Abstract:
Echo canceling method including the steps of determining an Euclidean norm of an echo-replica signal, determining an Euclidean norm of an input signal, determining a gradient step size, correcting the coefficients of an adaptive filter, deriving an updated echo-replica signal, and determining an updated error signal by subtracting the updated echo-replica signal from the input signal.

Description:
FIELD OF THE INVENTION 
     The present invention relates to methods and systems for processing sound signals in general, and to methods and systems for processing audio signals in the presence of echo conditions, in particular. 
     BACKGROUND OF THE INVENTION 
     Echo cancellation and suppression systems and methods therefor are known in the art. A speech transmission oriented system, such as a speech communication system, includes a sound production unit such as a speaker and a sound detection unit such as a microphone. The speaker produces sounds received from a remote source and the microphone detects voice sounds which are provided by the user. It is noted that the microphone also detects sounds from other sources such as noise and sounds, which are produced by the speaker (echo), after traveling through and acoustic path therebetween. 
     Methods and systems for “emphasizing” the voice portion over the noise and echo are known in the art. The common approaches use adaptive filtering to derive a replica of an echo-signal, which is further subtracted from the microphone signal. As a result, a level of the echo-signal is decreased significantly. Speech communication systems, especially hand-held devices, are limited in their processing and storage resources. This fact forces developers to search for new approaches for robust and high quality echo cancellation and echo suppression methods, which can function within these limited conditions. 
     In the article by A. Hirano et al. “A noise-robust stochastic gradient algorithm with an adaptive step-size suitable for mobile hands-free telephones”, Proc. ICASST-95, v.5, pp. 1392–1395, 1995, an adaptive step-size algorithm is proposed. The algorithm controls the step size of the standard NLMS method, based on the reference input signal power and the noise power. 
     U.S. Pat. No. 5,608,804 to Hirano, entitled “Method of and apparatus for identifying a system with adaptive filter” is directed to a method of and an apparatus for estimating characteristics of an unknown system, using an adaptive filter, in an echo canceller. A gradient step size, which is controlled dynamically, is a function of the power of the reference input signal. The gradient step size monotonously increases if the power of the reference input signal is smaller than a threshold and monotonously decreases if the power of the reference input signal is greater than the threshold. 
     U.S. Pat. No. 5,546,459 to Sih, et al., entitled “Variable block size adaptation algorithm for noise-robust acoustic echo cancellation” is directed to an apparatus for acoustic echo cancellation using an adaptive filter. The apparatus updates coefficients of the adaptive filter using a signal block of length L. The block size is adjusted in response to the instantaneous signal-to-noise ratio. 
     SUMMARY OF THE PRESENT INVENTION 
     It is an object of the present invention to provide a novel method and system for echo suppression, which alleviates the disadvantages of the prior art. 
     It is another object of the present invention to provide a novel method and system for echo canceling, which alleviates the disadvantages of the prior art. 
     In accordance with the present invention, there is thus provided an echo canceling apparatus, including a signal processor, an adaptation control unit, connected to the signal processor, an adaptive filter, connected to the signal processor and to the adaptation control unit, and a subtractor, connected to the signal processor, to the adaptation control unit and to the adaptive filter. 
     The signal processor determines an Euclidean norm of an echo-replica signal. The signal processor also determines an Euclidean norm of an input signal. The echo replica signal can be produced by processing a reference far-end signal with the adaptive filter. The adaptation control unit corrects the coefficients of the adaptive filter. The adaptive filter derives an updated echo-replica signal. The subtractor subtracts the updated echo-replica signal from the input signal and derives an updated error signal thereof. 
     The echo canceling apparatus can further include a digital-to-analog converter, connected to the signal processor, to the subtractor and to a source of the input signal such as a microphone. The analog-to-digital converter converts an analog input signal into a digital input signal. 
     In accordance with another aspect of the present invention, there is thus provided a echo canceling method, including the steps of; determining an Euclidean norm of an echo-replica signal, determining an Euclidean norm of an input signal, determining a gradient step size, correcting the coefficients of an adaptive filter, deriving an updated echo-replica signal, and determining an updated error signal by subtracting the updated echo-replica signal from the input signal. The echo replica signal can be produced by processing a reference far-end signal with an adaptive filter. 
     The input signal can include at least one of the list consisting of an echo-signal, a near-end speech signal, a noise signal and the like. The gradient step size can be a function of the Euclidean norms of the echo-replica and the input signals. According to one aspect of the invention, the value of the function decreases when the values of the near-end speech and noise signals increase, and the value of the function increases when the values of the near-end speech and noise signals decrease. 
     The correction of the coefficients of the adaptive filter can be performed by employing the gradient step size, the reference far-end signal and an error signal. According to a preferred embodiment of the present invention, the error signal can be determined at the previous adaptation step. 
     In accordance with a further aspect of the present invention, there is thus provided an echo suppression apparatus including a first amplitude estimation unit, a second amplitude estimation unit, a comparison unit, connected to the first amplitude estimation unit and to the second amplitude estimation unit, and a decision logic unit connected to the comparison unit. 
     The first amplitude estimation unit estimates an amplitude measure of a reference far-end signal. The second amplitude estimation unit estimates an amplitude measure of an error signal. The comparison unit compares between the values of the amplitude measure of the far-end signal and the amplitude measure of the error signal and produces a comparison result thereof. 
     The decision logic unit analyzes the comparison result and produces a control signal thereof. The error signal can be an output of an echo cancellation system, which is connected to the echo suppression apparatus. The far-end signal and the error signal are generally digital signal blocks, each containing at least one digital sample. 
     In accordance with yet a further aspect of the present invention, there is thus provided an echo suppression method, including the steps of; estimating an amplitude measure of a reference far-end signal, estimating an amplitude measure of an error signal, comparing between the values of the amplitude measures of the far-end and the error signals, thereby producing a comparison value, and analyzing the comparison value, thereby producing a control signal. The error signal can be received from an echo cancellation system. 
     The far-end signal and the error signal, are generally digital signal blocks, where each of the digital signal blocks contains at least one digital sample. 
     In accordance with another aspect of the present invention, there is thus provided an echo suppression apparatus, including a first amplitude estimation unit, a second amplitude estimation unit, a comparison unit, connected to the first amplitude estimation unit and to the second amplitude estimation unit, a delay unit, connected to the comparison unit, and a decision logic unit connected to the delay unit. 
     The first amplitude estimation unit produces at least two far-end signal amplitude measure values by estimating an amplitude measure of a reference far-end signal. The reference far-end signal is a sequence of at least two digital signal blocks, where each of the digital signal blocks contains at least one digital sample. The second amplitude estimation unit produces at least two error signal amplitude measure values by estimating an amplitude measure of an error signal. The error signal is a sequence of at least two digital signal blocks, where each of the digital signal blocks contains at least one digital sample. 
     The comparison unit determines a plurality of value pairs. Each of the pairs includes a selected one of the error signal amplitude measure values and a respective one of the far-end signal amplitude measure values. The comparison unit further compares between the far-end signal amplitude measure value and the error signal measure value, within each of the value pairs and produces at least two comparison results thereof. 
     The delay unit stores the comparison results. The decision logic unit analyzes the comparison results and produces a control signal thereof. 
     In accordance with yet another aspect of the present invention, there is provided an echo suppression method including the steps of:
         ξProducing at least two far-end signal amplitude measure values by estimating an amplitude measure of a reference far-end signal. The reference far-end signal is a sequence of at least two digital signal blocks. Each of the digital signal blocks contains at least one digital sample.   ξProducing at least two error signal amplitude measure values by estimating an amplitude measure of an error signal. The error signal includes a sequence of at least two digital signal blocks. Each of the digital signal blocks contains at least one digital sample.   ξDetermining a plurality of value pairs, where each pair includes a selected one of the error signal amplitude measure values and a respective one of the far-end signal amplitude measure values.   ξComparing between the far-end signal amplitude measure value and the error signal amplitude measure value, within each of the value pairs, thereby producing at least two comparison results.   ξAnalyzing the at least two comparison results, thereby producing a control signal.       

     In accordance with another aspect of the present invention, there is thus provided an echo suppression apparatus which includes a first amplitude estimation unit, a second amplitude estimation unit, a comparison unit, connected to the first amplitude estimation unit and to the second amplitude estimation unit, and a decision logic unit, connected to the comparison unit. 
     The first amplitude estimation unit estimates an amplitude measure of a reference far-end signal. The second amplitude estimation unit estimates an amplitude measure of an error signal. The comparison unit compares between the values of the amplitude measure of the far-end signal and the amplitude measure of the error signal and produces a comparison result thereof. 
     The decision logic unit analyzes the comparison result and produces a control signal thereof. The error signal can be the output of an echo cancellation system, which is connected to the echo suppression apparatus of the invention. 
     In accordance with a further aspect of the invention, there is provided an echo suppression apparatus, including a first amplitude estimation unit, a second amplitude estimation unit, a comparison unit, connected to the first amplitude estimation unit and to the second amplitude estimation unit, a delay unit, connected to the comparison unit, and a decision logic unit connected to the delay unit. The first amplitude estimation unit produces at least two far-end signal amplitude measure values by estimating an amplitude measure of a reference far-end signal, wherein the reference far-end signal is a sequence of at least two digital signal blocks. Each of the digital signal blocks contains at least one digital sample. 
     The second amplitude estimation unit produces at least two error signal amplitude measure values by estimating an amplitude measure of an error signal, wherein the error signal is a sequence of at least two digital signal blocks. Each of the digital signal blocks contains at least one digital sample. 
     The comparison unit determines a plurality of value pairs. Each of the pairs includes a selected one of the error signal amplitude measure values and a respective one of the far-end signal amplitude measure values. The comparison unit compares between the far-end signal amplitude measure value and the error signal measure value, within each of the value pairs and produces at least two comparison results thereof. The delay unit stores the at least two comparison results. The decision logic unit analyzes the comparison results and produces a control signal thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention will be understood and appreciated more fully from the following detailed description taken in conjunction with the drawings in which: 
         FIG. 1  is a schematic illustration of a speech communication system, constructed and operative in accordance with a preferred embodiment of the present invention; 
         FIG. 2  is a schematic illustration in detail of the echo-cancellation system of  FIG. 1 , constructed and operative in accordance with a further preferred embodiment of the present invention; 
         FIG. 3  is a schematic illustration of a method for operating the echo-cancellation system of  FIG. 2 , operative in accordance with a further preferred embodiment of the present invention; 
         FIG. 4  is a schematic illustration in detail of the echo-suppression system of  FIG. 1 , constructed and operative in accordance with a further preferred embodiment of the present invention; and 
         FIG. 5  is a schematic illustration of a method for operating the echo-suppression system of  FIG. 4 , operative in accordance with another preferred embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     The present invention alleviates the disadvantages of the prior art by providing better robustness of echo cancellation, whenever an additive background noise and/or near-end speech are present. It also provides a more accurate and computation-effective method of echo suppression. 
     Reference is now made to  FIG. 1 , which is a schematic illustration of a speech communication system, generally referenced  10 , constructed and operative in accordance with a preferred embodiment of the present invention. 
     Speech communication system  10  includes a receiver  12 , a loudspeaker  14 , a switch  16 , an echo-cancellation system  18 , a transmitter  20 , an echo-suppression system  22 , a comfort noise generator  24 , a digital-to-analog converter (D/A)  36  and a microphone  26 . 
     Echo-cancellation system  18  is connected to receiver  12 , to digital-to-analog converter  36 , to echo-suppression system  22 , and to microphone  26 . Echo-suppression system  22  is connected to D/A  36 , and to switch  16 . Transmitter  20  and comfort noise generator  24  are connected to switch  16 . Loudspeaker  14  is connected to D/A  36 . 
     Receiver  12  receives an RF signal, extracts the audio information embedded therein and produces a digital audio stream X k . Receiver  12  further provides the digital audio stream X k  to D/A  36 , to echo-suppression system  22  and to echo-cancellation system  18 . D/A  36  converts the digital audio stream X k  into an analog signal x(t) and provides it to loudspeaker  14 . Loudspeaker  14  converts further the signal x(t) into an analog sound signal and provides it to a near-end speaker  30 . Microphone  26  detects an echo-signal y′(t), which is a combination of the sound, radiated by loudspeaker  14  and a sound, reflected from a reflector  32 . Microphone  26  detects also a speech signal v′(t) from the near-end speaker  30  and a noise signal n′(t) from a noise source  28 . Microphone  26  converts acoustic signals y′(t), v′(t), and n′(t) into electric signals y(t), v(t) and n(t), respectively. Microphone  26  provides a signal d(t), which is a combination of the signals y(t), v(t) and n(t) to echo-cancellation system  18 . 
     Echo-cancellation system  18  analyzes the signals X k  and d(t), estimates an echo replica and performs echo cancellation. The echo replica estimation is performed by employing a novel adaptive filtering method, which will be described in detail hereinafter. Echo-cancellation system  18  provides an output digital signal E k  to echo-suppression system  22 . 
     Echo-suppression system  22  analyzes the signals from the loudspeaker  14  and echo-cancellation system  18 . Based on predetermined conditions, echo-suppression system  22  replaces, if necessary, the output signal E k  with an artificial noise signal  34 , generated by comfort noise generator  24 . Echo-suppression system  22  controls switch  16 , which performs the switching between the output signal E k  and the artificial noise signal. Switch  16  provides the output signal to transmitter  20 . 
     Reference is now made to  FIG. 2 , which is a schematic illustration in detail of echo-cancellation system  18  ( FIG. 1 ), constructed and operative in accordance with a further preferred embodiment of the present invention. 
     Echo-cancellation system  18  includes analog-to-digital converter (A/D)  58 , an adaptive filter  54 , an adaptation control unit  62 , a signal processor  60 , and a summator  56 . Adaptive filter  54  is connected to the first input of summator  56 , to signal processor  60 , and to adaptation control unit  62 . A/D converter  58  is connected to the second input of summator  56 , and to signal processor  60 . Adaptation control unit  62  is further connected to the output of summator  56 , and to signal processor  60 . 
     A/D converter  58  converts an analog signal d(t) into a digital signal D k . System  18  operates on digital signal blocks of length L, hence the signal D k  is a vector of length L, with components D k ( 0 ), D k ( 1 ) . . . D k (L−1). Index k denotes a block number and has values  0 ,  1 ,  2 , . . . . The signal d(t) is a sum of signals y(t), v(t) and n(t). Hence, signal D k  will be a sum of respective digital signals Y k , V k  and N k , each of them being a vector of length L. Accordingly, the signal X k  is a vector of length L, with components X k ( 0 ), X k ( 1 ) . . . X k (L−1). 
     Signal processor  60  receives near-end signal D k  and echo-replica signal Ŷ k , determines Euclidean norms of the signals D k  and Ŷ k , and provides it to adaptation control unit  62 . It is noted that the filter tap values, which are used for echo-replica signal Ŷ k  calculation, were determined during the previous iteration. 
     Adaptation control unit  62  receives far-end signal X k , error (residual) signal E k =D k −Ŷ k  and the Euclidean norms of the signals D k  and Ŷ k , and outputs filter tap correction values thereof. Adaptation control unit  62  provides further corrected filter tap values to adaptive filter  54 . 
     Adaptive filter  54  receives the far-end signal X k  and the corrected filter tap values, and outputs echo-replica signal Ŷ k . Adaptive filter  54  provides echo-replica signal Ŷ k  to summator  56 . 
     Summator  56  sums signals D k  and Ŷ k , produces an error signal E k  at the output, and provides signal E k  to echo-suppression system  22 , and to adaptation control unit  62 . 
     Reference is further made to  FIG. 3 , which is a schematic illustration of a method for operating echo-cancellation system  18  ( FIG. 2 ), operative in accordance with a further preferred embodiment of the present invention. 
     In step  70 , a far-end signal X k  is received by adaptation control unit  62 , and adaptive filter  54 . With reference to  FIG. 2 , a digital far-end signal X k , is provided by a far-end speaker to adaptation control unit  62 , and to adaptive filter  54 . 
     In step  72 , a near-end signal D k  is received by summator  56 , and signal processor  60 . With reference to  FIG. 2 , A/D converter  58  converts an analog near-end signal d(t) into a digital near-end signal D k , and provides it to summator  56 , and to signal processor  60 . 
     In step  74 , Euclidean norms ∥D k (n)∥ and ∥Y k (n)∥ of the signals D k  and Ŷ k  respectively, at time instance n, are calculated. With reference to  FIG. 2 , signal processor  60  receives signals D k  and Ŷ k , and determines Euclidean norms ∥D k (n)∥ and ∥Ŷ k (n)∥ according to the following expressions: 
                                  D   k     ⁡     (   n   )            2                               l   L     ⁢     0   1     ⁢           ⁢       D   k   2     ⁡     (     n   ⁢           ⁢   l     )         ,                Y   ^     k     ⁡     (   n   )            2                 0   1       l   L     ⁢           ⁢         Y   ^     k   2     ⁡     (     n   ⁢           ⁢   l     )         ,                 (   1   )               
Signal processor  60  provides further the Euclidean norms ∥D k (n)∥ and ∥Y k (n)∥ to adaptation control unit  62 .
 
     In step  76 , a gradient step size Π(n), at time instance n, is determined. With reference to  FIG. 2 , adaptation control unit  62  receives far-end signal block X k , residual signal block E k , and the Euclidean norms ∥D k (n)∥ and ∥Ŷ k (n)∥. Gradient step size Π(n) is further determined as: 
                     Π   ⁢           ⁢     (   n   )     ⁢           Π   0     ⁢            X   k     ⁡     (   n   )            ⁢           ⁢   ⁢           ⁢                Y   ^       k   ⁢               ⁡     (   n   )            2     ⁢     E     ≈   …     ≡       ⁢                          X   k     ⁡     (   n   )            2     ⁢   ⁢                Y   ^     k     ⁡     (   n   )            2     ⁢     E     ≈   …     ≡     ⁢                     D   k     ⁡     (   n   )            2     ⁢   Δ   ⁢                Y   ^     k     ⁡     (   n   )            2                  ,           (   2   )               
where             and E are constants. The value of           depends on hardware and software implementation of the system, and can be determined experimentally. The optimal value of           will be the one which effects in a maximal echo cancellation. The value of E can also be determined experimentally. The constant           is dimensionless, and the constant E has dimensions of energy. It is noted, that constants           and E can be multi-valued, i.e. they can be represented with vectors, having components           (i), E(j) (i,j=1, 2, 3, . . . ). The most appropriate values of           (i), E(j) can be selected, depending on the working conditions, and HW and SW implementation. It is appreciated, that other expressions for the gradient step size can be used, such as that proposed in the article by A. Hirano, et al. “A noise-robust stochastic gradient algorithm with an adaptive step-size suitable for mobile hands-free telephones”, Proc. ICASST-95, v.5, pp. 1392–1395, 1995.

     It is noted, that the value of the gradient step size Π(n) is controlled dynamically. It follows from equation (2), that Π(n) decreases with a rise in the near-end signal V k  or the noise signal N k . In other words, the adaptation process will be performed more accurately, and hence, the robustness of the system will improve. In an opposite case of low near-end and noise signals, Π(n) will rise, causing, in turn, a rise in adaptation speed. 
     Based on the gradient step size Π(n), adaptation control unit  62  further determines an updated vector H(n+1), at a time n+1. The components H q (n+1) of the vector H(n+1) represent filter tap coefficients and are determined according to the following recurrent expression:
 
H q (n 1) H q (n) Π(n) E k (n) X k (n q),  (3)
 
where q=0, 1, 2, . . . Q−1, and Q is the order of adaptive filter  54 . It is noted that the length L of the signal block X k , can be less than the order Q of adaptive filter  54 . In this case, additional Q L zero valued samples must be appended to the signal block X k , so that the dimension of the signal block X k , will be equal to dimension of the adaptive filter  54 . Adaptation control unit  62  provides the updated vector H to adaptive filter  54 .
 
     In step  80 , adaptive filter tap coefficients are updated and an echo-replica signal Ŷ k  is derived thereof. With reference to  FIG. 2 , adaptive filter  54  receives near-end signal block X k , and vector H, and derives the echo-replica signal Ŷ k (n), at a time instance n, according to the following expression: 
                           Y   k     ⁡     (   n   )               0   1       q   Q     ⁢       H   k     ⁡     (   q   )                   X   k     ⁡     (     n   ⁢           ⁢   q     )       ,                 (   4   )               
The echo-replica signal Ŷ k (n) is further provided to signal processor  60 , and to summator  56 .
 
     In step  82 , signals Ŷ k (n) and D k (n) are subtracted and an error signal E k (n) is determined thereof. With reference to  FIG. 2 , summator  56  receives signals Ŷ k (n) and D k (n) and derives the error signal E k (n) D k (n) Ŷ k (n) at the output. 
     In step  84 , error signal E k  at time n, is provided to echo-suppression system  22  ( FIG. 1 ), and to adaptation control unit  62 . With reference to  FIG. 2 , summator  56  provides the error signal E k  at time n, to adaptation control unit  62 , for the next iteration, and to echo-suppression system  22 . 
     Reference is now made to  FIG. 4 , which is a schematic illustration in detail of echo-suppression system  22  ( FIG. 1 ), constructed and operative in accordance with a further preferred embodiment of the present invention. 
     Echo-suppression system  22  includes amplitude estimation units  104  and  106 , a comparison unit  108 , a delay unit  110 , and a decision logic unit  112 . 
     Comparison unit  108  is connected to delay unit  110 , and to amplitude estimation units  104  and  106 . Decision logic unit  112  is connected to delay unit  110 . 
     Amplitude estimation units  104  and  106  receive signals E k  and X k , respectively. Each of the amplitude estimation units further determines an amplitude estimation A E   (k)  and A X   (k)  of the respective signals, and provide the result to comparison unit  108 . 
     Comparison unit  108  performs the comparison of amplitude estimations A E   (k)  and A X   (k) , and provides the result to delay unit  110 . Delay unit  110  already contains the results of comparisons made for the previous M-1 pairs of signal blocks. The output of delay unit  110  is a vector              k  with components            k ( 0 ),            k ( 1 ), . . .            k (M−1). Delay unit  110  provides vector            k  to decision logic unit  112 .
     Decision logic unit  112  analyzes vector              k , and produces a control signal P. Decision logic unit  112  provides the control signal P to switch  16  ( FIG. 1 ).
     Reference is further made to  FIG. 5 , which is a schematic illustration of a method for operating echo-suppression system  22  ( FIG. 4 ), operative in accordance with another preferred embodiment of the present invention. 
     In steps  150  and  152 , the far-end signal X k  and error signal E k  are received. With reference to  FIG. 4 , amplitude estimation unit  106  receives the far end signal X k , and amplitude estimation unit  104  receives error signal E k  from echo-cancellation system  18  ( FIG. 1 ). 
     In step  154 , amplitude measures of the far-end and residual signals are estimated. With reference to  FIG. 4 , amplitude estimation units  104  and  106  determine amplitude measures A E   (k)  and A X   (k)  for signals E k  and X k , respectively, according to the following expressions 
                         A   E     (   k   )                                 l   L     ⁢   ⁢     0   1     ⁢           ⁢            E   k     ⁡     (   l   )              ,     A   X     (   k   )                   0   1       l   L     ⁢           ⁢            X   k     ⁡     (   l   )              ,                 (   5   )               
where L is a length of signal blocks E k  and X k . Amplitude measures A E   (k)  and A X   (k)  are further provided to comparison unit  108 .
 
     It is appreciated that instead of amplitude measures A E   (k)  and A X   (k) , it is possible to use other signal measures, for example energy estimate. It is noted, that in distinction to energy estimation, which is quadratic in signal amplitude, amplitude measures A E   (k)  and A X   (k)  are linear functions of signal amplitudes E k  and X k , respectively. The use of A E   (k)  and A X   (k)  reduces the amount of the required calculations, and furthermore, increases their accuracy. In step  156 , amplitude measures A E   (k)  and A X   (k)  are compared. With reference to  FIG. 4 , comparison unit  108  receives amplitude measures A E   (k)  and A X   (k) , compares them, and produces a boolean output              k ( 0 ), according to the following conditional expressions:
                                   k           o   ↑     ⁢     (   0   )     ⁢           ⁢   1     ,               if     ⁢           ⁢     A   X     (   k   )       ⁢           ⁢       A   E     (   k   )       !     ⁢           ⁢   T     ,                             o   ↑     ⁢     -&gt;     k     ⁢     (   0   )     ⁢           ⁢   0     ,           otherwise                 (   6   )                 
where T is a threshold value. The value of T is determined experimentally. The value of              k ( 0 ) is further provided to delay unit  110 . Delay unit  110  already contains the results of comparisons made for the previous M−1 pairs of signal blocks. The output of delay unit  110  is a vector            k  with components            k ( 0 ),            k ( 1 ), . . .            k (M−1). Delay unit  110  provides vector            k  to decision logic unit  112 .

     In step  158 , the results of the current comparison, and previous M−1 comparisons of amplitude measures, are analyzed, and a respective control signal is derived thereof. With the reference to  FIG. 4 , decision logic unit  112  receives comparison vector              k , analyzes comparison vector            k , and derives appropriate control signal P for switch  16 . The decision process is accomplished in accordance with the following logical scheme:
       if (           k (0)==1,            k (1)==1, . . .            k (M−1)==1)   count=M   endif   count=count-1   if(count&gt;0)
           P==1   
           else
           P==O   
           endif       

     In step  160 , control signal P is provided to switch  16 . With reference to  FIG. 4 , decision logic unit  112  provides control signal P to switch  16 . In case P==1, switch  16  replaces signal E k  with comfort noise, otherwise signal E k  is provided to transmitter  20 . 
     It will be appreciated by persons skilled in the art that the present invention is not limited to what has been particularly shown and described hereinabove. Rather the scope of the present invention is defined only by the claims, which follow.