WO1998028857A1 - Double talk and echo path change detection in a telephony system - Google Patents
Double talk and echo path change detection in a telephony system Download PDFInfo
- Publication number
- WO1998028857A1 WO1998028857A1 PCT/SE1997/002076 SE9702076W WO9828857A1 WO 1998028857 A1 WO1998028857 A1 WO 1998028857A1 SE 9702076 W SE9702076 W SE 9702076W WO 9828857 A1 WO9828857 A1 WO 9828857A1
- Authority
- WO
- WIPO (PCT)
- Prior art keywords
- signal
- echo
- double talk
- echo path
- path change
- Prior art date
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Classifications
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04B—TRANSMISSION
- H04B3/00—Line transmission systems
- H04B3/02—Details
- H04B3/20—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other
- H04B3/23—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other using a replica of transmitted signal in the time domain, e.g. echo cancellers
- H04B3/234—Reducing echo effects or singing; Opening or closing transmitting path; Conditioning for transmission in one direction or the other using a replica of transmitted signal in the time domain, e.g. echo cancellers using double talk detection
-
- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M9/00—Arrangements for interconnection not involving centralised switching
- H04M9/08—Two-way loud-speaking telephone systems with means for conditioning the signal, e.g. for suppressing echoes for one or both directions of traffic
- H04M9/082—Two-way loud-speaking telephone systems with means for conditioning the signal, e.g. for suppressing echoes for one or both directions of traffic using echo cancellers
Definitions
- the present invention relates in general to the telecommunications field and, in particular, to echo cancellation in telephony systems.
- Echo is a phenomenon that can occur in a telephony system whenever a portion of transmitted speech signal energy is reflected back to a sender. These reflections are caused by impedance mismatches in analog portions of the telephony network.
- There can be many different sources of echo such as, for example, a hybrid circuit that converts a 4 -wire line to a 2 -wire line in a Public Switched Telephone Network (PSTN) subscriber interface, or acoustical cross-talk in a mobile radiotelephone.
- PSTN Public Switched Telephone Network
- the presence of echo along with a substantial delay can severely degrade the quality of the speech signals being processed.
- An echo canceller is a device that is commonly used in telephony systems to suppress or remove echoes in long distance traffic.
- PLMNs Public Land Mobile Networks
- MSCs mobile services switching centers
- Echo cancellers are also used in mobile radiotelephones and " andsfree" telephone equipment to compensate for acoustical echoes .
- a general description of an existing echo cancellation technique can be found in the paper entitled: "A Double Talk Detector Based on Coherence" by Gansler et al, Signal Processing Group, Dept . of Elec . Eng. and Comp . Science, Lund Universi ty, Sweden .
- FIGURE 1 is a simplified schematic block diagram of a conventional echo canceller (10) .
- the main component of such an echo canceller is an adaptive finite-impulse- response (FIR) filter 12.
- FIR finite-impulse- response
- filter 12 models the impulse response of the echo path.
- NLP non-linear processor
- DTD double-talk detector
- the block denoted by 18 represents the echo source in the telephony system which generates the "desired" signal, y(t), as a function of the "far end” signal, x(t), and the "near end” signal, v(t) .
- the quality of the echo path estimation made is determined primarily by the step size used in the adaptation algorithm. In order to obtain a small estimation error, a small step size can be used. However, one result of using a small step size is a slow adaptation rate, and fast adaptation is desired during the initial adaptation stages. A practical trade-off is to use a large step size during the initial adaptation stage (for a high adaptation rate) , and after a prescribed period of time, reduce the step size (to obtain a low estimation error) .
- Double talk is assumed to be present if the "desired" signal level, y L (t), exceeds the maximum “far end” signal level multiplied by the expected hybrid circuit attenuation, inside a window "length” that is equal to the filter "length”. In other words, double talk is assumed to be present if: y L (t) > ⁇ (max)(x L (t),... ,x L (t-N)) (3)
- Another approach used to solve a double talk problem is to determine if there is any linear dependence between the "far end" and "desired” signals. If there is a large amount of linear dependence between the two signals, it is assumed that the "desired” signal is dominated by echo, and the FIR filter (12) adaptation is advanced. If little or no linear dependence between the two signals is found, then it is assumed that double talk is present and the FIR filter (12) adaptation is inhibited.
- a double talk detector should be capable of distinguishing between actual double talk and a change in the echo path impulse response (referred to hereafter as "echo path change") .
- echo path change a change in the echo path impulse response
- both double talk and echo path changes appear as increases in residual echo power.
- the two results require opposite adaptation actions. For example, when double talk is present, the adaptation process should be inhibited, but when the echo path is changed, the adaptation rate should be increased so the FIR filter (12) can quickly model the new signal environment.
- the existing double talk detection approaches do not provide useful information about echo path changes .
- a double talk and echo path change detector that determines whether a residual signal is dominated by echo or by a "near end" signal. In order to make this determination, a first measure of linear dependency is computed between the residual signal and an echo estimate, and a second measure of linear dependency is computed between the residual signal and a desired signal. The two results are compared with each other, and if they are of about the same order, no further action is needed. However, if the comparison determines that the dependence between the residual signal and the desired signal is much stronger than the dependence between the residual signal and the echo estimate, the present detector assumes that double talk has been detected and an appropriate signal denoting that result is output.
- the detector assumes that an echo path change has been detected and an appropriate signal denoting that result is output.
- a reliable and computationally effective method and apparatus are provided to give an echo canceller useful information about double talk and echo path changes that have occurred.
- FIGURE 1 is a simplified schematic block diagram of a conventional echo canceller
- FIGURE 2 is a schematic block diagram of a double talk and echo path change detector, which can be used to implement the apparatus and method of the present invention.
- a double talk and echo path change detector which functions based on making a determination about whether the residual signal, e(t), is dominated by echo or the "near end" signal.
- a measure of linear dependency is computed between the residual signal, e(t), and the echo estimate, s"(t), on the one side, and between the residual signal, e(t), and the desired signal, y(t), on the other side.
- the two resulting measurements are then compared with each other, and if they are of about the same order, no further action is taken. However, if the comparison determines that the dependence between e(t) and y(t) is much stronger than the dependence between e(t) and s ⁇ (t) , then double talk is detected. On the other hand, if the dependence between e(t) and y(t) is much weaker than the dependence between e(t) and s ⁇ (t) , then an echo path change is detected. Albeit, these determinations are made only if the signals being processed have a significant amount of power.
- a linear dependence measurement can be obtained by considering the coherence function, ⁇ 2 xy (f) between the two stationary random processes, x(t) and y(t), which is defined by:
- S xy (f) l/T*E [X * (f) Y (f) ] is the cross-spectral density between x(t) and y(t), and S xx (f) and S ⁇ f) are the corresponding auto-spectral densities.
- the coherence function is a measure of a possible linear relationship between the stationary random processes involved, and it is bounded between zero and one. If a perfect linear relationship exists between those stationary random processes for some frequency, then the coherence will be equal to unity at that frequency. If no such linear relationship exists, then the coherence function will be equal to zero.
- a scalar linear dependence measure can be obtained by computing a mean value for the coherence function.
- An approach that can be used to detect echo path changes is based on the observation that under the principle of orthogonality, there can be no linear relationship between the residual signal, e(t), and the echo estimate, s ⁇ (t) , if the echo estimate is produced by an optimal estimator and, therefore, the coherence function is equal to zero.
- the coherence function will be close to zero if the adaptive algorithm is converged.
- a strong linear relationship will appear between the signals, e(t) and s ⁇ (t) , and therefore, the coherence function will be large.
- the coherence function will be small. If the "near end" signal is strong, then the residual signal is dominated by the "near end” component, v(t), and the coherence function will be close to unity. Notably, if the FIR filter's coefficients are initialized to zero, then a linear relationship will exist between the residual signal and the desired signal during the initial adaptation stage.
- test variable (d) In order to detect both double talk and echo path changes, the following test variable (d) can be used:
- f 1 and f 2 are the borders of the frequency band of interest.
- the value of the test variable, d will be close to zero during normal operation of the detector
- FIGURE 2 is a schematic block diagram of a double talk and echo path change detector, which can be used to implement the apparatus and method of the present invention.
- Detector 100 includes a first coherence measurement unit 102 and second coherence measurement unit 104.
- First coherence measurement unit 102 computes the linear dependency between the residual signal, e(t), and the echo path estimate, s ⁇ (t).
- Second coherence measurement unit 104 computes the linear dependency between the residual signal, e(t), and the desired signal, y(t) .
- the output of each of the two measurement units is compared at comparator 106. This comparison is accomplished in accordance with Equation 5 described above.
- the output of comparator 106 is coupled to an input of a first test unit 110 and a second test unit 112.
- First test unit 110 compares the output value (e.g., test variable d) of comparator 106 with a first predetermined threshold level, thl .
- Second test unit 112 compares the output value (d) of comparator 106 with a second predetermined threshold value, th2.
- the value of test variable, d is close to zero during a normal operation, and no filter adaptation occurs. If, however, the value of test variable, d, is greater than threshold level thl , then first test unit 110 outputs a signal (preferably to an appropriate stage in echo canceller 12) denoting that double talk has been detected. Consequently, the adaptive algorithm for the FIR filter (12) is inhibited.
- test variable, d If the value of test variable, d, is less than threshold level th2 , then second test unit 112 outputs a signal (preferably to an appropriate stage in echo canceller 12) denoting that an echo path change has been detected. Consequently, the adaptive algorithm for the FIR filter (12) is advanced.
- a timer 108 is used to decouple the output of comparator 106 to ensure that the filter adaptation process occurs during the initial test period. For this embodiment, timer 108 is set for about 20 times the "length" of the FIR filter used.
- the coherence functions can be estimated by replacing the mathematical expectations with time averages, and computing the spectra by using discrete Fourier transforms (or some other orthogonal transform) .
- the integration performed above in Equation 5 can be replaced by summation over discrete frequencies .
- the adaptive algorithm computes the coherence functions using an 8 -point Fast Fourier Transform (FFT) .
- FFT Fast Fourier Transform
- the present invention is not intended to be limited to any specific FFT value, and any appropriate value (e.g., any other positive value) can be used instead of "8".
- positive values should be used that allow the use of FFT algorithms (e.g., powers of 2) .
- time averaging is performed over 64 consecutive transforms, which overlap in seven samples
- an element of the discrete coherence vector can be defined as:
- a is a regularization constant, which controls errors in division when the denominator otherwise is small.
- the regularization constant can be a design variable that allows a measure of control over the detector's sensitivity.
- the value of the regularization constant is preferably comparable to the fourth power of the signal levels expected.
- Equation 9 results from an element-wise division of vectors, the elements of which have dimensions of the fourth powers of the corresponding signals. In other words, the dynamic range of these elements is four times greater than that of the signals involved. This result may be undesirable if the above-described math is to be implemented with fixed point arithmetic. Consequently, the following normalization vector may be used instead:
- the value of the regularization constant, a, should be selected to be comparable to the power levels of the signals expected to be involved.
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- Engineering & Computer Science (AREA)
- Signal Processing (AREA)
- Computer Networks & Wireless Communication (AREA)
- Cable Transmission Systems, Equalization Of Radio And Reduction Of Echo (AREA)
Abstract
Description
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Priority Applications (7)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
AU53516/98A AU5351698A (en) | 1996-12-20 | 1997-12-12 | Double talk and echo path change detection in a telephony system |
PL97334048A PL186221B1 (en) | 1996-12-20 | 1997-12-12 | Method of and apparatus for detecting back crosstalks and echo changes in a telephone system |
JP52867498A JP4205169B2 (en) | 1996-12-20 | 1997-12-12 | Double talk and echo path change detection in telephony systems |
DE69732917T DE69732917T2 (en) | 1996-12-20 | 1997-12-12 | INTERCOM AND ECHOPFAD CHANGE DETECTION IN A TELEPHONE SYSTEM |
EP97950543A EP0947057B1 (en) | 1996-12-20 | 1997-12-12 | Double talk and echo path change detection in a telephony system |
CA002275662A CA2275662C (en) | 1996-12-20 | 1997-12-12 | Double talk and echo path change detection in a telephony system |
BR9713754-5A BR9713754A (en) | 1996-12-20 | 1997-12-12 | Detector and process for detecting double talk and echo path variation. |
Applications Claiming Priority (2)
Application Number | Priority Date | Filing Date | Title |
---|---|---|---|
US08/770,908 US6035034A (en) | 1996-12-20 | 1996-12-20 | Double talk and echo path change detection in a telephony system |
US08/770,908 | 1996-12-20 |
Publications (1)
Publication Number | Publication Date |
---|---|
WO1998028857A1 true WO1998028857A1 (en) | 1998-07-02 |
Family
ID=25090083
Family Applications (1)
Application Number | Title | Priority Date | Filing Date |
---|---|---|---|
PCT/SE1997/002076 WO1998028857A1 (en) | 1996-12-20 | 1997-12-12 | Double talk and echo path change detection in a telephony system |
Country Status (12)
Country | Link |
---|---|
US (1) | US6035034A (en) |
EP (1) | EP0947057B1 (en) |
JP (1) | JP4205169B2 (en) |
CN (1) | CN1127813C (en) |
AU (1) | AU5351698A (en) |
BR (1) | BR9713754A (en) |
CA (1) | CA2275662C (en) |
DE (1) | DE69732917T2 (en) |
MY (1) | MY118091A (en) |
PL (1) | PL186221B1 (en) |
TR (1) | TR199901384T2 (en) |
WO (1) | WO1998028857A1 (en) |
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GB2371191A (en) * | 2001-01-11 | 2002-07-17 | Mitel Corp | Detecting double-talk and path changes in echo cancellers |
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- 1997-12-12 EP EP97950543A patent/EP0947057B1/en not_active Expired - Lifetime
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GB2371191A (en) * | 2001-01-11 | 2002-07-17 | Mitel Corp | Detecting double-talk and path changes in echo cancellers |
GB2371191B (en) * | 2001-01-11 | 2005-06-15 | Mitel Corp | Double-talk and path change detection using a matrix of correlation coefficients |
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Also Published As
Publication number | Publication date |
---|---|
PL186221B1 (en) | 2003-11-28 |
MY118091A (en) | 2004-08-30 |
AU5351698A (en) | 1998-07-17 |
CA2275662A1 (en) | 1998-07-02 |
DE69732917D1 (en) | 2005-05-04 |
CN1127813C (en) | 2003-11-12 |
CA2275662C (en) | 2006-06-13 |
DE69732917T2 (en) | 2006-04-13 |
CN1247652A (en) | 2000-03-15 |
PL334048A1 (en) | 2000-01-31 |
EP0947057B1 (en) | 2005-03-30 |
JP4205169B2 (en) | 2009-01-07 |
EP0947057A1 (en) | 1999-10-06 |
TR199901384T2 (en) | 1999-09-21 |
US6035034A (en) | 2000-03-07 |
JP2001508609A (en) | 2001-06-26 |
BR9713754A (en) | 2000-02-01 |
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