A METHOD FOR THE DETECTION OF A DISABLE TONE SIGNAL OF AN ECHO CANCELLER

Abstract: The invention relates to a disable tone detection method in an echo canceller. The disable tone signal is within a predetermined tolerance range from a nominal frequency and includes phase inversions occurring at predetermined intervals. The method according to the invention comprises steps of a) removing frequencies outside said tolerance range from an input signal (Sin, Rin) by band-pass filtering (33), b) sampling (34) in the filtered signal at a sampling frequency, said sampling frequency being about half of said nominal frequency, so as to aliase the sample signal to the vicinity of the zero frequency, c) creating a substantially in-phase version for said aliased sample signal component, d) delaying said in-phase version by about 2n sample signal periods, where n is a positive integer, e) detecting (34) a phase inversion in the disable tone signal by means of an XOR operation executed on the aliased sample signal component and the delayed version thereof.

Description: A method for the detection of a disable tone signal of an echo canceller Field of the Invention The invention relates to a method for the de- tection of a disable tone signal in an echo cancel¬ ler, said disable tone signal being within a prede¬ termined tolerance range from a nominal frequency and including phase inversions occurring at predetermined intervals. Background of the Invention End-to-end connections of a data transmission system, such as a telephone network, often show long transit time delays, in consequence of which echo is observed for instance in the case of normal speech, when a signal is reflected from the far end of a con¬ nection back to the talker. An echo canceller is an analog or digital device for processing a signal, such as a speech signal, so as to reduce echo by sub¬ tracting estimated echo from the echo (signal) . Normally, an echo canceller shall be capable of detecting a so-called disable signal. Upon detecting the disable tone, the echo canceller is switched to a "transparent" state, in which the echo canceller does not process a signal passing through. The charac¬ teristics of the disable tone are defined accurately in CCITT recommendation V.25. A disable tone is a signal of about 2100 Hz with phase inversions at in- tervals of (450 ± 25). The disable tone detection of the echo canceller shall respond to this particular signal but not e.g. to speech or a 2100 Hz signal with no phase inversions. Disable tones are monitored both at the receiving and transmitting end of each telephone channel. Prior art echo cancellers with disable tone detection are disclosed e.g. in U.S. Patents 5,029,204 and 5,029,204. Disclosure of the Invention The object of the invention is a method for the detection of a disable tone signal in an echo can¬ celler. This is achieved by means of the method of the invention, which is characterized by a) removing frequencies outside said tolerance range from an input signal by band-pass filtering, b) sampling the band-pass <filtered signal at a sampling frequency, said sampling frequency being about half of said nominal frequency, the resulting sample signal component being aliased to the vicinity of a zero frequency, c) creating a substantially in-phase version for said aliased sample signal component, d) delaying said in-phase version by about 2n sample signal periods, where n is a positive integer, e) detecting a phase inversion in said disable tone signal by means of a logical XOR operation exe¬ cuted on said aliased sample signal component and the delayed version thereof. According to the invention, all frequencies falling outside a monitored frequency band are remov¬ ed from a received signal by band-pass filtering. After this, the tendency of a signal of being aliased to lower frequencies, when the sampling frequency is lower than the Nyquist frequency, is utilized for the detection of a disable tone. To this end, the filter¬ ed signal is sampled at a sampling frequency which is only about half of the nominal frequency of the dis¬ able tone, resulting in an aliased sample signal co - ponent close to the zero frequency. When the sample rate of the original signal is for instance 8000 sam¬ ples/second (PCM signal) and the sampling frequency is 1000 Hz, then only every eighth original sample signal transmitted in the telephone channel is fur- ther processed. Thanks to this solution, a signal processor or another device applying the method of the invention is capable of processing both signal directions in several telephone channels. When a signal at the preset frequency and on a preset level is detected, the signal is tested in order to find a phase inversion of about 180 degrees. This takes place by means of an XOR operation exe¬ cuted between the aliased sample signal component and the version thereof delayed by two sample signal pe- riods. In-phase signals always have like signs (plus or minus), but if there exists a phase inversion in the disable tone, the original sample signal compo¬ nent and the delayed version thereof have temporarily unlike signs. By means of the method of the invention, it is possible to provide a very simple disable tone detec¬ tion requiring a short processing time. Brief Description of the Drawings The invention will be explained in the follow¬ ing in greater detail by means of illustrative embo¬ diments referring to the enclosed drawings, in which Figure 1 shows a block diagram of an echo can¬ celler according to the invention, Figure 2 shows a graph illustrating frequency characteristics of a disable tone detector, the ver- tical axis representing the signal level and the ho¬ rizontal axis the frequency, Figure 3 illustrates an aliasing in a frequency domain due to the sampling, and Figures 4A to 4C are signal diagrams illustrat¬ ing the XOR operation to be used for the detection. Detailed Description of the Invention Figure 1 shows a digital echo canceller, to which the inventive detection of a disable tone sig¬ nal can be applied. The echo canceller comprises an input port S in and an output port S out of a digital signal to be transmitted as well as an input port R in and an output port R out of a digital signal to be re¬ ceived. The echo canceller shall eliminate the echo only in the transmission « irection, in this applica¬ tion called far end (the echo canceller can alterna¬ tively be constructed to eliminate the echo in the receiving direction). The opposite direction is called near end. The ports S ln and R out are connected to the transmission path of the near end and the ports S out and R in to the transmission path of the far end. The echo to be cancelled is a component r(i) of a speech signal x(i) received at the port R in from the far end and transmitted forward over the port R out to the near end (to the echo path), which component is reflected from the near end. A transmission signal y(i) of the near end is linearized by an A-law converter 35A, processed by means of an adaptive FIR filter 21 and finally ap¬ plied to a change-over switch SW1. The switch SW1 controlled by a control unit 26 connects the output of the filter 21 either over a non-linear processor 27 or an A-law converter 36 to the output port S out of the echo canceller. The adaptive digital filter 21 is for instance a digital transversal filter, which models an impulse response of the echo path. A control unit 22 controls the operation, adaptation and updating of the adap- tive filter 21 on the basis of the levels of the sig¬ nals y(i) and x(i), which levels are obtained by means of level detectors 24 and 25, respectively, and on the basis of an enable tone detection, which is performed by means of an enable tone detection cir¬ cuitry 37. The signal x(i) is linearized by A-law convert¬ ers 35B, 35C and 35D, before being applied to a sam¬ ple buffer 23, the level detector 25 and the enable tone detection circuitry 37, respectively. Samples taken from the signal x(i) are stored in the sample buffer 23, from which they are applied, if needed, to the filter 21 for the calculation of a correlation between them and samples taken from the signal y(i). Coefficients a k of the filter 21 are updated on the basis of this correlation. In the echo canceller shown in Figure 1, the detection of a disable tone is performed by means of the circuitry 37. Firstly, the signal y(i) received from the converter 35A and/or the signal x(i) receiv¬ ed from the converter 35D is applied over a change¬ over switch SW2 to a band-pass filter 33, having a center-frequency of about 2100 Hz and a pass-band of for instance about 1900 to 2350 Hz, at least about 2079 to 2121 Hz. The filter 33 removes all the other frequencies except the ones within the frequency band to be examined. The frequency band required for the detector 37 is illustrated in Figure 2. From Figure 2 is seen that the detection always shall succeed at frequencies 2100±21 Hz (zone 10) of the disable tone signal, while the operation at fre¬ quencies 1900 to 2350 Hz (zone 11) is undefined, i.e. the detection may work, but it does not need to work. Outside the frequency range 1900 to 2350 Hz (zone 12) the detection must not work. The level of the output signal (i.e. enable tone signal) of the filter 37 is measured by a level detector 32. The input " signal of the filter 33 is also applied to a band stop filter 31, the stop band of which is substantially the same as the pass band of the filter 33 and which removes the 1900 to 2350 Hz detection band from the input signal and leaves a surrounding guard band for the level detection by means of a level detector 30. The power level of the guard band is measured in order that the speech can¬ not be misinterpreted as a disable tone. Again ac¬ cording to Figure 2, when white noise (300 to 3400 Hz) is applied to the detection circuitry 37 simul¬ taneously with a 2100 Hz signal having a level of -31 dBmO, the white noise shall prevent the detection, if the noise level rises close to the level of the 2100 Hz signal. When the level of the disable tone signal is raised, the white noise shall prevent a switching off of the echo canceller within the whole frequency range on levels lower than the disable tone level. When the control unit 22 detects a signal at the preset frequency and on a preset level on the basis of the outputs of the level detectors 30 and 32, the signal is tested to find a phase inversion of 180±25 degrees in the signal. For this purpose, the output signal of the filter 33 is applied also to a phase inversion detector 34. Firstly, the phase in¬ version detector 34 samples the signal at a sampling frequency which is about half of the 2100 Hz tone frequency. In the preferred embodiment of the inven¬ tion the sampling frequency is 1000 Hz. As illustrat¬ ed in Figure 3, an aliased sample signal component 40' results in the vicinity of the zero frequency in consequence of sampling an original signal 40 having a frequency of about 2100 Hz. Then the detector 34 generates a substantially in-phase component 40" de¬ layed by 2n signal periods, preferably 2 periods, with respect to the sample signal component 40' aliased in the vicinity of the zero frequency, where n is a positive integer, as illustrated in Figures 4A and 4B. Then the detector 34 detects a phase inver¬ sion in the disable tone by means of an XOR operation executed between the aliased sample signal component 40' and the delayed version 40" thereof. An XOR oper- ation can for instance be executed on the signs of the signals 40' and 40". When a logic XOR operation is executed on the sample signal component 40' and the delayed version 40", the signals 40' and 40" be¬ ing substantially in-phase and of the same sign, the result is a logic state 0, as illustrated in Figure 4C. When a phase inversion occurs in the signal 40' at time 41, a corresponding phase inversion does not occur in the delayed version 40" until approximately two periods later at time 41 ' . Then the XOR operation temporarily gives a logic state 1, because due to the phase inversion 41 in the disable tone signal a phase and sign of the delayed version 40" temporarily dif¬ fer from those of the original sample signal compo¬ nent 40', as illustrated in Figure 4C. The duration of the temporary change in the result of the XOR op¬ eration expresses the amount of the phase change in the disable tone signal. Before a phase inversion can be detected, a delay shall be searched for for the delayed sample signal component 40", by which delay the delayed sam¬ ple is located as accurately as possible at the dis¬ tance of two periods from the respective sample of the original signal. This is carried out by experi¬ menting with all delays between 7 to 20 sampling pe- riods. These delay differences correspond to a 140 to 50 Hz frequency range, to which 2050 to 2140 Hz fre¬ quencies are aliased and which thus is at the same time the frequency range within which a signal may be detected, while according to the specifications, fre- quencies between 2079 and 2121 Hz must be detected. A delay by which the XOR operation yields only one or no result with unlike signs during one signal period will be close to the actual frequency delayed by about one signal period, and the obtained delay can be used as a delay value in the subsequent stages of detection. If the XOR operation always gives a simi¬ lar result-, the signals are in-phase. On the other hand, if the signals are in-phase all the time, they also have the same frequencies. If a phase inversion occurs when a frequency is searched for, it results in an unsuccessful search in the event that the phase inversion is coincident with finding of the desired frequency. In that case the frequency search procedure restarts from the begin- ning. At renewal, no phase inversions should be pre¬ sent in the signal, because phase inversions shall occur at intervals of (450±30) ms and a search lasts about 100 ms at the most. The maximum detection time of a phase inversion is thus (2*100ms)+480ms = 680ms. In the preferred embodiment of the invention all the blocks shown in Figure 1 are realized by software in a digital signal processor. The invention has above been described in con¬ nection with particular exemplifying echo canceller types and digital transversal filters. The method of the invention can, however, be applied to all echo cancellers. The figures and the description relating to them are only intended to illustrate the present in- vention, anyway. As to the details, the method of the invention can vary within the scope of the enclosed claims.

Claims: Claims : 1. A method for the detection of a disable tone signal in an echo canceller, said disable tone signal being within a predetermined tolerance range from a nominal frequency and including phase inversions occurring at predetermined intervals, c h a r a c t e r i z e d by a) removing frequencies outside said tolerance range from an input signal by band-pass filtering, b) sampling the band-pass filtered signal at a sampling frequency, said sampling frequency being about half of said nominal frequency, the resulting sample signal component being aliased to the vicinity of a zero frequency, c) creating a substantially in-phase version for said aliased sample signal component, d) delaying said in-phase version by about 2n sample signal periods, where n is a positive integer, e) detecting a phase inversion in said disable tone signal by means of a logical XOR operation exe¬ cuted on said aliased sample signal component and the delayed version thereof. 2. A method according to claim 1, c h a r a c- t e r i z e d in that the logical XOR operation re¬ sults in a first logical state, when said sample sig¬ nal component and said delayed version are substan¬ tially in-phase and of the same sign, and in a second logical state, the delayed version having temporari- ly, in consequence of a phase inversion occurring in said disable tone signal, a phase and sign unlike those of the very sample signal component, and that the phase inversion is detected on the basis of said temporary change in the result yielded by the XOR operation. 3. A method according to claim 2, c h a r ¬ a c t e r i z e d in that the duration of said tem¬ porary change in the result yielded by the XOR opera¬ tion expresses the amount of the phase change in the disable tone signal. 4. A method according to claim 1, 2 or 3, c h a r a c t e r i z e d in that the delay for de¬ laying said sample signal component by about 2n peri¬ ods is searched for before the detection of the phase inversion, said search comprising testing with a suitable number of delays, each of which delays af¬ fecting a delay of about 2 signal periods for one of the frequency components aliased to the vicinity of the zero frequency, and selecting the delay by which the XOR operation executed on said sample signal com¬ ponent and said delayed version thereof gives only one or no result indicating the unlike phases of the signals during one period of the sample signal. 5. A method according to any of the preceding claims, c h a r a c t e r i z e d in that said nom¬ inal frequency is 2100 Hz, said tolerance range is 2079 to 2121 Hz, the sampling frequency is 1000 Hz, the frequency to which said nominal frequenzy aliases in the vicinity of the zero frequency being 100 Hz and the frequency range to which the tolerance range aliases in the vicinity of the zero frequency being 140 to 50 Hz. 6. A method according to claim 5, c h a r a c¬ t e r i z e d in that delays between 7 and 20 sam- pling periods are tested in the delay search. 7. A method according to any of the preceding claims, c h a r a c t e r i z e d in that a phase inversion of about 155 to 205 degrees occurs in the disable tone signal at intervals of about 420 to 480 ms.

Inventors: LAEHDEMAEKI HEIMO, LAEHDEMAEKI, HEIMO
Applicants: NOKIA TELECOMMUNICATIONS OY
Application year: 1992
Main classification: H04B3
IPC codes: H04B3