U.S. patent application number 11/422785 was filed with the patent office on 2006-09-21 for circuit and method for detecting ac voltage pulses.
Invention is credited to Thomas Hauser, Frank Hurtgen, Christian Kranz.
Application Number | 20060210055 11/422785 |
Document ID | / |
Family ID | 7662590 |
Filed Date | 2006-09-21 |
United States Patent
Application |
20060210055 |
Kind Code |
A1 |
Hauser; Thomas ; et
al. |
September 21, 2006 |
CIRCUIT AND METHOD FOR DETECTING AC VOLTAGE PULSES
Abstract
A detection circuit and method for detecting AC voltage pulses
at a defined frequency relate to first transforming an input signal
to a low-frequency signal by multiplying the input signal by a
mixing frequency. This down-mixed signal can then be filtered and
evaluated. The circuit is particularly suitable for identifying
charge unit signals in the telephone network.
Inventors: |
Hauser; Thomas; (Munchen,
DE) ; Hurtgen; Frank; (Dusseldorf, DE) ;
Kranz; Christian; (Ratingen Lintorf, DE) |
Correspondence
Address: |
SCHWEGMAN, LUNDBERG, WOESSNER & KLUTH, P.A.
P.O. BOX 2938
MINNEAPOLIS
MN
55402
US
|
Family ID: |
7662590 |
Appl. No.: |
11/422785 |
Filed: |
June 7, 2006 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
10431901 |
May 8, 2003 |
7085373 |
|
|
11422785 |
Jun 7, 2006 |
|
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Current U.S.
Class: |
379/399.01 |
Current CPC
Class: |
H04L 27/14 20130101 |
Class at
Publication: |
379/399.01 |
International
Class: |
H04M 1/00 20060101
H04M001/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 8, 2000 |
DE |
100 55 375.3 |
Oct 11, 2001 |
WO |
PCT/DE01/03890 |
Claims
1. A circuit for detecting AC voltage pulses at a defined
frequency, comprising: an analog/digital converter unit for
converting an input signal to a sequence of sample values; at least
one mixing stage for multiplying the sequence of sample values by a
mixing frequency signal, said mixing stage having a quadrature
signal path and an in-phase signal path; at least one filter stage
for defining a permissible intermediate frequency band and for
obtaining converted sample values by filtering the sequence of
sample values having been multiplied by the mixing frequency
signal; and an evaluation unit for determining whether an AC
voltage pulse is present by using the converted sample values, said
evaluation unit squaring, adding and comparing the converted sample
values in said quadrature signal path and in said in-phase signal
path with a threshold value.
2. The circuit according to claim 1, further comprising at least
one decimation stage for reducing a signal rate by omitting sample
values.
3. The circuit according to claim 2, wherein: in said quadrature
signal path the sequence of sample values is multiplied by a sine
signal at the mixing frequency signal; and in said in-phase signal
path the sequence of sample values is multiplied by a cosine signal
at the mixing frequency signal.
4. The circuit according to claim 3, wherein in said quadrature
signal path, the sequence of sample values is multiplied by the
sine signal by multiplying the sequence of sample values by a
sequence 0, 1, 0, -1, 0, . . . .
5. The circuit according to claim 3, wherein in said in-phase
signal path, the sequence of sample values is multiplied by the
cosine signal by multiplying the sequence of sample values by a
sequence 1, 0, -1, 0, 1, . . . .
6. The circuit according to claim 1, wherein the mixing frequency
signal of said mixing stage has a frequency corresponding to the
frequency of the AC voltage pulses that will be detected.
7. The circuit according to claim 1, further comprising: a first
mixing stage defining said mixing stage, said first mixing stage
for multiplying the sequence of sample values by a first mixing
frequency signal having a frequency corresponding to a frequency of
a first type of AC voltage pulses; and a second mixing stage
configured between said first mixing stage and said evaluation
unit; said second mixing stage for multiplying the sequence of
sample values by a second mixing frequency signal having a
frequency corresponding to a difference frequency formed by a
difference between the frequency of the first type of AC voltage
pulses and a frequency of a second type of AC voltage pulses.
8. The circuit according to claim 7, further comprising: a low-pass
filter stage having a cut-off frequency above the difference
frequency; said low-pass filter stage configured between said first
mixing stage and said second mixing stage.
9. The circuit according to claim 1, wherein the AC voltage pulse
is a charge unit signal in a telephone network.
10. A telephone portion, comprising: an analog/digital converter
unit for converting an input signal to a sequence of sample values;
at least one mixing stage coupled to receive the sequence of sample
values and to multiply the sequence of sample values by a mixing
frequency signal; at least one filter stage coupled to filter the
sequence of sample values multiplied by the mixing frequency signal
to define a permissible intermediate frequency band and to provide
converted sample values; an evaluation unit coupled to receive the
converted sample values to determine whether a charge unit signal
is present; and a speech path coupled to for performing audio
signal processing, the charge unit signal being detected via said
speech path.
11. The telephone portion according to claim 10, wherein: said
analog/digital converter unit is configured in the speech path; and
the charge unit signals are digitized by said analog/digital
converter unit.
12. The telephone portion according to claim 10, further comprising
at least one digital signal processor for implementing at least
said mixing stage and said filter stage.
13. A method for identifying different types of AC voltage pulses
each having a defined frequency, the method which comprises:
digitizing an input signal to form a sequence of digitized sample
values; obtaining a sequence of converted sample values by
converting the sequence of sample values using any desired sequence
of steps including multiplying the sequence of sample values by
predetermined mixing frequency signals and filtering the sequence
of sample values to define a permissible intermediate frequency
band; and using the sequence of converted sample values to
determine whether a specific type of AC voltage pulse is present by
squaring, adding and comparing the sequence of converted sample
values with a threshold value.
14. The method according to claim 13, which further comprises,
while performing the step of converting the sequence of sample
values, reducing a signal rate by reducing a number of the sample
values in the sequence of sample values.
15. The method according to claim 13, wherein the step of
multiplying the sequence of sample values by predetermined mixing
frequency signals includes multiplying the sequence of sample
values by a mixing frequency signal having a frequency
corresponding to a frequency of the AC voltage pulses being
identified.
16. The method according to claim 13, wherein the step of
multiplying the sequence of sample values by predetermined mixing
frequency signals includes first multiplying the sequence of sample
values by a first mixing frequency signal having a frequency
corresponding to a frequency of a first type of AC voltage pulses,
and subsequently multiplying the sequence of sample values by a
second mixing frequency signal having a frequency corresponding to
a difference between the frequency of the first type of AC voltage
pulses and a frequency of a second type of AC voltage pulses.
17. The method according to claim 13, wherein the AC voltage pulses
are charge unit signals in a telephone network.
18. A detection circuit, comprising: an analog/digital converter to
convert an input signal to a sequence of sampled values; a mixing
stage having a quadrature signal path and an in-phase signal path
and multiplying said sequence of sampled values by a mixing
frequency signal to provide of mixed sampled values; a filter stage
coupled to receive said mixed sampled values and to filter said
mixed sampled values to converted sample values; and an evaluation
unit coupled to receive mixed sampled values, the evaluation unit
including a circuit to square, and add said converted sample values
and compare results with a threshold value to determine whether an
AC voltage pulse is present.
19. A telephone, comprising: an analog/digital converter to convert
an input signal in a speech path of the telephone to a sequence of
sampled values; a mixing stage to multiply said sequence of sampled
values by a mixing frequency signal to provide mixed sampled values
at an output of the mixing stage, the mixing frequency having a
value determined by a frequency of a charge unit signal for the
telephone; a filter stage coupled to receive said mixed sampled
values and filter said mixed sampled values to converted sampled
values; an evaluation unit coupled to receive said converted sample
values and determine whether the charge unit signal is present.
20. A method of processing an input signal to detect an AC pulse
having a predetermined frequency, comprising: digitizing the input
signal to a sequence of sample values; multiplying said sequence of
sample values by a mixing frequency signal having a frequency
determined by the frequency of the AC pulse to provide mixed sample
values, using a mixing stage having a quadrature signal path and an
in phase signal path; filtering said mixed sample values to provide
converted sample values; and squaring, adding and comparing said
converted sample values with a threshold value, and determining
whether the AC voltage pulse is present.
21. A circuit for detecting AC voltage pulses at a defined
frequency, comprising: an analog/digital converter unit for
converting an input signal to a sequence of sample values; at least
one mixing stage for multiplying the sequence of sample values by a
mixing frequency signal, said mixing stage has a quadrature signal
path in which the sequence of sample values is multiplied by a sine
signal at the mixing frequency; and said mixing stage has an
in-phase signal path in which the sequence of sample values is
multiplied by a cosine signal at the mixing frequency; at least one
filter stage for defining a permissible intermediate frequency band
and for obtaining converted sample values by filtering the sequence
of sample values having been multiplied by the mixing frequency
signal; an evaluation unit for determining whether an AC voltage
pulse is present by using the converted sample values wherein said
evaluation unit squares, adds and compares converted sample values
in said quadrature signal path and in said in-phase signal path
with a threshold value; and at least one decimation stage for
reducing a signal rate by omitting sample values.
22. The circuit according to claim 21, wherein in said quadrature
signal path, the sequence of sample values is multiplied by the
sine signal by multiplying the sequence of sample values by a
sequence 0, 1, 0, -1, 0, . . . .
23. The circuit according to claim 21 , wherein in said in-phase
signal path, the sequence of sample values is multiplied by the
cosine signal by multiplying the sequence of sample values by a
sequence 1, 0, -1, 0, 1, . . . .
24. The circuit according to claim 21, wherein the mixing frequency
signal of said mixing stage has a frequency corresponding to the
frequency of the AC voltage pulses that will be detected.
25. The circuit according to claim 21, further comprising: a first
mixing stage defining said mixing stage, said first mixing stage
for multiplying the sequence of sample values by a first mixing
frequency signal having a frequency corresponding to a frequency of
a first type of AC voltage pulses; and a second mixing stage
configured between said first mixing stage and said evaluation
unit; said second mixing stage for multiplying the sequence of
sample values by a second mixing frequency signal having a
frequency corresponding to a difference frequency formed by a
difference between the frequency of the first type of AC voltage
pulses and a frequency of a second type of AC voltage pulses.
26. The circuit according to claim 25, further comprising: a
low-pass filter stage having a cut-off frequency above the
difference frequency; said low-pass filter stage configured between
said first mixing stage and said second mixing stage.
27. The circuit according to claim 21, wherein the AC voltage pulse
is a charge unit signal in a telephone network.
28. A telephone portion including a circuit for detecting charge
unit signals at a defined frequency, the circuit comprising: an
analog/digital converter unit for converting an input signal to a
sequence of sample values; at least one mixing stage for
multiplying the sequence of sample values by a mixing frequency
signal; at least one filter stage for defining a permissible
intermediate frequency band and for obtaining converted sample
values by filtering the sequence of sample values having been
multiplied by the mixing signal; an evaluation unit for determining
whether a charge unit signal is present by using the converted
sample values; and a speech path via which audio signal processing
is performed; the charge unit signals being detected via the speech
path.
29. The telephone portion according to claim 28, wherein: said
analog/digital converter unit is configured in the speech path; and
the charge unit signals are digitized by said analog/digital
converter unit.
30. The telephone portion according to claim 29, further
comprising: at least one digital signal processor for implementing
at least said mixing stage and said filter stage.
31. A method for identifying different types of AC voltage pulses
each having a defined frequency, the method which comprises:
digitizing an input signal to form a sequence of digitized sample
values; obtaining a sequence of converted sample values by
converting the sequence of sample values using any desired sequence
of steps including multiplying the sequence of sample values by
predetermined mixing frequency signals and filtering the sequence
of sample values to define a permissible intermediate frequency
band and which further comprises, while performing the step of
converting the sequence of sample values, reducing a signal rate by
reducing a number of the sample values in the sequence of sample
values; and using the sequence of converted sample values to
determine whether a specific type of AC voltage pulse is present
wherein the step of determining whether a specific type of AC
voltage pulse is present includes squaring, adding and comparing
the sequence of converted sample values with a threshold value.
32. The method according to claim 31, wherein the step of
multiplying the sequence of sample values by predetermined mixing
frequency signals includes multiplying the sequence of sample
values by a mixing frequency signal having a frequency
corresponding to a frequency of the AC voltage pulses being
identified.
33. The method according to claim 31, wherein the step of
multiplying the sequence of sample values by predetermined mixing
frequency signals includes first multiplying the sequence of sample
values by a first mixing frequency signal having a frequency
corresponding to a frequency of a first type of AC voltage pulses,
and subsequently multiplying the sequence of sample values by a
second mixing frequency signal having a frequency corresponding to
a difference between the frequency of the first type of AC voltage
pulses and a frequency of a second type of AC voltage pulses.
34. The method according to claim 31, wherein the AC voltage pulses
are charge unit signals in a telephone network.
Description
PRIORITY INFORMATION
[0001] This application is a Continuation of U.S. Application Ser.
No. 10/431,901, filed May 8, 2003, now published as U.S. Patent
Application Publication No. 2003/0207677 A1 on Nov. 6, 2003, which
application and publication are incorporated herein by
reference.
BACKGROUND
Field of the Invention
[0002] The invention relates to a circuit for detecting AC voltage
pulses at a defined frequency, to a telephone having such a
circuit, and to a method for identifying AC voltage pulses at a
defined frequency.
[0003] In the European analog telephone network, charge unit pulses
are transmitted to the subscriber terminals, in order to allow the
accumulating charges to be detected at the subscriber end. The
charge unit pulses are short AC voltage pulses at a frequency of 16
kHz, or in some countries at about 12 kHz. These charge pulses must
be identified and counted in the subscriber device.
[0004] Originally, the charge unit signals were output from the
reception path by a bandpass filter, and were supplied to a
mechanical meter, which was incremented by one step for each pulse.
More recent solutions provide for the charge unit signals to be
output from the reception path which is provided for audio signal
processing, and to be filtered by a bandpass filter. The charge
unit signals are then converted by a comparator to square-wave
signals, whose period duration is determined digitally by a
counter. If the digitally detected length of the charge pulse is
within a predefined tolerance band, a charge pulse is
registered.
[0005] This solution has the disadvantage of requiring a relatively
large number of discrete components for outputting and bandpass
filtering the charge unit signals. Discrete components are
expensive and impede further miniaturization of the subscriber
terminals.
SUMMARY OF THE INVENTION
[0006] It is accordingly an object of the invention to provide a
circuit for detecting AC voltage pulses at a defined frequency, a
telephone having such a circuit, and a method for identifying AC
voltage pulses at a defined frequency, which overcome the
above-mentioned disadvantages of the prior art apparatus and
methods of this general type.
[0007] In particular, it is an object of the invention to further
reduce the number of discrete components required to detect AC
voltage pulses at a defined frequency.
[0008] With the foregoing and other objects in view there is
provided, in accordance with the invention, a circuit for detecting
AC voltage pulses at a defined frequency. The circuit includes: an
analog/digital converter unit for converting an input signal to a
sequence of sample values; at least one mixing stage for
multiplying the sequence of sample values by a mixing frequency
signal; at least one filter stage for defining a permissible
intermediate frequency band and for obtaining converted sample
values by filtering the sequence of sample values having been
multiplied by the mixing signal; and an evaluation unit for
determining whether an AC voltage pulse is present by using the
converted sample values.
[0009] The detection circuit has an analog/digital converter unit
for converting the input signal to a sequence of sample values. The
sequence of sample values is multiplied by a mixing frequency
signal using at least one mixing stage. Furthermore, the detection
circuit has at least one filter stage for defining the permissible
intermediate frequency band. The converted sample values are used
by an evaluation unit to determine whether an AC voltage pulse is
present.
[0010] In contrast to the situation in the prior art, the incoming
AC voltage pulse is first of all digitized; the rest of the
processing is then carried out in digital form. Both the mixing
stage or the mixing stages and the filter stage or the filter
stages are in the form of digital circuits or a digital signal
processor.
[0011] The superheterodyne principle is used in order to allow
frequency-specific detection of AC voltage pulses. This is done by
multiplying the sample values of the input signal by a mixing
frequency signal at the frequency .omega..sub.M. An AC voltage
pulse at the frequency .omega..sub.0 is thus on the one hand
up-mixed to the frequency .omega..sub.0+.omega..sub.M, and on the
other hand is down-mixed to the frequency
.omega..sub.0-.omega..sub.M. A downstream filter stage defines the
permissible intermediate frequency band and thus the tolerance band
for the frequency of the AC voltage pulse. For example, the
up-mixed component .omega..sub.0+.omega..sub.M can be suppressed by
a low-pass filter.
[0012] A low-pass filter can also be used to define the permissible
intermediate frequency band for the down-mixed component
.omega..sub.0-.omega..sub.M, because
|.omega..sub.0-.omega..sub.M|.ltoreq..omega..sub.G and thus
.omega..sub.M-.omega..sub.G.ltoreq..omega..sub.0.ltoreq..omega..sub.M+.om-
ega..sub.G at a cut-off frequency .omega..sub.G. Instead of a
low-pass filter, a filter with a different characteristic, for
example, a bandpass filter, may also be used to define the
permissible intermediate frequency band.
[0013] The AC voltage signal to be detected is thus first of all
down-mixed to the intermediate frequency band, and the frequency
band within which AC voltage pulses are intended to be detected is
then defined by limiting the intermediate frequency band. The
evaluation unit then uses the converted sample values to determine
whether an AC voltage pulse has occurred in the frequency window
defined in this way.
[0014] The use of the superheterodyne principle for detecting AC
voltage pulses at a defined frequency means that there is no need
for filter arrangements with discrete components. The signal
processing is carried out completely digitally and can be carried
out by digital signal processors, thus allowing the evaluation
circuit to be implemented in a space-saving and cost-effective
manner.
[0015] According to one advantageous embodiment of the invention,
the circuit for detecting AC voltage pulses has at least one
decimation stage, which reduces the signal rate by omitting sample
values. Decimation stages such as these allow the sampling rate for
the sample values to be reduced in steps; the actual signal
processing can then be carried out at a lower sampling frequency.
More processing time is thus available for signal processing for
each sample value, and the requirements for the performance of the
digital signal processor or of the digital evaluation circuit can
therefore be made less stringent. A further advantage of a low
processing frequency is that the power consumption of the DSP or of
the evaluation circuit is reduced.
[0016] A further advantageous embodiment of the invention provides
for the mixing stage or the mixing stages each to have a quadrature
signal path and an in-phase signal path. The sequence of sample
values is multiplied by the sine signal at the mixing frequency in
the quadrature signal path, and the sequence of sample values is
multiplied by the cosine signal at the mixing frequency in the
in-phase signal path.
[0017] The cosine signal of the mixing frequency, which is used in
the in-phase signal path, is phase-shifted through 90.degree.
relative to the sine signal which is used in the quadrature signal
path. An AC voltage signal which is applied to the input of the
evaluation circuit can thus be processed irrespective of its phase
angle relative to the mixing frequency signal. Either the
quadrature signal path or the in-phase signal path produces the
stronger signal, depending on the phase angle of the input
signal.
[0018] In this case, it is particularly advantageous for the
multiplication by the sine signal at the mixing frequency to be
carried out by multiplying the sequence of sample values by the
sequence 0, 1, 0, -1, 0. . . . This value sequence corresponds to
the profile of the sine signal at the mixing frequency in the
situation where four times the mixing frequency is chosen as the
sampling frequency. If the mixing frequency is 16 kHz, the sampling
frequency must therefore be chosen to be 64 kHz. The multiplication
of the sequence of sample values by the sequence 0, 1, 0, -1, 0 . .
. can be carried out in a simple manner by every alternate sample
value being set to zero and every fourth sample value being
inverted. The multiplication of the sequence of sample values by
the sequence 0, 1, 0, -1, 0 . . . can be linked in a simple manner
to a decimation step, by first of all omitting every alternate
sample value. Every alternate sample value of the remaining sample
values is then inverted.
[0019] In a corresponding manner the multiplication by the cosine
signal at the mixing frequency can be carried out by multiplying
the sequence of sample values by the sequence 1, 0, -1, 0, 1, . . .
, if the sampling frequency is four times as great as the mixing
frequency. This takes account of the 90.degree. phase shift between
the sine signal and the cosine signal.
[0020] It is advantageous for the evaluation unit to in each case
square, add and compare with a threshold value, the converted
sample values in the quadrature signal path and in the in-phase
signal path. Once the AC voltage pulse has passed through the
various mixing and filter stages, the square of the magnitude of
the signal can be obtained by squaring and adding the converted
sample values from the quadrature signal path and from the in-phase
signal path. The splitting of the signal path into a quadrature
signal path and an in-phase signal path always results in the same
square of the magnitude irrespective of the phase angle of the
input signal relative to the mixing signal.
[0021] In the situation where the AC voltage pulse is within the
permissible tolerance band and passes through the various mixing
and filter stages, the threshold value which is defined in the
evaluation unit is exceeded, which means that an AC voltage pulse
of a specific type has been received. If, on the other hand, the
frequency of the AC voltage pulse is not within the predefined
frequency window, then the signal is so severely attenuated by the
mixing and filter stages that the square of the magnitude of the
signal is below the threshold value.
[0022] It is advantageous for the mixing stage to multiply the
sequence of sample values by a mixing frequency signal whose
frequency corresponds to the frequency of the AC voltage pulses to
be detected. The mixing frequency signal .omega..sub.M results in
an AC voltage pulse at the frequency .omega..sub.0 being down-mixed
to the frequency .omega..sub.0-.omega..sub.M. If the frequency of
the mixing signal corresponds to the frequency of the AC voltage
pulses to be detected, then the AC voltage signal which is present
initially is down-mixed to 0 Hz, or to a very low frequency.
[0023] This down-mixed signal can be processed more easily than the
original relatively high-frequency signal. In accordance with the
Nyquist theorem, the down-mixed signal can be detected using
relatively low sampling rates. The down-mixing of the input signal
thus allows the sampling frequency to be reduced. A further
advantage is that the down-mixed, low-frequency signal can be
band-limited by using a low-pass filter stage, in order in this way
to define the permissible frequency window for the AC voltage
signal which was present initially. This frequency window extends
from .omega..sub.M-.omega..sub.G to .omega..sub.M+.omega..sub.G
where .omega..sub.M is the mixing frequency and .omega..sub.G is
the cut-off frequency of the low-pass filter.
[0024] According to a further advantageous embodiment of the
invention, the circuit has a first mixing stage which multiplies
the sequence of sample values by a first mixing frequency signal
having a frequency corresponding to the frequency of a first type
of AC voltage pulses. Furthermore, the circuit has a second mixing
stage, which is arranged between the first mixing stage and the
evaluation unit, and which multiplies the sequence of sample values
by a second mixing frequency signal. The frequency of the second
mixing frequency signal corresponds to the difference between the
frequencies of the first type and of a second type of AC voltage
pulses.
[0025] An AC voltage pulse of the first type which is applied to
the input is down-mixed by the first mixing stage to a frequency of
0 Hz, or to a very low frequency, and can then be processed further
by a first detection path. If, however, an AC voltage pulse of the
second type is applied to the input of the first mixing stage, then
this AC voltage pulse is down-mixed to a frequency which
corresponds to the difference frequency between the first and the
second type of AC voltage pulses. This signal is supplied to the
downstream second mixing stage, whose mixing frequency corresponds
precisely to this difference frequency. The second mixing stage
then down-mixes the applied difference frequency signal to 0 Hz or
to a very low frequency. The signal which is produced at the output
of the second mixing stage is processed further by the second
detection path. Filter stages can be arranged both in the first
detection path (for the first type of AC voltage pulses) and in the
second detection path (for the second type of AC voltage pulses) in
order in each case to define the permissible frequency bands for
the first type and for the second type of AC voltage pulses.
[0026] It is particularly advantageous for a low-pass filter stage
whose cut-off frequency is above the difference frequency to be
arranged between the first mixing stage and the second mixing
stage. Not only the down-mixed signal .omega..sub.0-.omega..sub.M,
but also an up-mixed signal at the frequency
.omega..sub.0+.omega..sub.M are produced at the output of the
second mixing stage. The low-pass filter stage, which is arranged
between the first mixing stage and the second mixing stage,
suppresses this up-mixed signal component, since it is not required
for the further signal processing.
[0027] It is particularly advantageous for the circuit to be used
for detecting charge unit signals in the telephone network. The
charge unit signals are short AC voltage pulses at a defined
frequency of about 16 kHz, or in some countries at about 12 kHz. If
the detection circuit is designed exclusively for processing 16 kHz
pulses, then a single mixing stage with a mixing frequency of 16
kHz is sufficient to down-mix the input signal to low frequencies.
In a corresponding way, a mixing stage which operates at a mixing
frequency of 12 kHz is sufficient for detecting 12 kHz pulses. If,
on the other hand, a single detection circuit is intended to be
used to detect both 12 kHz and 16 kHz pulses, then the input signal
can first of all be down-mixed by a first mixing stage, which is
operated at a mixing frequency of 16 kHz, to 0 Hz (when the signal
is at 16 kHz), or to 4 kHz (when the signal is as 12 kHz). The 16
kHz signal which has been down-mixed to 0 Hz can be evaluated
immediately. The 12 kHz signal which has been down-mixed to 4 kHz
can likewise be down-mixed to 0 Hz by a second mixing stage, which
is operated at a mixing frequency of 4 kHz, and can then be
evaluated.
[0028] The detection circuit for AC voltage pulses at a defined
frequency is particularly suitable for being used in a telephone,
in order to detect the charge unit signals that are transmitted via
the telephone network. A telephone that is equipped with the
detection circuit can be matched to all the charge unit signal
standards that are used in Europe. The production costs are in this
case lower than for previous solutions.
[0029] In this case, it is particularly advantageous for the charge
unit signals to be detected via the speech path, via which the
audio signal processing is also carried out. In contrast to the
situation with previous solutions, there is no need to output the
charge unit pulses from the speech path, and filter arrangements
with discrete components are superfluous for the inventive
solution.
[0030] In particular, it is advantageous for the charge unit
signals to be digitized by the analog/digital converter unit in the
speech path. Modem telephones have an analog/digital converter
unit, which converts the incoming audio signal to digital sample
values. In the inventive solution, this analog/digital converter
unit also carries out the task of digitizing the incoming charge
unit signals. No separate analog/digital converter module is
therefore required for the analog/digital conversion of the charge
unit signals.
[0031] It is particularly advantageous for the various processing
stages for converting the sequence of sample values to be in the
form of one or more digital signal processors. The mixing stages,
filter stages and decimation stages which are required for
evaluating the charge unit signals may be formed completely by
digital signal processors. A digital signal processor can also be
used for the evaluation unit in which the sample values are squared
and added.
[0032] With the foregoing and other objects in view there is
provided, in accordance with the invention, a telephone portion
including a circuit for detecting charge unit signals at a defined
frequency. The circuit includes: an analog/digital converter unit
for converting an input signal to a sequence of sample values; at
least one mixing stage for multiplying the sequence of sample
values by a mixing frequency signal; at least one filter stage for
defining a permissible intermediate frequency band and for
obtaining converted sample values by filtering the sequence of
sample values having been multiplied by the mixing signal; and an
evaluation unit for determining whether a charge unit signal is
present by using the converted sample values.
[0033] In accordance with an added feature of the invention, a
speech path is provided via which audio signal processing is
performed; and the charge unit signals are detected via the speech
path.
[0034] In accordance with an additional feature of the invention,
the analog/digital converter unit is configured in the speech path;
and the charge unit signals are digitized by the analog/digital
converter unit.
[0035] In accordance with another feature of the invention, at
least one digital signal processor is provided for implementing at
least the mixing stage and the filter stage.
[0036] With the foregoing and other objects in view there is
provided, in accordance with the invention, a method for
identifying different types of AC voltage pulses each having a
defined frequency. The method includes steps of: digitizing an
input signal to form a sequence of digitized sample values;
obtaining a sequence of converted sample values by converting the
sequence of sample values using any desired sequence of steps
including multiplying the sequence of sample values by
predetermined mixing frequency signals and filtering the sequence
of sample values to define a permissible intermediate frequency
band; and using the sequence of converted sample values to
determine whether a specific type of AC voltage pulse is
present.
[0037] In accordance with an added feature of the invention, the
method includes, while performing the step of converting the
sequence of sample values, reducing a signal rate by reducing a
number of the sample values in the sequence of sample values.
[0038] In accordance with an additional feature of the invention,
the step of determining whether a specific type of AC voltage pulse
is present includes squaring, adding and comparing the sequence of
converted sample values with a threshold value.
[0039] In accordance with another feature of the invention, the
step of multiplying the sequence of sample values by predetermined
mixing frequency signals includes multiplying the sequence of
sample values by a mixing frequency signal having a frequency
corresponding to a frequency of the AC voltage pulses being
identified.
[0040] In accordance with a further feature of the invention, the
step of multiplying the sequence of sample values by predetermined
mixing frequency signals includes first multiplying the sequence of
sample values by a first mixing frequency signal having a frequency
corresponding to a frequency of a first type of AC voltage pulses,
and subsequently multiplying the sequence of sample values by a
second mixing frequency signal having a frequency corresponding to
a difference between the frequency of the first type of AC voltage
pulses and a frequency of a second type of AC voltage pulses.
[0041] In accordance with a further added feature of the invention,
the AC voltage pulses are charge unit signals in a telephone
network.
[0042] In the method for identifying different types of AC voltage
pulses, each at a defined frequency, the input signal is digitized
in a first step. The sequence of digitized sample values obtained
in this way is then converted by carrying out various steps
including the multiplication of the sequence of sample values by
predetermined mixing frequency signals and filtering the sequence
of sample values. The permissible intermediate frequency band is
defined by filtering the sequence of sample values. It is then
possible to use the sequence of converted sample values to
determine whether a specific type of AC voltage pulse is
present.
[0043] The inventive method allows completely digital selective
identification of various types of AC voltage pulses. In the
method, the input signal is down-mixed to low frequencies by being
multiplied by different mixing frequency signals. First, this has
the advantage that the signal can be filtered considerably more
easily in the low-frequency intermediate frequency band than in the
original input signal frequency band. A further advantage is that
it is possible to use considerably lower signal rates once the
input signal has been transformed to the low-frequency band. This
means that less powerful signal processors can be used and that the
power consumption of the circuit is reduced.
[0044] Other features which are considered as characteristic for
the invention are set forth in the appended claims.
[0045] Although the invention is illustrated and described herein
as embodied in an evaluation circuit for AC voltage pulses, it is
nevertheless not intended to be limited to the details shown, since
various modifications and structural changes may be made therein
without departing from the spirit of the invention and within the
scope and range of equivalents of the claims.
[0046] The construction and method of operation of the invention,
however, together with additional objects and advantages thereof
will be best understood from the following description of specific
embodiments when read in connection with the accompanying
drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0047] The sole drawing FIG. 1 shows a detection circuit that can
detect both 16 kHz and 12 kHz charge unit signals.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0048] Referring now to the sole drawing FIG. 1, there is shown a
detection circuit for detecting AC voltage pulses at a defined
frequency. This detection circuit is particularly suitable for
detecting the charge unit signals that are transmitted via the
telephone network. The circuit and the detection principle are not,
however, restricted to this application, but can be used wherever
identification pulses at a defined frequency should be selectively
detected.
[0049] The analog input signal 1 is first of all supplied to the
analog/digital converter A/D and is converted to a sequence of
digital sample values x(k). If the circuit in a telephone is used
for detecting the charge unit pulses, then the charge unit signals
are digitized by the same analog/digital converter that is also
used to convert the speech signal to digital values, because the
charge unit pulses are not output from the speech path in this
inventive solution.
[0050] The digitized sample values x(k) are now supplied to the
first mixing stage, and for this purpose are split into a
quadrature signal path 2 and an in-phase signal path 3. In the
mixer 4 for the quadrature signal path, the signal is multiplied by
the sine signal sin(.omega..sub.M1t) at the mixing frequency
.omega..sub.M1, where 1 2 .times. .times. .pi. 16 ##EQU1##
.omega..sub.M1 is chosen to be 16 kHz, thus corresponding to the
frequency of the 16 kHz charge unit pulses.
[0051] In the mixer 5 for the in-phase signal path, the signal is
multiplied by the cosine signal cos(.omega..sub.M1t) at the mixing
frequency .omega. M .times. .times. 1 = 1 2 .times. .times. .pi. 16
##EQU2## kHz, that is to say by a mixing signal which has been
phase-shifted through 90.degree.. The charge unit pulses can
therefore be detected independently of their phase angle relative
to the mixing frequency signals.
[0052] The signal rate at the output of the analog/digital
converter and in the mixers 4 and 5 is 64 kHz and is therefore four
times the mixing frequency f.sub.M1. Sampling is therefore carried
out four times in each clock cycle of the mixing frequency,
therefore resulting in the sequence 0, 1, 0, -1, 0, . . . ( sin
.times. .times. ( .pi. k 2 ) , where .times. .times. k = 0 , 1 , 2
, ) ##EQU3## as the sine signal at the mixing frequency. In a
corresponding manner, the sequence 1, 0, -1, 0, 1, . . . ( cos
.times. .times. ( .pi. k 2 ) , where .times. .times. k = 0 , 1 , 2
, ) ##EQU4## is obtained for the cosine signal at the mixing
frequency. The multiplication of the input signal by the sine
sequence in the mixer 4 thus results in a signal sequence in which
every alternate value is set to be equal to zero. A signal sequence
in which every alternate value is equal to zero is also produced at
the output of the mixer 5 of the in-phase signal path. In the
subsequent decimation stages 6 and 7, the signal rate is in each
case reduced from f.sub.S=64 kHz to f.sub.S=32 kHz, by omitting
those values which have been set to zero in the signal
sequence.
[0053] An up-mixed signal in each case appears at the outputs of
the mixers 4 and 5, whose frequency corresponds to the sum of the
frequencies of the input signal and of the mixing signal, as well
as a down-mixed signal whose frequency corresponds to the
difference frequency between the input signal and the mixing
signal. A 16 kHz pulse is on the one hand up-mixed to 32 kHz by the
mixing frequency f.sub.M1=16 kHz, and is on the other hand
down-mixed to 0 kHz (superheterodyne principle). In a corresponding
manner, a 12 kHz pulse which is applied to the input of the first
mixing stage is on the one hand up-mixed to 28 kHz, and is on the
other hand down-mixed to 4 kHz, by the mixing frequency of 16 kHz.
The-low-pass filters 8 and 9 (TP1) have the task of filtering out
the up-mixed frequency components and of allowing only the 0 kHz
component of the 16 kHz pulse as well as the 4 kHz component of the
12 kHz pulse to pass. It is therefore worthwhile to define the
cut-off frequency of the low-pass filters 8 and 9 (TP1) in the
quadrature and in-phase path, respectively, to be 4.5 kHz. The
low-pass filters 8 and 9 are followed by the decimation stages 10
and 11, which once again reduce the signal from f.sub.S=32 kHz to
f.sub.S=16 kHz. The repeated reduction in the signal rate allows
the further signal processing to be significantly simplified.
[0054] The following text is based on the assumption that the
detection circuit for charge unit pulses has been set to identify
16 kHz pulses. In this case, there is no need to use a second
mixing frequency to once again up-mix or down-mix the signals which
are produced at the output of the decimation stages 10 and 11,
because the 16 kHz pulses have already been down-mixed by the
mixers 4 and 5 from 16 kHz to 0 kHz.
[0055] The mixer 12 is thus switched to be inactive, or multiplies
every value applied to it by 1. The actual band limiting of the
down-mixed signal is carried out by the low-pass filters 13 and 14
(TP2/TP3, TP2). The low-pass filters 13, 14 are used to define the
intermediate frequency band within which AC voltage pulses are
intended to be detected. If, for example, the cut-off frequency of
the low-pass filters 13 and 14 is fixed at 0.5 kHz, then AC
voltages at frequencies from -0.5 kHz to +0.5 kHz can pass through
the low-pass filters. This corresponds to a permissible frequency
band from 15.5 kHz to 16.5 kHz in the input signal. The calculation
example shows that the permissible band range for AC voltage pulses
to be detected can be defined in a simple manner by low-pass
filtering the intermediate frequencies.
[0056] The sampling frequency is once again halved in the
subsequent decimation stages 15 and 16 (from f.sub.S=16 kHz to
f.sub.S=8 kHz), by omitting every alternate signal value. The
signal values which then remain are supplied to the multipliers 17
and 18, which square each signal value. The adder 19 in each case
forms the sum of the squares of the quadrature and in-phase signal
values, and thus determines the square of the magnitude of the
respective signal value, to be precise independently of the phase
angle of the input signal. The threshold value decision device 20
compares the result supplied from the adder 19 with a predetermined
threshold value. If this threshold value is exceeded, then the
detection signal y.sub.16(k) from the threshold value decision
device 20 indicates that a 16 kHz pulse has been received.
[0057] Charge unit pulses at a frequency of about 12 kHz are used
in some European countries. If the circuit illustrated in the
drawing FIG. 1 is intended to evaluate such 12 kHz pulses, then, in
addition to the first mixing frequency of f.sub.M1=16 kHz, a second
mixing frequency of f.sub.M2=4 kHz must also be used. For this
purpose, the quadrature signal which is produced at the output of
the decimation stage 10 and has been down-mixed by the mixer 4 to
an intermediate frequency of 4 kHz is supplied to the mixers 12 and
21. The mixers 12 and 21 multiply the signal applied to them by the
sine signal sequence and cosine signal sequence, respectively, for
the mixing frequency f.sub.M2=4 kHz.
[0058] Since the sampling frequency f.sub.S is in this case 16 kHz
and is thus four times the mixing frequency, the cosine signal
sequence once again becomes 1. 0, -1, 0, 1, . . . ( cos .times.
.times. ( .pi. k 2 ) , where .times. .times. k = 0 , 1 , 2 , ) ,
##EQU5## and the sine sequence becomes 0. 1, 0, -1, 0, . . . ( sin
.times. .times. ( .pi. k 2 ) , where .times. .times. k = 0 , 1 , 2
, ) . ##EQU6##
[0059] Multiplication by a mixing frequency signal at the frequency
4 kHz results in the intermediate frequency signal which is applied
to the mixers 12 and 21 on the one hand being up-mixed to 8 kHz and
on the other hand being down-mixed to 0 kHz. The low-pass filters
13 and 22 (TP2/TP3, TP3) which are arranged downstream in the
signal path on the one hand eliminate the up-mixed 8 kHz component.
On the other hand, with regard to the signal component which has
been down-mixed to 0 kHz, the cut-off frequency of the low-pass
filters 13 and 22 defines the permissible intermediate band range
within which pulses are detected.
[0060] The downstream decimation stages 15 and 23 halve the signal
rate from f.sub.S=16 kHz to f.sub.S=8 kHz by omitting every
alternate sample value. The evaluation is carried out by the
multipliers 17 and 24, by the adder 25 and by the threshold value
decision device 26. If the sum of the squares of the signal values
is greater than a predetermined threshold, then a 12 kHz pulse is
present. This is indicated by the detection signal y.sub.12(k)
being produced at the output of the threshold value decision device
26.
* * * * *