CA1220586A - Apparatus for multifrequency signalling tone detection - Google Patents
Apparatus for multifrequency signalling tone detectionInfo
- Publication number
- CA1220586A CA1220586A CA000475008A CA475008A CA1220586A CA 1220586 A CA1220586 A CA 1220586A CA 000475008 A CA000475008 A CA 000475008A CA 475008 A CA475008 A CA 475008A CA 1220586 A CA1220586 A CA 1220586A
- Authority
- CA
- Canada
- Prior art keywords
- input signal
- tone
- filter
- band
- filters
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired
Links
Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04Q—SELECTING
- H04Q1/00—Details of selecting apparatus or arrangements
- H04Q1/18—Electrical details
- H04Q1/30—Signalling arrangements; Manipulation of signalling currents
- H04Q1/44—Signalling arrangements; Manipulation of signalling currents using alternate current
- H04Q1/444—Signalling arrangements; Manipulation of signalling currents using alternate current with voice-band signalling frequencies
- H04Q1/46—Signalling arrangements; Manipulation of signalling currents using alternate current with voice-band signalling frequencies comprising means for distinguishing between a signalling current of predetermined frequency and a complex current containing that frequency, e.g. speech current
Abstract
APPARATUS FOR MULTIFREQUENCY SIGNALLING TONE DETECTION
Abstract of the Disclosure Apparatus for detection of the presence of signalling tones in telephony is provided wherein the output levels for four band-pass filters in the VF band are processed by means of logic circuits to yield a "tone present" decision. Selection of two critical thresholds permits the apparatus to operate optimally within its environment by trading off talk-off immunity with minimum acceptable signal-to-noise ratio.
Abstract of the Disclosure Apparatus for detection of the presence of signalling tones in telephony is provided wherein the output levels for four band-pass filters in the VF band are processed by means of logic circuits to yield a "tone present" decision. Selection of two critical thresholds permits the apparatus to operate optimally within its environment by trading off talk-off immunity with minimum acceptable signal-to-noise ratio.
Description
Q5~
APPARATUS FOR MULTIFREQUENCY SIGNALLING TONE DETECTION
Field of the Invention The present invention relates generally to signal detectors and particularly to multifrequency signalling tone detectors. More particularly, it relates to the detection of signalling tones in telephony in the presence of speech, echoes and low signal-to-noise ratios without sacriFicing good talk-oFf immunity from near-end speech.
Background of the Invention Multi~requency tone signalling is an important function in telephone systems. For example, the digits of a telephone number are encoded as combinations oF eight diFferent signal frequency tones within the voice frequency (VF) band. It is thus possible to encode a maximum of sixteen diF-Ferent signalling states of which ten represent the ten digits zero to nine. Four of the signalling tones are in the lower portion and the other -Four are in the upper portion of the VF band. Since these signalling tones fa1l within the speech (VF) band~ the signalling tone detectors or receivers must be able to recognize the occasional presence of a speech signal component at one of the eight signalling tone Frequencies as such. This is particularly difficult in the presence of near-end talk-oFf speech, where high energy signa1s are oFten found in the lower portion of the VF band, below, say 800 Hz.
The adverse e-Ffects o-F talk-off speech may be mitigated by raising the signal-to-noise (S/N) ratio threshold below which a signal is not accepted as a signalling tone. This, however, would reduce the sensitivity of tone detection in valid situations i8f~
where the (S/N) rat10 is low, as for examp1e far-end tones 1n the presence of near-end speech echoes, The choice of threshold (which yields a trade-of~
between immunity and minimum S/N ratio necessary for operation) is a matter for the system designer. It should be selected to best suit the environment in which the tone receiver operates. For example, where speech echo is not a problem (where an echo canceller is on the telephone line), the minimum operational S/N ratio may be raised to permit better talk-off immunity.
Whatever the required values of talk-off immunity and minimum S/N ratio, the apparatus of the present invention is a practical device for selecting the optimal combination for the particular system environment.
Summary of the Invention The key components of the apparatus provided by the present invention are four frequency bandpass filters:
- Low-band filter;
- Full-band filter;
- Low-tone -Filter; and - High-tone filter.
All four filters yield a-t their ultimate outputs an average of the input signal level within the respective band over a predetermined period of time, nominally elght milliseconds (8 msec).
The average levels from the four filters are processed to yield an instantaneous decision at the circuit output indicatins presence of a signalling tone.
Accordingly, the present invention provides apparatus ... .
S~6 for indicating presence of signalling tones in an input signal in presence of noise, comprising: a plurality of Filter means each having an output level representative of spectral power within a predetermined frequency band of said input signal; first summing means for subtracting output levels of a firs-t predetermined pair o-f Filters in said plurality of filter means to yield a power difference level; means for calculating the input signal level from output levels of a second predetermined pair of filters in said plurality of filter means; second summing means for subtracting said input signal level from the output level of a predetermined one of said plurality of filter means to yield a signal-to-noise ratio related level; and means for indicating presence of a signalling tone in said input signal when said first and second summing means simultaneously yield levels exceeding predetermined first and second thresholds, respectively.
In particular, the plurality oF filter means comprises a Full-band Filter for said input signal; a low-band filter For a low frequency por-tion of said input signal; a high-tone fil-ter -for high frequency signalliny -tones in said inpu-t signal; and a low-tone fil-ter for low frequency tones in said inpu-t signal, thereby permitting adjustment to trade off -talk-off immuni-ty and signal-to-noise ra-tio.
Brief Descr~iption of the Drawings The preferred embodimen-t of the present invention will now be described in detail in conjunc-tion with the attached drawings, in which:
Figure 1 is a functional block diagram of the . lZZ~S~
tone detection apparatus according to the present invention;
Figure 2 is a band-wave realization of a portion of the apparatus shown in Figure 1, and Figure 3 is a functional block-diagram of the signal calculator shown in Figure 1.
Detailed Description oF the Preferred Embodiment Turning to Figure 1 of the drawings, the tone detection apparatus comprises a low-band filter 10 for passing frequencies below 800 Hz, a full-band filter 11 for passing the VF
band, a low-tone filter 12 for passing the portion of the VF band containing the low-tone frequency of concern, and a high-tone filter 13 for passing the portion of the VF band containing the high-tone frequency of concern. The two filters 12 and 13 are the two out of eight filters that show the highest power at their outputs and, hence, could be any two if no actual signal is present. Each oF
the filters 10, 11, 12 and 13 is fed by the input signal and has connected to its output a respective one of four rectiFier/averager (R/A) circuits 14, 15, 16 and 17, the time-constant of which is 8 msec. The R/A 14 therefore outputs an instantaneous level B
representative of the average (over 8 msec) signal power (in the input signal) in the low-band below 800 Hz. Similarly, the other R/As 15, 16 and 17 output instantaneous levels T, F1 and F2, respectively. The four levels B, T, F1 and F2 are processed algebraically and logically either by means of software in a microprocessor (not shown) or by means of hard-wired circuitry.
Functionally, the levels B and T are processed as shown in block 18, a~6 wherein (T-B) is calculated and if found less than a predetermined power difference Cl a NO TONE decision is made. If on the other hand (T-B) is equal to or greater than Cl, then a cond1tional YES is given. In -this preferred embodimen~ Cl = 7.5 dB, for a choice of talk-off immunity o~ 52%.
The levels Fl and F2 are simultaneously calculated to yield a signal level S in block 19. The level S is defined as follows:
S = max (FI, F2) - 3 IFl - F2¦+ 3 dB, where ¦F1 - F2¦= abs (Fl - F2) and is known in the art as TWIST. In block 20 the difference (T - S) is calculated and if found less than a predetermined constant C2 a NO TONE decision is made. If, however, (T - S) is equal to or greater than C2, then a conditional YES is given. In this preferred embodiment C2 = 1 dB.
A ~inal TONE P~ESENT decision is only glven if both the decision blocks 18 and 20 yield simultaneously a conditional YES. This is a co-function and is indicated by AND yate 21.
As will be appreciated by those skilled in the art, the power difference (T - B) - Cl is a power representative of speech content, while the power difference (T - S) = C2 is representative of the signal-to-noise (S/N) ratio. It can be shown -that (T - S) = 10 log (1 + 10 O.l(S/N)) S/N, which yields C2 = 1 dB for S/N = 6 dB.
The advantage of using software to perform the decisions is the ease with which the constants Cl and C2 may be altered to suit the particular environment in which the apparatus :Q z;~
is operating, preferably aFter some field trial.
For some applications, possibly for reasons of cost and/or reliability, full hardware realization oF -the apparatus may be desirable, In that case, the decision blocks 18, 19 and 20 may be realized as shown in Figures 2 and 3.
Figure 2 shows a simple hardware realization of the blocks 18 and 20. A differential amplifier 30 generates the difference (T - B) (or (T - S)), which difference is compared in comparator 31 to a reference level representing C1 (or C2), and which is set by potentiometer 32. The output of the comparator yields YES when C1 (or C2) is exceeded.
In Figure 3, realization of the signal calculator 19 is shown schematically. The levels Fl and F2 are connected via a transfer switch 40, which is controlled by comparator 41, to multiplexers 42 and 43, respectively, or vice versa, depending on whe-ther Fl>F2 or F1<F2. Summer 44 subtracts the output of the multiplexer 42 from that of the multiplexer 43, which diFFerence is divided by eight in divider 45. The result from the divider 45 is subtracted from three to yield the signal level S in summer 46. The circuit shown in Figure 3 calculates as follows:
For F1~F2 S = - F1 + ~ F2 + 3 dB
For F1<F2 S = - F2 + 8 F1 + 3 dB.
APPARATUS FOR MULTIFREQUENCY SIGNALLING TONE DETECTION
Field of the Invention The present invention relates generally to signal detectors and particularly to multifrequency signalling tone detectors. More particularly, it relates to the detection of signalling tones in telephony in the presence of speech, echoes and low signal-to-noise ratios without sacriFicing good talk-oFf immunity from near-end speech.
Background of the Invention Multi~requency tone signalling is an important function in telephone systems. For example, the digits of a telephone number are encoded as combinations oF eight diFferent signal frequency tones within the voice frequency (VF) band. It is thus possible to encode a maximum of sixteen diF-Ferent signalling states of which ten represent the ten digits zero to nine. Four of the signalling tones are in the lower portion and the other -Four are in the upper portion of the VF band. Since these signalling tones fa1l within the speech (VF) band~ the signalling tone detectors or receivers must be able to recognize the occasional presence of a speech signal component at one of the eight signalling tone Frequencies as such. This is particularly difficult in the presence of near-end talk-oFf speech, where high energy signa1s are oFten found in the lower portion of the VF band, below, say 800 Hz.
The adverse e-Ffects o-F talk-off speech may be mitigated by raising the signal-to-noise (S/N) ratio threshold below which a signal is not accepted as a signalling tone. This, however, would reduce the sensitivity of tone detection in valid situations i8f~
where the (S/N) rat10 is low, as for examp1e far-end tones 1n the presence of near-end speech echoes, The choice of threshold (which yields a trade-of~
between immunity and minimum S/N ratio necessary for operation) is a matter for the system designer. It should be selected to best suit the environment in which the tone receiver operates. For example, where speech echo is not a problem (where an echo canceller is on the telephone line), the minimum operational S/N ratio may be raised to permit better talk-off immunity.
Whatever the required values of talk-off immunity and minimum S/N ratio, the apparatus of the present invention is a practical device for selecting the optimal combination for the particular system environment.
Summary of the Invention The key components of the apparatus provided by the present invention are four frequency bandpass filters:
- Low-band filter;
- Full-band filter;
- Low-tone -Filter; and - High-tone filter.
All four filters yield a-t their ultimate outputs an average of the input signal level within the respective band over a predetermined period of time, nominally elght milliseconds (8 msec).
The average levels from the four filters are processed to yield an instantaneous decision at the circuit output indicatins presence of a signalling tone.
Accordingly, the present invention provides apparatus ... .
S~6 for indicating presence of signalling tones in an input signal in presence of noise, comprising: a plurality of Filter means each having an output level representative of spectral power within a predetermined frequency band of said input signal; first summing means for subtracting output levels of a firs-t predetermined pair o-f Filters in said plurality of filter means to yield a power difference level; means for calculating the input signal level from output levels of a second predetermined pair of filters in said plurality of filter means; second summing means for subtracting said input signal level from the output level of a predetermined one of said plurality of filter means to yield a signal-to-noise ratio related level; and means for indicating presence of a signalling tone in said input signal when said first and second summing means simultaneously yield levels exceeding predetermined first and second thresholds, respectively.
In particular, the plurality oF filter means comprises a Full-band Filter for said input signal; a low-band filter For a low frequency por-tion of said input signal; a high-tone fil-ter -for high frequency signalliny -tones in said inpu-t signal; and a low-tone fil-ter for low frequency tones in said inpu-t signal, thereby permitting adjustment to trade off -talk-off immuni-ty and signal-to-noise ra-tio.
Brief Descr~iption of the Drawings The preferred embodimen-t of the present invention will now be described in detail in conjunc-tion with the attached drawings, in which:
Figure 1 is a functional block diagram of the . lZZ~S~
tone detection apparatus according to the present invention;
Figure 2 is a band-wave realization of a portion of the apparatus shown in Figure 1, and Figure 3 is a functional block-diagram of the signal calculator shown in Figure 1.
Detailed Description oF the Preferred Embodiment Turning to Figure 1 of the drawings, the tone detection apparatus comprises a low-band filter 10 for passing frequencies below 800 Hz, a full-band filter 11 for passing the VF
band, a low-tone filter 12 for passing the portion of the VF band containing the low-tone frequency of concern, and a high-tone filter 13 for passing the portion of the VF band containing the high-tone frequency of concern. The two filters 12 and 13 are the two out of eight filters that show the highest power at their outputs and, hence, could be any two if no actual signal is present. Each oF
the filters 10, 11, 12 and 13 is fed by the input signal and has connected to its output a respective one of four rectiFier/averager (R/A) circuits 14, 15, 16 and 17, the time-constant of which is 8 msec. The R/A 14 therefore outputs an instantaneous level B
representative of the average (over 8 msec) signal power (in the input signal) in the low-band below 800 Hz. Similarly, the other R/As 15, 16 and 17 output instantaneous levels T, F1 and F2, respectively. The four levels B, T, F1 and F2 are processed algebraically and logically either by means of software in a microprocessor (not shown) or by means of hard-wired circuitry.
Functionally, the levels B and T are processed as shown in block 18, a~6 wherein (T-B) is calculated and if found less than a predetermined power difference Cl a NO TONE decision is made. If on the other hand (T-B) is equal to or greater than Cl, then a cond1tional YES is given. In -this preferred embodimen~ Cl = 7.5 dB, for a choice of talk-off immunity o~ 52%.
The levels Fl and F2 are simultaneously calculated to yield a signal level S in block 19. The level S is defined as follows:
S = max (FI, F2) - 3 IFl - F2¦+ 3 dB, where ¦F1 - F2¦= abs (Fl - F2) and is known in the art as TWIST. In block 20 the difference (T - S) is calculated and if found less than a predetermined constant C2 a NO TONE decision is made. If, however, (T - S) is equal to or greater than C2, then a conditional YES is given. In this preferred embodiment C2 = 1 dB.
A ~inal TONE P~ESENT decision is only glven if both the decision blocks 18 and 20 yield simultaneously a conditional YES. This is a co-function and is indicated by AND yate 21.
As will be appreciated by those skilled in the art, the power difference (T - B) - Cl is a power representative of speech content, while the power difference (T - S) = C2 is representative of the signal-to-noise (S/N) ratio. It can be shown -that (T - S) = 10 log (1 + 10 O.l(S/N)) S/N, which yields C2 = 1 dB for S/N = 6 dB.
The advantage of using software to perform the decisions is the ease with which the constants Cl and C2 may be altered to suit the particular environment in which the apparatus :Q z;~
is operating, preferably aFter some field trial.
For some applications, possibly for reasons of cost and/or reliability, full hardware realization oF -the apparatus may be desirable, In that case, the decision blocks 18, 19 and 20 may be realized as shown in Figures 2 and 3.
Figure 2 shows a simple hardware realization of the blocks 18 and 20. A differential amplifier 30 generates the difference (T - B) (or (T - S)), which difference is compared in comparator 31 to a reference level representing C1 (or C2), and which is set by potentiometer 32. The output of the comparator yields YES when C1 (or C2) is exceeded.
In Figure 3, realization of the signal calculator 19 is shown schematically. The levels Fl and F2 are connected via a transfer switch 40, which is controlled by comparator 41, to multiplexers 42 and 43, respectively, or vice versa, depending on whe-ther Fl>F2 or F1<F2. Summer 44 subtracts the output of the multiplexer 42 from that of the multiplexer 43, which diFFerence is divided by eight in divider 45. The result from the divider 45 is subtracted from three to yield the signal level S in summer 46. The circuit shown in Figure 3 calculates as follows:
For F1~F2 S = - F1 + ~ F2 + 3 dB
For F1<F2 S = - F2 + 8 F1 + 3 dB.
Claims (5)
EXCLUSIVE PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. Apparatus for indicating presence of signalling tones in an input signal in presence of noise, comprising:
a plurality of filter means each having an output level representative of spectral power within a predetermined frequency band of said input signal:
first summing means for subtracting output levels of a first predetermined pair of filters in said plurality of filter means to yield a power difference level;
means for calculating the input signal level from output levels of a second predetermined pair of filters in said plurality of filter means;
second summing means for subtracting said input signal level from the output level of a predetermined one of said plurality of filter means to yield a signal-to-noise ratio related level; and means for indicating presence of a signalling tone in said input signal when said first and second summing means simultaneously yield levels exceeding predetermined first and second thresholds, respectively.
a plurality of filter means each having an output level representative of spectral power within a predetermined frequency band of said input signal:
first summing means for subtracting output levels of a first predetermined pair of filters in said plurality of filter means to yield a power difference level;
means for calculating the input signal level from output levels of a second predetermined pair of filters in said plurality of filter means;
second summing means for subtracting said input signal level from the output level of a predetermined one of said plurality of filter means to yield a signal-to-noise ratio related level; and means for indicating presence of a signalling tone in said input signal when said first and second summing means simultaneously yield levels exceeding predetermined first and second thresholds, respectively.
2. The apparatus for indicating presence of signalling tones in an input signal in the presence of noise as defined in claim 1, said plurality of filter means comprising:
a full-band filter for said input signal;
a low-band filter for a low frequency portion of said input signal;
a high-tone filter for high frequency signalling tones in said input signal, and a low-tone filter for low frequency tones in said input signal, thereby permitting adjustment to trade off talk-off immunity and signal-to-noise ratio.
a full-band filter for said input signal;
a low-band filter for a low frequency portion of said input signal;
a high-tone filter for high frequency signalling tones in said input signal, and a low-tone filter for low frequency tones in said input signal, thereby permitting adjustment to trade off talk-off immunity and signal-to-noise ratio.
3. The apparatus for indicating presence of signalling tones in an input signal in the presence of noise as defined in claim 2, said first predetermined pair of filters being said full-band and low-band filters.
4. The apparatus for indicating presence of signalling tones in an input signal in the presence of noise as claimed in claim 3, such second predetermined pair of filters being said low-tone and high-tone filters.
5. The apparatus for indicating presence of signalling tones in an input signal in the presence of noise as claimed in claim 4, said predetermined one of said plurality of filter means being said full-band filter.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000475008A CA1220586A (en) | 1985-02-22 | 1985-02-22 | Apparatus for multifrequency signalling tone detection |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA000475008A CA1220586A (en) | 1985-02-22 | 1985-02-22 | Apparatus for multifrequency signalling tone detection |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA1220586A true CA1220586A (en) | 1987-04-14 |
Family
ID=4129894
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA000475008A Expired CA1220586A (en) | 1985-02-22 | 1985-02-22 | Apparatus for multifrequency signalling tone detection |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA1220586A (en) |
-
1985
- 1985-02-22 CA CA000475008A patent/CA1220586A/en not_active Expired
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| MKEX | Expiry |