Specifically, the lower-side cutoff
frequency "fc" and the higher-side cutoff frequency "fc" of the PKG
are in the ranges as follows.
[0088] Low Frequency Band: 20 Hz.ltoreq.fc<100 Hz
[0089] High Frequency Band: 100 Hz.ltoreq.fc.ltoreq.20 kHz
[0090] The memory section 3 stores a table in which as shown in
FIG. 7, sets of previously-calculated filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2 are assigned to different
cutoff frequencies respectively. In the table, the gain (boot
amount) and Q factor of the PKG may also be listed in relation to
different cutoff frequencies. The filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2 are expressed by the
equations (7)-(11) derived from the transfer function of the
digital filter in the equation (6).
[0091] The memory section 4 stores a table in which as shown in
FIG. 8, sets of precalculated basic coefficients ".alpha." and
".beta." are assigned to different cutoff frequencies respectively.
In the table, the gain (boot amount) and Q factor of the PKG may
also be listed in relation to different cutoff frequencies. The
precalculated basic coefficients ".alpha." and ".beta." are
expressed by the equations (12) and (13).
[0092] Overall operation of the digital filter device in the first
embodiment of this invention will be explained. A command signal
representative of a desired cutoff frequency is fed via the input
section 1 to the judgement section 2. The judgment section 2
decides whether the desired cutoff frequency represented by the
command signal is in the low frequency band (20 Hz to 100 Hz) or
the high frequency band (100 Hz to 20 kHz).
[0093] In the case where the judgment section 2 decides that the
desired cutoff frequency is in the low frequency band (20 Hz to 100
Hz), the memory section 3 is accessed in response to the desired
cutoff frequency so that a signal of a set of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 which corresponds
to the desired cutoff frequency is read out therefrom through a
table look-up process. Then, the signal of the set of the filter
coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 is
written into the first section of the memory within the filtering
section 6 which is assigned to a lower-side cutoff frequency. As a
result, the actual lower-side cutoff frequency of the filtering
section 6 is set or equalized to the desired cutoff frequency
represented by the command signal.
[0094] In the case where the judgment section 2 decides that the
desired cutoff frequency is in the high frequency band (100 Hz to
20 kHz), the memory section 4 is accessed in response to the
desired cutoff frequency so that a signal of a set of precalculated
basic coefficients ".alpha." and ".beta." which corresponds to the
desired cutoff frequency is read out therefrom through a table
look-up process. In addition, a signal of a PKG gain and a signal
of a PKG Q factor may be read out from the memory section 4. Then,
the readout signal of the set of the precalculated basic
coefficients ".alpha." and ".beta.", the read-out signal of the PKG
gain, and the read-out signal of the PKG Q factor are fed to the
coefficient expanding section 5. The coefficient expanding section
5 calculates a set of final filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 from the set of the precalculated
basic coefficients ".alpha." and ".beta.", the PKG gain, and the
PKG Q factor according to a predetermined coefficient expansion
algorithm. The predetermined coefficient expansion algorithm uses
the previously-mentioned expansion equations for calculating final
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2
from precalculated basic coefficients ".alpha." and ".beta.", a Q
factor, and a gain "k". The expansion equations include the
equations (7)-(11). The coefficient expanding section 5 generates a
signal of the calculated set of the final filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2. Then, the signal
of the set of the final filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 is written into the second section of
the memory within the filtering section 6 which is assigned to a
higher-side cutoff frequency. As a result, the actual higher-side
cutoff frequency of the filtering section 6 is set or equalized to
the desired cutoff frequency represented by the command signal.
[0095] As previously mentioned, when the desired cutoff frequency
represented by the command signal is in the low frequency band,
filter coefficients to be set in the filtering section 6 are
determined by the table look-up process in which the memory section
3 is accessed. Thus, it is possible to prevent the occurrence of
errors in the filter coefficients set in the filtering section 6
which might be caused by a calculation process. Accordingly, the
filtering section 6 can implement an accurate filtering process. In
addition, it is possible to reduce an amount of calculation work.
On the other hand, when the desired cutoff frequency represented by
the command signal is in the high frequency band, filter
coefficients to be set in the filtering section 6 are determined by
the table look-up process accessing the memory section 3 and also
the coefficient expansion process using the common terms (the
equations (12) and (13)) in the coefficient-representing equations
(7)-(11). The coefficient expansion process makes it sufficient
that the memory section 4 has a small capacity.
Second Embodiment
[0096] A second embodiment of this invention is similar to the
first embodiment (FIGS. 4-8) thereof except for design changes
indicated hereinafter.
[0097] FIG. 9 is an operation flow diagram of a digital filter
device in the second embodiment of this invention. With reference
to FIG. 9, an averaging section (interpolating section) 7 is
provided between a memory section 3 and a filtering section 6. In
addition, an averaging section (interpolating section) 8 is
provided between a memory section 4 and a coefficient expanding
section 5.
[0098] In the case where a desired lower-side cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies assigned to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, an interpolation process is implemented as follows. A
signal of a first set of filter coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of filter coefficients assigned to a cutoff
frequency immediately above the desired cutoff frequency are read
out from the memory section 3. The averaging section 7 calculates
mean filter coefficients between the filter coefficients in the
first set and the filter coefficients in the second set.
[0099] Specifically, the mean filter coefficients
b.sub.0[{(m-1)+m}/2], b.sub.1[{(m-1)+m}/2], b.sub.2[{(m-1)+m}/2],
a.sub.1[{(m-1)+m}/2], and a.sub.2[{(m-1)+m}/2] are expressed as
follows.
b.sub.0[{(m-1)+m}/2]={b.sub.0(m-1)+b.sub.0(m)}/2 (14)
b.sub.1[{(m-1)+m}/2]={b.sub.1(m-1)+b.sub.1(m)}/2 (15)
b.sub.2[{(m-1)+m}/2]={b.sub.2(m-1)+b.sub.2(m)}/2 (16)
a.sub.1[{(m-1)+m}/2]={a.sub.1(m-1)+a.sub.1(m)}/2 (17)
a.sub.2[{(m-1)+m}/2]={a.sub.2(m-1)+a.sub.2(m)}/2 (18)
[0100] where b.sub.0(m-1), b.sub.1(m-1), b.sub.2(m-1),
a.sub.1(m-1), and a.sub.2(m-1) denote the filter coefficients in
the first set, and b.sub.0(m), b.sub.1(m), b.sub.2(m), a.sub.1(m),
and a.sub.2(m) denote the filter coefficients in the second
set.
[0101] In the case where a desired higher-side cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies assigned to respective sets of precalculated basic
coefficients ".alpha." and ".beta." in the memory section 4, an
interpolation process is implemented as follows. A signal of a
first set of precalculated basic coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of precalculated basic coefficients assigned
to a cutoff frequency immediately above the desired cutoff
frequency are read out from the memory section 4. The averaging
section 8 calculates mean basic coefficients between the
precalculated basic coefficients in the first set and the
precalculated basic coefficients in the second set.
[0102] FIG. 10 shows sets of filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 which include sets of filter
coefficients provided by the averaging section 7. As shown in FIG.
10, the filter coefficient b.sub.0 assigned to a cutoff frequency
Fc(m) of 21 Hz is equal to a mean between the filter coefficient
b.sub.0 assigned to a cutoff frequency Fc(m) of 20 Hz and the
filter coefficient bo assigned to a cutoff frequency Fc(m) of 22.4
Hz. Also, each of the filter coefficients b.sub.1, b.sub.2,
a.sub.1, and a.sub.2 assigned to a cutoff frequency Fc(m) of 21 Hz
is equal to a similar mean. In addition, each of the filter
coefficients assigned to a cutoff frequency Fc(m) of 24 Hz, . . . ,
or 85 Hz is equal to a similar mean.
[0103] FIG. 11 shows sets of basic coefficients ".alpha." and
".beta." which include sets of basic coefficients provided by the
averaging section 8. As shown in FIG. 11, the basic coefficient
".alpha." assigned to a cutoff frequency Fc(k) of 105 Hz is equal
to a mean between the basic coefficient ".alpha." assigned to a
cutoff frequency Fc(k) of 100 Hz and the basic coefficient
".alpha." assigned to a cutoff frequency Fc(k) of 112 Hz. Also, the
basic coefficient ".beta." assigned to a cutoff frequency Fc(k) of
105 Hz is equal to a similar mean. In addition, each of the basic
coefficients assigned to a cutoff frequency Fc(k) of . . . 19.2 kHz
is equal to a similar mean.
[0104] The averaging section 7 doubles the total number of
different usable sets of filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 without increasing the capacity of
the memory section 3. The averaging section 8 doubles the total
number of different usable sets of basic filter coefficients
".alpha." and ".beta." without increasing the capacity of the
memory section 4.
[0105] Overall operation of the digital filter device in the second
embodiment of this invention will be explained. A command signal
representative of a desired cutoff frequency is fed via an input
section 1 to a judgement section 2. The judgment section 2 decides
whether the desired cutoff frequency represented by the command
signal is in a low frequency band (20 Hz to 100 Hz) or a high
frequency band (100 Hz to 20 kHz).
[0106] In the case where the desired cutoff frequency is in the low
frequency band (20 Hz to 100 Hz), the judgment section 2 further
decides whether or not the desired cutoff frequency is equal to one
of cutoff frequencies corresponding to respective sets of filter
coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the
memory section 3.
[0107] When the desired cutoff frequency is equal to one of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, a signal of a set of filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2 which corresponds to the
desired cutoff frequency is read out from the memory section 3
through a table look-up process. Then, the signal of the set of the
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2
is transmitted through the averaging section 7, and is written into
a first section of a memory within the filtering section 6 which is
assigned to a lower-side cutoff frequency. As a result, the actual
lower-side cutoff frequency of the filtering section 6 is set or
equalized to the desired cutoff frequency represented by the
command signal.
[0108] When the desired cutoff frequency is equal to none of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, an interpolation process is implemented as follows. A
signal of a first set of filter coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of filter coefficients assigned to a cutoff
frequency immediately above the desired cutoff frequency are read
out from the memory section 3. The signal of the first set of the
filter coefficients and the signal of the second set of the filter
coefficients are fed to the averaging section 7. The averaging
section 7 calculates mean filter coefficients between the filter
coefficients in the first set and the filter coefficients in the
second set. Then, a signal of a set of the calculated mean filter
coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 is
written into the first section of the memory within the filtering
section 6. As a result, the actual lower-side cutoff frequency of
the filtering section 6 is substantially set or equalized to the
desired cutoff frequency represented by the command signal.
[0109] In the case where the desired cutoff frequency is in the
high frequency band (100 Hz to 20 kHz), the judgment section 2
further decides whether or not the desired cutoff frequency is
equal to one of cutoff frequencies corresponding to respective sets
of precalculated basic coefficients ".alpha." and ".beta." in the
memory section 4.
[0110] When the desired cutoff frequency is equal to one of cutoff
frequencies corresponding to respective sets of precalculated basic
coefficients ".alpha." and ".beta." in the memory section 4, a
signal of a set of precalculated basic coefficients ".alpha." and
".beta." assigned to the desired cutoff frequency is read out from
the memory section 4 through a table look-up process. In addition,
a signal of a PKG gain and a signal of a PKG Q factor may be read
out from the memory section 4. Then, the read-out signal of the set
of the precalculated basic coefficients ".alpha." and ".beta.", the
read-out signal of the PKG gain, and the read-out signal of the PKG
Q factor are transmitted through the averaging section 8, and are
fed to the coefficient expanding section 5. The coefficient
expanding section 5 calculates a set of final filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 from the set of the
precalculated basic coefficients ".alpha." and ".beta.", the PKG
gain, and the PKG Q factor according to a predetermined coefficient
expansion algorithm. The predetermined coefficient expansion
algorithm uses the previously-mentioned expansion equations for
calculating final filter coefficients a.sub.1, a.sub.2, b.sub.0,
b.sub.1, and b.sub.2 from precalculated basic coefficients
".alpha." and ".beta.", a Q factor, and a gain "k". The expansion
equations include the equations (7)-(11). The coefficient expanding
section 5 generates a signal of the calculated set of the final
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and
b.sub.2. Then, the signal of the set of the final filter
coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 is
written into a second section of the memory within the filtering
section 6 which is assigned to a higher-side cutoff frequency. As a
result, the actual higher-side cutoff frequency of the filtering
section 6 is set or equalized to the desired cutoff frequency
represented by the command signal.
[0111] When the desired cutoff frequency is equal to none of cutoff
frequencies corresponding to respective sets of precalculated basic
coefficients ".alpha." and ".beta." in the memory section 4, an
interpolation process is implemented as follows. A signal of a
first set of precalculated basic coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of precalculated basic coefficients assigned
to a cutoff frequency immediately above the desired cutoff
frequency are read out from the memory section 4. The signal of the
first set of the precalculated basic coefficients and the signal of
the second set of the precalculated basic coefficients are fed to
the averaging section 8. The averaging section 8 calculates mean
basic coefficients between the basic coefficients in the first set
and the basic coefficients in the second set. Then, a signal of a
set of the calculated mean basic coefficients ".alpha." and
".beta." is fed to the coefficient expanding section 5. In
addition, the signal of the PKG gain and the signal of the PKG Q
factor may be read out from the memory section 4 before being fed
to the coefficient expanding section 5 through the averaging
section 8. The coefficient expanding section 5 calculates a set of
final filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and
b.sub.2 from the set of the mean basic coefficients ".alpha." and
".beta.", the PKG gain, and the PKG Q factor according to a
predetermined coefficient expansion algorithm. The predetermined
coefficient expansion algorithm uses the previously-mentioned
expansion equations for calculating final filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 from basic
coefficients ".alpha." and ".beta.", a Q factor, and a gain "k".
The expansion equations include the equations (7)-(11). The
coefficient expanding section 5 generates a signal of the
calculated set of the final filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2. Then, the signal of the set of the
final filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and
b.sub.2 is written into the second section of the memory within the
filtering section 6. As a result, the actual higher-side cutoff
frequency of the filtering section 6 is substantially set or
equalized to the desired cutoff frequency represented by the
command signal.
[0112] As previously explained, the averaging section 7 doubles the
total number of different usable sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 without increasing
the capacity of the memory section 3. The averaging section 8
doubles the total number of different usable sets of basic
coefficients ".alpha." and ".beta." without increasing the capacity
of the memory section 4.
Third Embodiment
[0113] A third embodiment of this invention is similar to the first
embodiment (FIGS. 4-8) thereof except for design changes indicated
hereinafter.
[0114] FIG. 12 is an operation flow diagram of a digital filter
device in the third embodiment of this invention. With reference to
FIG. 12, a logarithmically averaging section (logarithmically
interpolating section) 9 is provided between a memory section 3 and
a filtering section 6. In addition, a logarithmically averaging
section (logarithmically interpolating section) 10 is provided
between a memory section 4 and a coefficient expanding section
5.
[0115] In the case where a desired lower-side cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, an interpolation process is implemented as follows. A
signal of a first set of filter coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of filter coefficients assigned to a cutoff
frequency immediately above the desired cutoff frequency are read
out from the memory section 3. The logarithmically averaging
section 9 calculates mean filter coefficients between the filter
coefficients in the first set and the filter coefficients in the
second set according to a logarithmically averaging process.
[0116] Specifically, the mean filter coefficients
b.sub.0[{(m-1)+m}/2], b.sub.1[{(m-1)+m}/2], b.sub.2[{(m-1)+m}/2],
a.sub.1[{(m-1)+m}/2], and a.sub.2[{(m-1)+m}/2] are expressed as
follows.
b.sub.0[{(m-1)+m}/2]=log{b.sub.0(m-1).cndot.b.sub.0(m)}/2 (19)
b.sub.1[{(m-1)+m}/2]=log{b.sub.1(m-1).cndot.b.sub.1(m)}/2 (20)
b.sub.2[{(m-1)+m}/2]=log{b.sub.2(m-1).cndot.b.sub.2(m)}/2 (21)
a.sub.1[{(m-1)+m}/2]=log{a.sub.1(m-1).cndot.a.sub.1(m)}/2 (22)
a.sub.2[{(m-1)+m}/2]=log{a.sub.2(m-1).cndot.a.sub.2(m)}/2 (23)
[0117] where b.sub.0(m-1), b.sub.1(m-1), b.sub.2(m-1),
a.sub.1(m-1), and a.sub.2(m-1) denote the filter coefficients in
the first set, and b.sub.0(m), b.sub.1(m), b.sub.2(m), a.sub.1(m),
and a.sub.2(m) denote the filter coefficients in the second
set.
[0118] In the case where a desired higher-side cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of precalculated basic
coefficients ".alpha." and ".beta." in the memory section 4, an
interpolation process is implemented as follows. A signal of a
first set of precalculated basic coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of precalculated basic coefficients assigned
to a cutoff frequency immediately above the desired cutoff
frequency are read out from the memory section 4. The
logarithmically averaging section 10 calculates mean basic
coefficients between the basic coefficients in the first set and
the basic coefficients in the second set according to the
logarithmically averaging process. Specifically, the mean basic
coefficients .alpha.[{(m-1)+m}/2] and .beta.[{(m-1)+m}/2] are
expressed as follows.
.alpha.[{(m-1)+m}/2]=log{.alpha.(m-1).cndot..alpha.(m)}/2 (24)
.beta.[{(m-1)+m}/2]=log{.cndot..beta.(m-1).cndot..beta.(m)}/2
(25)
[0119] where .alpha.(m-1) and .beta.(m-1) denote the precalculated
basic coefficients in the first set, and .alpha.(m) and .beta.(m)
denote the precalculated basic coefficients in the second set.
[0120] The logarithmically averaging section 9 doubles the total
number of different usable sets of filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2 without increasing the
capacity of the memory section 3.
[0121] The logarithmically averaging section 10 doubles the total
number of different usable sets of basic coefficients ".alpha." and
".beta." without increasing the capacity of the memory section 4.
The logarithmically averaging processes implemented by the
logarithmically averaging sections 9 and 10 provide a good auditory
sensation about a change of cutoff frequencies of the filtering
section 6.
[0122] Overall operation of the digital filter device in the third
embodiment of this invention will be explained. A command signal
representative of a desired cutoff frequency is fed via an input
section 1 to a judgement section 2. The judgment section 2 decides
whether the desired cutoff frequency represented by the command
signal is in a low frequency band (20 Hz to 100 Hz) or a high
frequency band (100 Hz to 20 kHz).
[0123] In the case where the desired cutoff frequency is in the low
frequency band (20 Hz to 100 Hz), the judgment section 2 further
decides whether or not the desired cutoff frequency is equal to one
of cutoff frequencies corresponding to respective sets of filter
coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the
memory section 3.
[0124] When the desired cutoff frequency is equal to one of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, a signal of a set of filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2 which corresponds to the
desired cutoff frequency is read out from the memory section 3
through a table look-up process. Then, the signal of the set of the
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2
is transmitted through the logarithmically averaging section 9, and
is written into a first section of a memory within the filtering
section 6 which is assigned to a lower-side cutoff frequency. As a
result, the actual lower-side cutoff frequency of the filtering
section 6 is set or equalized to the desired cutoff frequency
represented by the command signal.
[0125] When the desired cutoff frequency is equal to none of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, an interpolation process is implemented as follows. A
signal of a first set of filter coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of filter coefficients assigned to a cutoff
frequency immediately above the desired cutoff frequency are read
out from the memory section 3. The signal of the first set of the
filter coefficients and the signal of the second set of the filter
coefficients are fed to the logarithmically averaging section 9.
The logarithmically averaging section 9 calculates mean filter
coefficients between the filter coefficients in the first set and
the filter coefficients in the second set according to the
logarithmically averaging process. Then, a signal of a set of the
calculated mean filter coefficients a.sub.1, a.sub.2, b.sub.0,
b.sub.1, and b.sub.2 is written into the first section of the
memory within the filtering section 6. As a result, the actual
lower-side cutoff frequency of the filtering section 6 is
substantially set or equalized to the desired cutoff frequency
represented by the command signal.
[0126] In the case where the desired cutoff frequency is in the
high frequency band (100 Hz to 20 kHz), the judgment section 2
further decides whether or not the desired cutoff frequency is
equal to one of cutoff frequencies corresponding to respective sets
of precalculated basic coefficients ".alpha." and ".beta." in the
memory section 4.
[0127] When the desired cutoff frequency is equal to one of cutoff
frequencies corresponding to respective sets of precalculated basic
coefficients ".alpha." and ".beta." in the memory section 4, a
signal of a set of precalculated basic coefficients ".alpha." and
".beta." assigned to the desired cutoff frequency is read out from
the memory section 4 through a table look-up process. In addition,
a signal of a PKG gain and a signal of a PKG Q factor may be read
out from the memory section 4.
[0128] Then, the read-out signal of the set of the precalculated
basic coefficients ".alpha." and ".beta.", the read-out signal of
the PKG gain, and the read-out signal of the PKG Q factor are
transmitted through the logarithmically averaging section 10, and
are fed to the coefficient expanding section 5. The coefficient
expanding section 5 calculates a set of final filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 from the set of the
precalculated basic filter coefficients ".alpha." and ".beta.", the
PKG gain, and the PKG Q factor according to a predetermined
coefficient expansion algorithm. The predetermined coefficient
expansion algorithm uses the previously-mentioned expansion
equations for calculating final filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2 from precalculated basic
coefficients ".alpha." and ".beta.", a Q factor, and a gain "k".
The expansion equations include the equations (7)-(11). The
coefficient expanding section 5 generates a signal of the
calculated set of the final filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2. Then, the signal of the set of the
final filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and
b.sub.2 is written into a second section of the memory within the
filtering section 6 which is assigned to a higher-side cutoff
frequency. As a result, the actual higher-side cutoff frequency of
the filtering section 6 is set or equalized to the desired cutoff
frequency represented by the command signal.
[0129] When the desired cutoff frequency is equal to none of cutoff
frequencies corresponding to respective sets of precalculated basic
coefficients ".alpha." and ".beta." in the memory section 4, an
interpolation process is implemented as follows. A signal of a
first set of precalculated basic coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of precalculated basic coefficients assigned
to a cutoff frequency immediately above the desired cutoff
frequency are read out from the memory section 4. The signal of the
first set of the precalculated basic coefficients and the signal of
the second set of the precalculated basic coefficients are fed to
the logarithmically averaging section 10. The logarithmically
averaging section 10 calculates mean basic coefficients between the
basic coefficients in the first set and the basic coefficients in
the second set according to the logarithmically averaging process.
Then, a signal of a set of the calculated mean basic coefficients
".alpha." and ".beta." is fed to the coefficient expanding section
5. In addition, the signal of the PKG gain and the signal of the
PKG Q factor may be read out from the memory section 4 before being
fed to the coefficient expanding section 5 through the
logarithmically averaging section 10. The coefficient expanding
section 5 calculates a set of final filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2 from the set of the mean
basic coefficients ".alpha." and ".beta.", the PKG gain, and the
PKG Q factor according to a predetermined coefficient expansion
algorithm. The predetermined coefficient expansion algorithm uses
the previously-mentioned expansion equations for calculating final
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2
from basic coefficients ".alpha." and ".beta.", a Q factor, and a
gain "k". The expansion equations include the equations (7)-(11).
The coefficient expanding section 5 generates a signal of the
calculated set of the final filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2. Then, the signal of the set of the
final filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and
b.sub.2 is written into the second section of the memory within the
filtering section 6. As a result, the actual higher-side cutoff
frequency of the filtering section 6 is substantially set or
equalized to the desired cutoff frequency represented by the
command signal.
[0130] As previously explained, the logarithmically averaging
processes implemented by the logarithmically averaging sections 9
and 10 provide a good auditory sensation about a change of cutoff
frequencies of the filtering section 6.
Fourth Embodiment
[0131] A fourth embodiment of this invention is similar to the
first embodiment (FIGS. 4-8) thereof except for design changes
indicated hereinafter.
[0132] FIG. 13 is an operation flow diagram of a digital filter
device in the fourth embodiment of this invention. With reference
to FIG. 13, a memory section 11 and a coefficient expanding section
5 are provided between a judgment section 2 and a filtering section
6. In addition, a calculating section 13 is provided between the
judgment section 2 and the filtering section 6.
[0133] In the fourth embodiment of this invention, the frequency
range in which the cutoff frequencies of the filtering section 6
(that is, the cutoff frequencies of a PKG) vary is separated into a
predetermined low frequency band, a predetermined intermediate
frequency band, and a predetermined high frequency band. The low
frequency band is lower in frequency than the intermediate
frequency band. The intermediate frequency band is lower in
frequency than the high frequency band.
[0134] The memory section 11 stores signals of sets of
precalculated basic coefficients ".alpha." and ".beta." (see the
equations (12) and (13)) which correspond to different cutoff
frequencies in the intermediate frequency band, respectively. The
memory section 11 is followed by the coefficient expanding section
12.
[0135] The coefficient expanding section 12 calculates a set of
final filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and
b.sub.2 from the set of the precalculated basic filter coefficients
".alpha." and ".beta.", a Q factor, and a gain "k" according to a
predetermined coefficient expansion algorithm. The predetermined
coefficient expansion algorithm uses the previously-mentioned
expansion equations for calculating final filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 from precalculated
basic coefficients ".alpha." and ".beta.", a Q factor, and a gain
"k". The Q factor and the gain "k" may be preset or variable.
Information of the Q factor and the gain "k" may be fed from an
external device (not shown).
[0136] The coefficient expanding section 12 generates a signal of
the calculated set of the final filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2.
[0137] The calculating section 13 computes a set of filter
coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 from a
desired cutoff frequency, a Q factor, and a gain "k" according to a
predetermined coefficient calculation algorithm. The Q factor and
the gain "k" may be preset or variable.
[0138] Information of the Q factor and the gain "k" may be fed from
an external device (not shown). The calculating section 13
generates a signal of the calculated set of the coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2. Overall operation
of the digital filter device in the fourth embodiment of this
invention will be explained. A command signal representative of a
desired cutoff frequency is fed via an input section 1 to the
judgement section 2. The judgment section 2 decides which of the
low frequency band, the intermediate frequency band, and the high
frequency band the desired cutoff frequency represented by the
command signal exists in.
[0139] In the case where the judgment section 2 decides that the
desired cutoff frequency is in the low frequency band, a memory
section 3 is accessed in response to the desired cutoff frequency
so that a signal of a set of filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 which corresponds to the desired
cutoff frequency is read out therefrom through a table look-up
process. Then, the signal of the set of the filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 is written into a
first section of a memory within the filtering section 6 which is
assigned to a lower-side cutoff frequency. As a result, the actual
lower-side cutoff frequency of the filtering section 6 is set or
equalized to the desired cutoff frequency represented by the
command signal.
[0140] In the case where the judgment section 2 decides that the
desired cutoff frequency is in the intermediate frequency band, the
memory section 11 is accessed in response to the desired cutoff
frequency so that a signal of asset of precalculated basic
coefficients ".alpha." and ".beta." which corresponds to the
desired cutoff frequency is read out therefrom through a table
look-up process. In addition, a signal of a PKG gain and a signal
of a PKG Q factor may be read out from the memory section 11. Then,
the read-out signal of the set of the precalculated basic
coefficients ".alpha." and ".beta.", the read-out signal of the PKG
gain, and the read-out signal of the PKG Q factor are fed to the
coefficient expanding section 12. The coefficient expanding section
12 calculates a set of final filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 from the set of the precalculated
basic filter coefficients ".alpha." and ".beta.", the PKG gain, and
the PKG Q factor according to a predetermined coefficient expansion
algorithm. The predetermined coefficient expansion algorithm uses
the previously-mentioned expansion equations for calculating final
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2
from precalculated basic coefficients ".alpha." and ".beta.", a Q
factor, and a gain "k". The expansion equations include the
equations (7)-(11). The coefficient expanding section 12 generates
a signal of the calculated set of the final filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2. Then, the signal
of the set of the final filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 is written into a second section of
the memory within the filtering section 6 which is assigned to an
intermediate/higher-side cutoff frequency. As a result, the actual
intermediate/higher-side cutoff frequency of the filtering section
6 is set or equalized to the desired cutoff frequency represented
by the command signal.
[0141] In the case where the judgment section 2 decides that the
desired cutoff frequency is in the high frequency band, the
judgment section 2 informs the calculating section 13 of the
desired cutoff frequency. In addition, a signal of a gain "k" and a
signal of a Q factor are fed to the calculating section 13. Then,
the calculating section 13 computes a set of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b .sub.1, and b.sub.2 from the desired
cutoff frequency, the Q factor, and the gain "k" according to a
predetermined coefficient calculation algorithm. The Q factor and
the gain "k" may be preset or variable.
[0142] Information of the Q factor and the gain "k" may be fed from
an external device (not shown). The calculating section 13
generates a signal of the calculated set of the coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2Subsequently, the
signal of the calculated set of the filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2 is written into the second
section of the memory within the filtering section 6. As a result,
the actual intermediate/higher-side cutoff frequency of the
filtering section 6 is set or equalized to the desired cutoff
frequency represented by the command signal.
[0143] As previously mentioned, when the desired cutoff frequency
represented by the command signal is in the low frequency band,
filter coefficients to be set in the filtering section 6 are
determined by the table look-up process in which the memory section
3 is accessed. Thus, it is possible to prevent the occurrence of
errors in the filter coefficients set in the filtering section 6
which might be caused by a calculation process. Accordingly, the
filtering section 6 can implement an accurate filtering process. In
addition, it is possible to reduce an amount of calculation
work.
[0144] On the other hand, when the desired cutoff frequency
represented by the command signal is in the intermediate frequency
band, filter coefficients to be set in the filtering section 6 are
determined by the table look-up process accessing the memory
section 11 and also the coefficient expansion process using the
common terms (the equations (12) and (13)) in the
coefficient-representing equations (7)-(11). The coefficient
expansion process makes it sufficient that the memory section 11
has a small capacity.
[0145] When the desired cutoff frequency represented by the command
signal is in the high frequency band, filter coefficients to be set
in the filtering section 6 are determined by the fully calculation
process. Accordingly, it is unnecessary to provide a ROM storing
signals of sets of filter coefficients for cutoff frequencies in
the high frequency band.
Fifth Embodiment
[0146] A fifth embodiment of this invention is similar to the
fourth embodiment (FIG. 13) thereof except for design changes
indicated hereinafter.
[0147] FIG. 14 is an operation flow diagram of a digital filter
device in the fifth embodiment of this invention. With reference to
FIG. 14, an averaging section (interpolating section) 7 is provided
between a memory section 3 and a filtering section 6. In addition,
an averaging section (interpolating section) 8 is provided between
a memory section 11 and a coefficient expanding section 12. The
averaging sections 7 and 8 are similar to those in FIG. 9.
[0148] In the case where a desired lower-side cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, an interpolation process is implemented as follows. A
signal of a first set of filter coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of filter coefficients assigned to a cutoff
frequency immediately above the desired cutoff frequency are read
out from the memory section 3. The averaging section 7 calculates
mean filter coefficients between the filter coefficients in the
first set and the filter coefficients in the second set.
[0149] In the case where a desired intermediate cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of precalculated basic
filter coefficients ".alpha." and ".beta." in the memory section
11, an interpolation process is implemented as follows. A signal of
a first set of precalculated basic coefficients assigned to a
cutoff frequency immediately below the desired cutoff frequency and
a signal of a second set of precalculated basic coefficients
assigned to a cutoff frequency immediately above the desired cutoff
frequency are read out from the memory section 11. The averaging
section 8 calculates mean basic coefficients between the basic
coefficients in the first set and the basic coefficients in the
second set.
[0150] Operation of the digital filter device in the fifth
embodiment of this invention will be explained. A command signal
representative of a desired cutoff frequency is fed via an input
section 1 to a judgement section 2. The judgment section 2 decides
which of a predetermined low frequency band, a predetermined
intermediate frequency band, and a predetermined high frequency
band the desired cutoff frequency represented by the command signal
exists in.
[0151] In the case where the desired cutoff frequency is in the low
frequency band, the judgment section 2 further decides whether or
not the desired cutoff frequency is equal to one of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3.
[0152] When the desired cutoff frequency is equal to one of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, a signal of a set of filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2 which corresponds to the
desired cutoff frequency is read out from the memory section 3
through a table look-up process. Then, the signal of the set of the
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2
is transmitted through the averaging section 7, and is written into
a first section of a memory within the filtering section 6 which is
assigned to a lower-side cutoff frequency. As a result, the actual
lower-side cutoff frequency of the filtering section 6 is set or
equalized to the desired cutoff frequency represented by the
command signal.
[0153] When the desired cutoff frequency is equal to none of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, an interpolation process is implemented as follows. A
signal of a first set of filter coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of filter coefficients assigned to a cutoff
frequency immediately above the desired cutoff frequency are read
out from the memory section 3. The signal of the first set of the
filter coefficients and the signal of the second set of the filter
coefficients are fed to the averaging section 7. The averaging
section 7 calculates mean filter coefficients between the filter
coefficients in the first set and the filter coefficients in the
second set. Then, a signal of a set of the calculated mean filter
coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 is
written into the first section of the memory within the filtering
section 6. As a result, the actual lower-side cutoff frequency of
the filtering section 6 is substantially set or equalized to the
desired cutoff frequency represented by the command signal.
[0154] In the case where the desired cutoff frequency is in the
intermediate frequency band, the judgment section 2 further decides
whether or not the desired cutoff frequency is equal to one of
cutoff frequencies corresponding to respective sets of
precalculated basic coefficients ".alpha." and ".beta." in the
memory section 11.
[0155] When the desired cutoff frequency is equal to one of cutoff
frequencies corresponding to respective sets of precalculated basic
coefficients ".alpha." and ".beta." in the memory section 11, a
signal of a set of precalculated basic coefficients ".alpha." and
".beta." which corresponds to the desired cutoff frequency is read
out from the memory section 11 through a table look-up process. In
addition, a signal of a PKG gain and a signal of a PKG Q factor may
be read out from the memory section 11. Then, the read-out signal
of the set of the precalculated basic coefficients ".alpha." and
".beta.", the read-out signal of the PKG gain, and the read-out
signal of the PKG Q factor are transmitted through the averaging
section 8, and are fed to the coefficient expanding section 12. The
coefficient expanding section 12 calculates a set of final filter
coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 from
the set of the precalculated basic filter coefficients ".alpha."
and ".beta.", the PKG gain, and the PKG Q factor according to a
predetermined coefficient expansion algorithm. The predetermined
coefficient expansion algorithm uses the previously-mentioned
expansion equations for calculating final filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 from precalculated
basic coefficients ".alpha." and ".beta.", a Q factor, and a gain
"k". The expansion equations include the equations (7)-(11). The
coefficient expanding section 12 generates a signal of the
calculated set of the final filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2. Then, the signal of the set of the
final filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and
b.sub.2 is written into a second section of the memory within the
filtering section 6 which is assigned to an
intermediate/higher-side cutoff frequency. As a result, the actual
intermediate/higher-side cutoff frequency of the filtering section
6 is set or equalized to the desired cutoff frequency represented
by the command signal.
[0156] When the desired cutoff frequency is equal to none of cutoff
frequencies corresponding to respective sets of precalculated basic
coefficients ".alpha." and ".beta." in the memory section 11, an
interpolation process is implemented as follows. A signal of a
first set of precalculated basic coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of precalculated basic coefficients assigned
to a cutoff frequency immediately above the desired cutoff
frequency are read out from the memory section 11. The signal of
the first set of the precalculated basic coefficients and the
signal of the second set of the precalculated basic coefficients
are fed to the averaging section 8. The averaging section 8
calculates mean basic coefficients between the basic coefficients
in the first set and the basic coefficients in the second set.
Then, a signal of a set of the calculated mean basic coefficients
".alpha." and ".beta." is fed to the coefficient expanding section
12. In addition, the signal of the PKG gain and the signal of the
PKG Q factor may be read out from the memory section 11 before
being fed to the coefficient expanding section 12 through the
averaging section 8. The coefficient expanding section 12
calculates a set of final filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 from the set of the mean basic
coefficients ".alpha." and ".beta.", the PKG gain, and the PKG Q
factor according to a predetermined coefficient expansion
algorithm. The predetermined coefficient expansion algorithm uses
the previously-mentioned expansion equations for calculating final
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2
from basic coefficients ".alpha." and ".beta.", a Q factor, and a
gain "k". The expansion equations include the equations (7)-(11).
The coefficient expanding section 12 generates a signal of the
calculated set of the final filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2. Then, the signal of the set of the
final filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and
b.sub.2 is written into the second section of the memory within the
filtering section 6. As a result, the actual
intermediate/higher-side cutoff frequency of the filtering section
6 is substantially set or equalized to the desired cutoff frequency
represented by the command signal.
[0157] The averaging section 7 doubles the total number of
different usable sets of filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 without increasing the capacity of
the memory section 3. The averaging section 8 doubles the total
number of different usable sets of basic coefficients ".alpha." and
".beta." without increasing the capacity of the memory section
11.
Sixth Embodiment
[0158] A sixth embodiment of this invention is similar to the
fourth embodiment (FIG. 13) thereof except for design changes
indicated hereinafter.
[0159] FIG. 15 is an operation flow diagram of a digital filter
device in the sixth embodiment of this invention. With reference to
FIG. 15, a logarithmically averaging section (logarithmically
interpolating section) 9 is provided between a memory section 3 and
a filtering section 6. In addition, a logarithmically averaging
section (logarithmically interpolating section) 10 is provided
between a memory section 11 and a coefficient expanding section 12.
The logarithmically averaging sections 9 and 10 are similar to
those in FIG. 12.
[0160] In the case where a desired lower-side cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, an interpolation process is implemented as follows. A
signal of a first set of filter coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of filter coefficients assigned to a cutoff
frequency immediately above the desired cutoff frequency are read
out from the memory section 3. The logarithmically averaging
section 9 calculates mean filter coefficients between the filter
coefficients in the first set and the filter coefficients in the
second set according to a logarithmically averaging process.
[0161] In the case where a desired intermediate cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of precalculated basic
filter coefficients ".alpha." and ".beta." in the memory section
11, an interpolation process is implemented as follows. A signal of
a first set of precalculated basic coefficients assigned to a
cutoff frequency immediately below the desired cutoff frequency and
a signal of a second set of precalculated basic coefficients
assigned to a cutoff frequency immediately above the desired cutoff
frequency are read out from the memory section 11. The
logarithmically averaging section 10 calculates mean basic
coefficients between the basic coefficients in the first set and
the basic coefficients in the second set according to the
logarithmically averaging process.
[0162] Operation of the digital filter device in the sixth
embodiment of this invention will be explained. A command signal
representative of a desired cutoff frequency is fed via an input
section 1 to a judgement section 2. The judgment section 2 decides
which of a predetermined low frequency band, a predetermined
intermediate frequency band, and a predetermined high frequency
band the desired cutoff frequency represented by the command signal
exists in.
[0163] In the case where the desired cutoff frequency is in the low
frequency band, the judgment section 2 further decides whether or
not the desired cutoff frequency is equal to one of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3.
[0164] When the desired cutoff frequency is equal to one of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, a signal of a set of filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2 which corresponds to the
desired cutoff frequency is read out from the memory section 3
through a table look-up process. Then, the signal of the set of the
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2
is transmitted through the logarithmically averaging section 9, and
is written into a first section of a memory within the filtering
section 6 which is assigned to a lower-side cutoff frequency. As a
result, the actual lower-side cutoff frequency of the filtering
section 6 is set or equalized to the desired cutoff frequency
represented by the command signal.
[0165] When the desired cutoff frequency is equal to none of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, an interpolation process is implemented as follows. A
signal of a first set of filter coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of filter coefficients assigned to a cutoff
frequency immediately above the desired cutoff frequency are read
out from the memory section 3. The signal of the first set of the
filter coefficients and the signal of the second set of the filter
coefficients are fed to the logarithmically averaging section 9.
The logarithmically averaging section 9 calculates mean filter
coefficients between the filter coefficients in the first set and
the filter coefficients in the second set according to the
logarithmically averaging process. Then, a signal of a set of the
calculated mean filter coefficients a.sub.1, a.sub.2, b.sub.0,
b.sub.1, and b.sub.2 is written into the first section of the
memory within the filtering section 6. As a result, the actual
lower-side cutoff frequency of the filtering section 6 is
substantially set or equalized to the desired cutoff frequency
represented by the command signal.
[0166] In the case where the desired cutoff frequency is in the
intermediate frequency band, the judgment section 2 further decides
whether or not the desired cutoff frequency is equal to one of
cutoff frequencies corresponding to respective sets of
precalculated basic coefficients ".alpha." and ".beta." in the
memory section 11.
[0167] When the desired cutoff frequency is equal to one of cutoff
frequencies corresponding to respective sets of precalculated basic
coefficients ".alpha." and ".beta." in the memory section 11, a
signal of a set of precalculated basic coefficients ".alpha." and
".beta." which corresponds to the desired cutoff frequency is read
out from the memory section 11 through a table look-up process. In
addition, a signal of a PKG gain and a signal of a PKG Q factor may
be read out from the memory section 11. Then, the read-out signal
of the set of the precalculated basic coefficients ".alpha." and
".beta.", the read-out signal of the PKG gain, and the read-out
signal of the PKG Q factor are transmitted through the
logarithmically averaging section 10, and are fed to the
coefficient expanding section 12. The coefficient expanding section
12 calculates a set of final filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 from the set of the precalculated
basic filter coefficients ".alpha." and ".beta.", the PKG gain, and
the PKG Q factor according to a predetermined coefficient expansion
algorithm. The predetermined coefficient expansion algorithm uses
the previously-mentioned expansion equations for calculating final
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2
from precalculated basic coefficients ".alpha." and ".beta.", a Q
factor, and a gain "k". The expansion equations include the
equations (7)-(11).
[0168] The coefficient expanding section 12 generates a signal of
the calculated set of the final filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2.
[0169] Then, the signal of the set of the final filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 is written into a
second section of the memory within the filtering section 6 which
is assigned to an intermediate/higher-side cutoff frequency. As a
result, the actual intermediate/higher-side cutoff frequency of the
filtering section 6 is set or equalized to the desired cutoff
frequency represented by the command signal.
[0170] When the desired cutoff frequency is equal to none of cutoff
frequencies corresponding to respective sets of precalculated basic
coefficients ".alpha." and ".beta." in the memory section 11, an
interpolation process is implemented as follows. A signal of a
first set of precalculated basic coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of precalculated basic coefficients assigned
to a cutoff frequency immediately above the desired cutoff
frequency are read out from the memory section 11. The signal of
the first set of the precalculated basic coefficients and the
signal of the second set of the precalculated basic coefficients
are fed to the logarithmically averaging section 10. The
logarithmically averaging section 10 calculates mean basic
coefficients between the basic coefficients in the first set and
the basic coefficients in the second set according to the
logarithmically averaging process. Then, a signal of a set of the
calculated mean basic coefficients ".alpha." and ".beta." is fed to
the coefficient expanding section 12. In addition, the signal of
the PKG gain and the signal of the PKG Q factor may be read out
from the memory section 11 before being fed to the coefficient
expanding section 12 through the logarithmically averaging section
10. The coefficient expanding section 12 calculates a set of final
filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2
from the set of the mean basic filter coefficients ".alpha." and
".beta.", the PKG gain, and the PKG Q factor according to a
predetermined coefficient expansion algorithm. The predetermined
coefficient expansion algorithm uses the previously-mentioned
expansion equations for calculating final filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 from basic
coefficients ".alpha." and ".beta.", a Q factor, and a gain "k".
The expansion equations include the equations (7)-(11). The
coefficient expanding section 12 generates a signal of the
calculated set of the final filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2. Then, the signal of the set of the
final filter coefficients a.sub.1, a.sub.2, b.sub.0, b.sub.1, and
b.sub.2 is written into the second section of the memory within the
filtering section 6. As a result, the actual
intermediate/higher-side cutoff frequency of the filtering section
6 is substantially set or equalized to the desired cutoff frequency
represented by the command signal.
[0171] The logarithmically averaging processes implemented by the
logarithmically averaging sections 9 and 10 provide a good auditory
sensation about a change of cutoff frequencies of the filtering
section 6.
Seventh Embodiment
[0172] A seventh embodiment of this invention is similar to the
first embodiment (FIGS. 4-8) thereof except for design changes
indicated hereinafter.
[0173] FIG. 16 is an operation flow diagram of a digital filter
device in the seventh embodiment of this invention. With reference
to FIG. 16, an averaging section (interpolating section) 14 is
provided between a memory section 3 and a filtering section 6. In
addition, an averaging section (interpolating section) 15 is
provided between a memory section 4 and a coefficient expanding
section 5.
[0174] The averaging section 14 includes a cascade combination
(series combination) of 1-st to N-th partial averaging blocks where
N denotes a predetermined natural number equal to or greater than
2. The averaging section 15 includes a cascade combination (series
combination) of 1-st to N-th partial averaging blocks.
[0175] In the case where a desired lower-side cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, an interpolation process is implemented as follows. A
signal of a first set of filter coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of filter coefficients assigned to a cutoff
frequency immediately above the desired cutoff frequency are read
out from the memory section 3. The 1-st partial averaging block in
the averaging section 14 calculates mean filter coefficients
between the filter coefficients in the first set and the filter
coefficients in the second set as the averaging section 7 in FIG. 9
does. Each of the 2-nd to N-th partial averaging blocks in the
averaging section 14 similarly calculates mean filter coefficients
with respect to inputted filter coefficients assigned to two
neighboring cutoff frequencies. The averaging calculation
implemented by each of the 1-st to N-th partial averaging blocks in
the averaging section 14 uses equations similar to the equations
(14)-(18).
[0176] In the case where a desired higher-side cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of precalculated basic
filter coefficients ".alpha." and ".beta." in the memory section 4,
an interpolation process is implemented as follows. A signal of a
first set of precalculated basic coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of precalculated basic coefficients assigned
to a cutoff frequency immediately above the desired cutoff
frequency are read out from the memory section 4. The 1-st
averaging block in the averaging section 15 calculates mean basic
coefficients between the basic coefficients in the first set and
the basic coefficients in the second set as the averaging section 8
in FIG. 9 does. Each of the 2-nd to N-th partial averaging blocks
in the averaging section 15 similarly calculates mean basic
coefficients with respect to inputted basic coefficients assigned
to two neighboring cutoff frequencies.
[0177] Operation of the digital filter device in the seventh
embodiment of this invention is basically similar to the operation
of the digital filter device in the second embodiment (FIG. 9) of
this invention. The averaging section 14 multiplies the total
number of different usable sets of filter coefficients a.sub.1,
a.sub.2, b.sub.0, b.sub.1, and b.sub.2 by a factor of 2N without
increasing the capacity of the memory section 3. The averaging
section 15 multiplies the total number of different usable sets of
basic coefficients ".alpha." and ".beta." by a factor of 2N without
increasing the capacity of the memory section 4.
Eighth Embodiment
[0178] An eighth embodiment of this invention is similar to the
first embodiment (FIGS. 4-8) thereof except for design changes
indicated hereinafter.
[0179] FIG. 17 is an operation flow diagram of a digital filter
device in the eighth embodiment of this invention. With reference
to FIG. 17, a logarithmically averaging section (logarithmically
interpolating section) 16 is provided between a memory section 3
and a filtering section 6. In addition, a logarithmically averaging
section (logarithmically interpolating section) 17 is provided
between a memory section 4 and a coefficient expanding section
5.
[0180] The logarithmically averaging section 16 includes a cascade
combination (series combination) of 1-st to N-th partial
logarithmically averaging blocks where N denotes a predetermined
natural number equal to or greater than 2. The logarithmically
averaging section 17 includes a cascade combination (series
combination) of 1-st to N-th partial logarithmically averaging
blocks.
[0181] In the case where a desired lower-side cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, an interpolation process is implemented as follows. A
signal of a first set of filter coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of filter coefficients assigned to a cutoff
frequency immediately above the desired cutoff frequency are read
out from the memory section 3. The 1-st logarithmically averaging
block in the logarithmically averaging section 16 calculates mean
filter coefficients between the filter coefficients in the first
set and the filter coefficients in the second set as the
logarithmically averaging section 9 in FIG. 12 does. Each of the
2-nd to N-th partial logarithmically averaging blocks in the
averaging section 16 similarly calculates mean filter coefficients
with respect to inputted filter coefficients assigned to two
neighboring cutoff frequencies. The logarithmically averaging
calculation implemented by each of the 1-st to N-th partial
logarithmically averaging blocks in the averaging section 16 uses
equations similar to the equations (19)-(23).
[0182] In the case where a desired higher-side cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of precalculated basic
filter coefficients ".alpha." and ".beta." in the memory section 4,
an interpolation process is implemented as follows. A signal of a
first set of precalculated basic coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of precalculated basic coefficients assigned
to a cutoff frequency immediately above the desired cutoff
frequency are read out from the memory section 4. The 1-st
logarithmically averaging block in the logarithmically averaging
section 17 calculates mean basic coefficients between the basic
coefficients in the first set and the basic coefficients in the
second set as the logarithmically averaging section 10 in FIG. 12
does. Each of the 2-nd to N-th partial logarithmically averaging
blocks in the logarithmically averaging section 17 similarly
calculates mean basic coefficients with respect to inputted basic
coefficients assigned to two neighboring cutoff frequencies. The
logarithmically averaging calculation implemented by each of the
1-st to N-th partial logarithmically averaging blocks in the
averaging section 17 uses equations similar to the equations (24)
and (25).
[0183] Operation of the digital filter device in the eighth
embodiment of this invention is basically similar to the operation
of the digital filter device in the third embodiment (FIG. 12) of
this invention. The logarithmically averaging section 16 multiplies
the total number of different usable sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 by a factor of 2N
without increasing the capacity of the memory section 3. The
logarithmically averaging section 17 multiplies the total number of
different usable sets of basic coefficients ".alpha." and ".beta."
by a factor of 2N without increasing the capacity of the memory
section 4. The logarithmically averaging processes implemented by
the logarithmically averaging sections 16 and 17 provide a good
auditory sensation about a change of cutoff frequencies of the
filtering section 6.
Ninth Embodiment
[0184] A ninth embodiment of this invention is similar to the
fourth embodiment (FIG. 13) thereof except for design changes
indicated hereinafter.
[0185] FIG. 18 is an operation flow diagram of a digital filter
device in the ninth embodiment of this invention. With reference to
FIG. 18, an averaging section (interpolating section) 18 is
provided between a memory section 3 and a filtering section 6. In
addition, an averaging section (interpolating section) 19 is
provided between a memory section 11 and a coefficient expanding
section 12.
[0186] The averaging section 18 includes a cascade combination
(series combination) of 1-st to N-th partial averaging blocks where
N denotes a predetermined natural number equal to or greater than
2. The averaging section 19 includes a cascade combination (series
combination) of 1-st to N-th partial averaging blocks.
[0187] In the case where a desired lower-side cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, an interpolation process is implemented as follows. A
signal of a first set of filter coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of filter coefficients assigned to a cutoff
frequency immediately above the desired cutoff frequency are read
out from the memory section 3. The 1-st partial averaging block in
the averaging section 18 calculates mean filter coefficients
between the filter coefficients in the first set and the filter
coefficients in the second set as the averaging section 7 in FIG. 9
does. Each of the 2-nd to N-th partial averaging blocks in the
averaging section 18 similarly calculates mean filter coefficients
with respect to inputted filter coefficients assigned to two
neighboring cutoff frequencies. The averaging calculation
implemented by each of the 1-st to N-th partial averaging blocks in
the averaging section 18 uses equations similar to the equations
(14)-(18).
[0188] In the case where a desired intermediate cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of precalculated basic
filter coefficients ".alpha." and ".beta." in the memory section
11, an interpolation process is implemented as follows. A signal of
a first set of precalculated basic coefficients assigned to a
cutoff frequency immediately below the desired cutoff frequency and
a signal of a second set of precalculated basic coefficients
assigned to a cutoff frequency immediately above the desired cutoff
frequency are read out from the memory section 11. The 1-st
averaging block in the averaging section 19 calculates mean basic
coefficients between the basic coefficients in the first set and
the basic coefficients in the second set as the averaging section 8
in FIG. 9 does. Each of the 2-nd to N-th partial averaging blocks
in the averaging section 19 similarly calculates mean basic
coefficients with respect to inputted basic coefficients assigned
to two neighboring cutoff frequencies.
[0189] Operation of the digital filter device in the ninth
embodiment of this invention is basically similar to the operation
of the digital filter device in the fourth embodiment (FIG. 14) of
this invention.
[0190] The averaging section 18 multiplies the total number of
different usable sets of filter coefficients a.sub.1, a.sub.2,
b.sub.0, b.sub.1, and b.sub.2 by a factor of 2N without increasing
the capacity of the memory section 3. The averaging section 19
multiplies the total number of different usable sets of basic
coefficients ".alpha." and ".beta." by a factor of 2N without
increasing the capacity of the memory section 11.
Tenth Embodiment
[0191] A tenth embodiment of this invention is similar to the
fourth embodiment (FIG. 13) thereof except for design changes
indicated hereinafter.
[0192] FIG. 19 is an operation flow diagram of a digital filter
device in the tenth embodiment of this invention. With reference to
FIG. 19, a logarithmically averaging section (logarithmically
interpolating section) 20 is provided between a memory section 3
and a filtering section 6. In addition, a logarithmically averaging
section (logarithmically interpolating section) 21 is provided
between a memory section 11 and a coefficient expanding section
12.
[0193] The logarithmically averaging section 20 includes a cascade
combination (series combination) of 1-st to N-th partial
logarithmically averaging blocks where N denotes a predetermined
natural number equal to or greater than 2. The logarithmically
averaging section 21 includes a cascade combination (series
combination) of 1-st to N-th partial logarithmically averaging
blocks.
[0194] In the case where a desired lower-side cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 in the memory
section 3, an interpolation process is implemented as follows. A
signal of a first set of filter coefficients assigned to a cutoff
frequency immediately below the desired cutoff frequency and a
signal of a second set of filter coefficients assigned to a cutoff
frequency immediately above the desired cutoff frequency are read
out from the memory section 3. The 1-st logarithmically averaging
block in the logarithmically averaging section 20 calculates mean
filter coefficients between the filter coefficients in the first
set and the filter coefficients in the second set as the
logarithmically averaging section 9 in FIG. 12 does. Each of the
2-nd to N-th partial logarithmically averaging blocks in the
averaging section 20 similarly calculates mean filter coefficients
with respect to inputted filter coefficients assigned to two
neighboring cutoff frequencies. The logarithmically averaging
calculation implemented by each of the 1-st to N-th partial
logarithmically averaging blocks in the averaging section 20 uses
equations similar to the equations (19)-(23).
[0195] In the case where a desired intermediate cutoff frequency
represented by a command signal is equal to none of cutoff
frequencies corresponding to respective sets of precalculated basic
filter coefficients ".alpha." and ".beta." in the memory section
11, an interpolation process is implemented as follows. A signal of
a first set of precalculated basic coefficients assigned to a
cutoff frequency immediately below the desired cutoff frequency and
a signal of a second set of precalculated basic coefficients
assigned to a cutoff frequency immediately above the desired cutoff
frequency are read out from the memory section 11. The 1-st
logarithmically averaging block in the logarithmically averaging
section 21 calculates mean basic coefficients between the basic
coefficients in the first set and the basic coefficients in the
second set as the logarithmically averaging section 10 in FIG. 12
does. Each of the 2-nd to N-th partial logarithmically averaging
blocks in the logarithmically averaging section 21 similarly
calculates mean basic coefficients with respect to inputted basic
coefficients assigned to two neighboring cutoff frequencies. The
logarithmically averaging calculation implemented by each of the
1-st to N-th partial logarithmically averaging blocks in the
averaging section 21 uses equations similar to the equations (24)
and (25).
[0196] Operation of the digital filter device in the tenth
embodiment of this invention is basically similar to the operation
of the digital filter device in the sixth embodiment (FIG. 15) of
this invention. The logarithmically averaging section 20 multiplies
the total number of different usable sets of filter coefficients
a.sub.1, a.sub.2, b.sub.0, b.sub.1, and b.sub.2 by a factor of 2N
without increasing the capacity of the memory section 3. The
logarithmically averaging section 21 multiplies the total number of
different usable sets of basic coefficients ".alpha." and ".beta."
by a factor of 2N without increasing the capacity of the memory
section 11. The logarithmically averaging processes implemented by
the logarithmically averaging sections 20 and 21 provide a good
auditory sensation about a change of cutoff frequencies of the
filtering section 6.
Eleventh Embodiment
[0197] An eleventh embodiment of this invention is similar to one
of the first to tenth embodiment thereof except that an IIR filter
is of a given-number-order type other than the second-order
type.
Twelfth Embodiment
[0198] A twelfth embodiment of this invention is similar to one of
the first to tenth embodiments thereof except that the PKG is
replaced by an SHL (shelving low pass filter), an SHH (shelving
high pass filter), an HPF (high pass filter), or an LPF (low pass
filter).
Thirteenth Embodiment
[0199] A thirteenth embodiment of this invention is similar to one
of the second, third, fifth, sixth, seventh, eighth, ninth, and
tenth embodiments thereof except that the averaging process is
implemented after the coefficient expanding process.
Fourteenth Embodiment
[0200] A fourteenth embodiment of this invention is similar to the
first to thirteenth embodiments thereof except that the DSP is
replaced by a CPU or another hardware device forming a filtering
section.
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