U.S. patent application number 09/482293 was filed with the patent office on 2002-01-03 for separation of plural band pass filters.
Invention is credited to Allen, Justin L., Thomasson, Samuel L..
Application Number | 20020000875 09/482293 |
Document ID | / |
Family ID | 23915495 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020000875 |
Kind Code |
A1 |
Allen, Justin L. ; et
al. |
January 3, 2002 |
Separation of plural band pass filters
Abstract
The separation of a adjacent band pass filters is improved,
without changing the filters, by inverting the output signals from
alternate filters and not inverting the remaining output signals.
All the output signals are then summed. The result is a deeper
notch in the frequency response of adjacent filters.
Inventors: |
Allen, Justin L.; (Mesa,
AZ) ; Thomasson, Samuel L.; (Gilbert, AZ) |
Correspondence
Address: |
Paul F Wille
6407 E Clinton Street
Scottsdale
AZ
85254
US
|
Family ID: |
23915495 |
Appl. No.: |
09/482293 |
Filed: |
January 13, 2000 |
Current U.S.
Class: |
327/557 |
Current CPC
Class: |
H03H 11/1217 20130101;
H03H 11/1295 20130101 |
Class at
Publication: |
327/557 |
International
Class: |
H03B 001/00 |
Claims
What is claimed as the invention is:
1. An audio processing circuit comprising: a plurality of band pass
filters; a difference amplifier having an inverting input and a
non-inverting input; wherein alternate band pass filters are
coupled to said non-inverting input and the remaining band pass
filters are coupled to said inverting input.
2. The audio processing circuit as set forth in claim 1 wherein
said band pass filters each included two notch filters and a
difference amplifier.
3. The audio processing circuit as set forth in claim 2 wherein
each notch filter includes a twin-T filter with positive
feedback.
4. A method for improving the frequency separation of a plurality
of band pass filters, said method comprising the steps of: applying
a first signal to the plurality of band pass filters to produce a
plurality of output signals; inverting the output signal from
alternate band pass filters, leaving the remaining output signals
non-inverted; and combining the inverted output signals with the
non-inverted output signals.
5. The method as set forth in claim 4 wherein said inverting step
includes the step of: coupling the output signals from alternate
band pass filters to an inverting input of an operational amplifier
and coupling the remaining output signals to a non-inverting input
of the difference amplifier.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application includes disclosure contained in
application Ser. No. 09/466,313, filed Dec. 17, 1999, entitled
"Band Pass Filter from Two Notch Filters", assigned to the assignee
of this invention, and now U.S. Pat. No. ______. The entire
contents of the earlier application is incorporated herein by
reference.
BACKGROUND OF THE INVENTION
[0002] This invention relates to band pass filters and, in
particular, to improving the separation of parallel band pass
filters without changing the filters themselves.
[0003] Frequently, a plurality of band pass filters are coupled in
parallel and the outputs are summed at an active or passive
summation node. As more fully described below, this has the effect
of broadening the response curve of each filter in a pair of
adjoining filters.
[0004] Parallel band pass filters are used in many and diverse
applications, such as equalizers, hearing aids, telephones, and
other audio and radio frequency applications. To consider but one
example, a single telephone may include several sets of filters,
e.g. for detecting multiple tone dialing signals, for noise
reduction, and for echo cancellation. Devices known as
complementary comb filters have been used to eliminate echoes by
having the signal to a speaker filtered through the pass bands of a
first comb filter, thereby falling within the stop bands of a
second, complementary comb filter coupled to a microphone.
[0005] Comb filters are used primarily because the "Q" of most
filters is relatively low, less than twenty and typically about
ten. One definition of "Q" is the ratio of the bandwidth at -3 dB
to the center frequency. The center frequencies in a comb filter
are widely spaced, relative to the bandwidth, and band reject or
stop band filters are used in between pairs of band pass filters.
The "Q" of a filter is not a very good description of the frequency
response of a filter because Q does not describe the shape of the
response curve of the filter, particularly the "skirts" of the
curve. It is desired that the skirts be as close to vertical as
possible, although a vertical skirt can only be approached as a
limit.
[0006] In view of the foregoing, it is therefore an object of the
invention to improve the separation of parallel band pass filters
without changing the filters themselves.
[0007] Another object of the invention is to reduce the bandwidth
of parallel filters without affecting the nominal "Q" of the
filter.
SUMMARY OF THE INVENTION
[0008] The foregoing objects are achieved in this invention in
which the separation of a adjacent band pass filters is improved,
without changing the filters, by inverting the output signals from
alternate filters and not inverting the remaining output signals.
All the output signals are then summed. The result is a deeper
notch in the frequency response of adjacent filters.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] A more complete understanding of the invention can be
obtained by considering the following detailed description in
conjunction with the accompanying drawings, in which:
[0010] FIG. 1 is a block diagram of parallel band pass filters
constructed in accordance with the prior art;
[0011] FIG. 2 is a block diagram of parallel band pass filters
arranged in accordance with the invention;
[0012] FIG. 3 is a chart of curves comparing the frequency
responses of the circuits illustrated in FIGS. 1 and 2;
[0013] FIG. 4 is a schematic of a preferred embodiment of a band
pass filter;
[0014] FIG. 5 is a partial block diagram of a preferred embodiment
of the invention.
DETAILED DESCRIPTION OF THE INVENTION
[0015] In FIG. 1, band pass filters 11, 12, 13, 14, and 15 have
progressively higher center frequencies and are connected in
parallel between input 10 and summation circuit 17. The center
frequencies of the filters are typically geometrically related,
e.g. each center frequency is 1.7 times the next lower center
frequency, and the "Q" of the filters is typically ten. Other
apparatus can be included in such circuitry, depending upon
application. For example, an equalizer also includes a variable
gain stage coupled to the output of each filter.
[0016] FIG. 2 is a block diagram of an audio processing circuit
constructed in accordance with the invention in which alternate
filters are coupled through an inverting amplifier to the summation
network. As illustrated in FIG. 2, the output of filter 12 is
coupled through inverting amplifier 21 to summation circuit 17 and
the output of filter 14 is coupled through inverting amplifier 22
to summation circuit 17. Either filters 11, 13, and 15 or filters
12 and 14 could have their outputs inverted. The effect of
inverting alternate outputs is illustrated in FIG. 3.
[0017] In FIG. 3, curve 31 represents the frequency response of
filter 11, curve 32 represents the frequency response of filter 12,
curve 33 represents the frequency response of filter 13, curve 34
represents the frequency response of filter 14, and curve 35
represents the frequency response of filter 15. The frequency
responses of the filters overlap, although the point at which the
response curves intersect may be far down on the curve. The
response curves intersect approximately half-way between the center
frequencies of the filters.
[0018] The sum of the outputs of filters 11 and 12 follows curve 37
in the region between the center frequencies. Note that the nadir
of curve 37 is distinctly higher than the point at which curves 31
and 32 intersect. The outputs of the remaining filters combine to
produce similar curves.
[0019] When alternate outputs are inverted in accordance with the
invention, the sum of the outputs of filters 11 and 12 follows
curve 38 in the region between the center frequencies. The
difference, represented by double ended arrow 39, is approximately
8.5 dB in one embodiment of the invention. Not only is this a large
additional attenuation between the bands but the skirts of the
response curves are also steeper below approximately -10 dB.
[0020] FIG. 4 is a schematic of a band pass filter incorporating
the invention disclosed and claimed in the application
cross-referenced above. Specifically, a preferred band pass filter
is made by subtracting the outputs from two notch filters. The
particular notch filter chosen is not critical. The notch filter
illustrated in FIG. 4 is known as a twin-T filter with positive
feedback; see "Electronic Filter Design Handbook" by Williams and
Taylor, Third Edition, McGraw-Hill, Inc., 1995, pages 6.38 and
6.39. This particular filter was chosen because of its simplicity,
depth of notch, and because the gain can be adjusted easily, by
changing the ratio of resistors R.sub.7 and R.sub.8 or R.sub.13 and
R.sub.14, to modify the frequency response of the resulting band
pass filter. Band pass filter 40 includes two channels, 41 and 42,
each containing a notch filter and each connected to input 43. The
outputs of the channels are subtracted in amplifier 44.
[0021] FIG. 5 is a partial schematic of a preferred embodiment of
the invention including five filters such as filter 40 (FIG. 4)
each having a different center frequency. In one embodiment of the
invention, the center frequencies were 316 Hz, 534 Hz, 900 Hz, 1525
Hz, 2577 Hz, and the filters were one-third octave.
[0022] Input 50 is coupled to each of band pass filters 51, 52, 53,
54, and 55. The outputs of filters 51, 53, and 55 are coupled to
the non-inverting input of operational amplifier 59, which
preferably has unity gain. The outputs of filters 52 and 54 are
coupled to the inverting input of amplifier 59. This is a simpler
circuit than that illustrated in FIG. 2 but achieves the same
result; namely, subtracting alternate bands from the remaining
bands.
[0023] In operation, band pass filters constructed in accordance
with FIG. 4 can have the skirts of the response curve individually
adjusted and, in particular made steeper than obtainable with
filters of the prior art. Even so, the circuit of FIG. 5 increases
the depth of the notch between filters by approximately 8.5 dB, a
significant improvement over filters of the prior art. Despite the
increased depth between pass bands, a signal passing through two
such filters, as in a telephone for example, showed 3 dB less
attenuation than identical filters in which alternate pass bands
were not inverted. In other words, the output signal was twice as
loud with the invention as without the invention.
[0024] The invention thus reduces the bandwidth of parallel filters
without affecting the nominal "Q" of the filter and provides
improved separation of parallel band pass filters without changing
the filters themselves. Thus, the invention can be used with any
band pass filter.
[0025] Having thus described the invention, it will be apparent to
those of skill in the art that various modifications can be made
within the scope of the invention. For example, the invention can
be used at any frequency and can be implemented in analog or
digital form; e.g. with impedance elements as shown or as a finite
impulse response (FIR) filter or as an infinite impulse response
(IIR) filter. The inverting amplifier could precede the filter in
alternate channels but this is not preferred. Depending upon the
application and the number of filters, one may not invert the
output of every other filter. For example, if fifteen band pass
filters were used, one might invert the outputs of only filters
five, seven, nine, and eleven or invert the outputs of only filters
one, three, five and nine.
* * * * *