Abstract:
An echo canceler having a predictive filter that generates an echo replica signal and an adder that subtracts the echo replica signal from a send input signal also has a receive input amplifier and a clipping circuit. The clipping circuit clips the input to the predictive filter at a threshold level determined by increasing the receive input amplifier gain until distortion of the echo component causes echo cancellation performance to degrade. Alternatively, the echo canceler may have a send input amplifier, a send output amplifier, and an echo replica amplifier that amplifies the predicted echo replica signal. The gain of the send input amplifier is increased to determine the gain at which echo cancellation performance begins to deteriorate due to echo amplification, and the gain of the echo replica amplifier is set to compensate for the echo amplification.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an echo canceler used in, for example, a hands-free communication device. 
   2. Description of the Related Art 
     FIG. 4  shows an example of the structure of a hands-free communication device having a conventional echo canceler. The echo canceler  1  includes a predictive filter  2  and an adder  3 . The device also comprises a speaker amplifier  4  with a volume control, a loudspeaker  5 , and a microphone  6 . 
   The predictive filter  2  generates an echo replica signal by predicting the echo of the received far-end signal that will enter the microphone  6  via an acoustic coupling path or echo path from the loudspeaker  5 . The echo replica signal is subtracted from the actual echo signal by the adder  3 , thereby canceling the echo. 
   A problem in a hands-free communication device with the structure described above is that the echo is inadequately cancelled in the following two situations.
         (1) As shown in  FIG. 5 , if the speaker amplifier  4  is adjusted to so high a volume level that the output of the speaker amplifier  4  saturates, causing parts of the output waveform to be clipped at the saturation amplitude level, the echo signal that reaches the sending input terminal Sin of the echo canceler  1  via the echo path is distorted. The predictive filter  2  cannot anticipate this distortion and predicts the echo signal inaccurately, increasing the difference between the waveform of the echo replica signal and the waveform of the echo signal. Echo cancellation performance suffers accordingly.   (2) If the attenuation on the echo path is so small that the level of the echo signal input to the sending input terminal Sin of the echo canceler  1  is higher than the level of the signal output from the receiving output terminal Rout, as shown in  FIG. 6 , then unless the precision of the predictive filter is increased by increasing the number of bits used to represent signal values, the prediction error is amplified and echo cancellation performance suffers severely. Increasing the number of bits, however, increases the size of the predictive filter, leading to increased cost.       

   SUMMARY OF THE INVENTION 
   One object of the present invention is therefore to provide an echo canceler that can maintain high echo cancellation performance even if the echo signal input to the echo canceler via the echo path has a distorted waveform. 
   Another object is to provide an echo canceler that can maintain high echo cancellation performance even if the echo signal input to the echo canceler has a higher level than the receive signal output from the echo canceler. 
   A further object is to provide an echo canceler that can maintain high echo cancellation performance even if the echo signal input to the echo canceler has both a distorted waveform and a higher level than the receive signal output from the echo canceler. 
   The invention provides an echo canceler of the type having a predictive filter for predicting the echo that will result from a receive input signal and generating an echo replica signal, and an adder for subtracting the echo replica signal from a send input signal, thereby canceling the echo component of the send input signal without canceling the near-end component. 
   According to a first aspect of the invention, besides having the predictive filter and adder, the echo canceler has an amplifier for modifying the level of the receive input signal, a clipping circuit for clipping the modified receive input signal output from the amplifier and supplying the clipped modified receive input signal to the predictive filter, and a gain controller for controlling the gain of the amplifier and the clipping threshold level of the clipping circuit. At a time when the receive input signal is present and the send input signal includes no near-end component, the gain controller increases the gain of the amplifier until a state is reached in which the output level of the adder exceeds a predetermined value, and sets the threshold level of the clipping circuit according to the modified level of the receive input signal in this state. 
   According to a second aspect of the invention, besides having the predictive filter and adder, the echo canceler has a first amplifier for modifying the level of the send input signal according to a first gain, a second amplifier for modifying the level of the output of the adder according to a second gain, a third amplifier for modifying the level of the echo replica signal according to a third gain, and a gain controller for specifying the first gain, second gain, and third gain. At a time when the receive input signal is present and the send input signal includes no near-end component, the gain controller gradually increases the first gain and decreases the second gain until a state is reached in which the output level of the adder exceeds a predetermined value, and sets the third gain to a level responsive to the second gain in this state. 
   The first and second aspects of the invention can be combined in a single echo canceler. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the attached drawings: 
       FIG. 1  shows the structure of a hands-free communication device including a first echo canceler embodying the present invention; 
       FIG. 2  shows the structure of a hands-free communication device including a second echo canceler embodying the invention; 
       FIG. 3  shows the structure of a hands-free communication device including a third echo canceler embodying the invention; 
       FIG. 4  shows the structure of a hands-free communication device including a conventional echo canceler; and 
       FIGS. 5 and 6  illustrate problems in the operation of the hands-free communication device in FIG.  4 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Embodiments of the invention will now be described with reference to the attached drawings. Each embodiment is an echo canceler used in a hands-free communication device including a speaker amplifier with a volume control, a loudspeaker, and a microphone. 
   Referring to  FIG. 1 , the echo canceler  10  in the first embodiment comprises an amplifier  11 , a gain controller  12 , a clipping circuit  13 , a predictive filter  14 , and an adder  15 , each having two input terminals and one output terminal. 
   The echo canceler  10  has a receiving input terminal Rin, which is connected to one input terminal of the amplifier  11 , and a receiving output terminal Rout, which is connected to the output terminal of the amplifier  11 , to one input terminal of the gain controller  12 , and to one input terminal of the clipping circuit  13 . The output terminal of the clipping circuit  13  is connected to one input terminal of the predictive filter  14 . The output terminal of the predictive filter  14  is connected to a negative input terminal of the adder  15 . The echo canceler  10  also has a sending input terminal Sin, which is connected to the positive input terminal of the adder  15 , and a sending output terminal Sout, which is connected to the output terminal of the adder  15  and to the other input terminals of the gain controller  12  and predictive filter  14 . The gain controller  12  has a gain control output terminal connected to the other input terminal of the amplifier  11 , and a clipping control output terminal connected to the other input terminal of the clipping circuit  13 . 
   The receiving output terminal Rout of the echo canceler  10  is connected to the input terminal of the speaker amplifier  4 . The output terminal of the speaker amplifier  4  is connected to the input terminal of the loudspeaker  5 . The output terminal of the microphone  6  is connected to the sending input terminal Sin of the echo canceler  10 . 
   The operation of the hands-free communication device with this structure will now be described. It is assumed that the maximum permitted input level (amplitude) of the receive input signal at the receiving input terminal Rin is zero millivolt decibels (0 dBm), the maximum gain and maximum output level of the speaker amplifier  4  are fourteen decibels (14 dB) and 6 dBm, respectively, the attenuation factor of the echo path is −10 dB, and the actual level of the receive input signal at the receiving input terminal Rin is −6 dBm. 
   The receive input signal will also be referred to below as the far-end signal. The send input signal received at the sending input terminal Sin includes both a near-end component and an echo component. 
   The gain controller  12  initially sets the gain value of the amplifier  11  to −8 dB in accordance with a built-in control program, so that the speaker amplifier  4  will not saturate, even if the receive input signal has the maximum permitted level (0 dBm) and the speaker amplifier  4  is set to its maximum gain (14 dB). The value of −8 dB is obtained by subtracting the maximum gain (14 dB) from the maximum speaker amplifier output level (6 dBm). The gain controller  12  also sets an initial clipping threshold value of 0 dBm in the clipping circuit  13 . With these settings, the clipping circuit  13  does not clip the receive input signal, even if the receive input signal has the maximum possible input level of 0 dBm, because the level of the signal input to the clipping circuit  13  less than the clipping threshold value. 
   In this state, as in the conventional echo canceler, during times when the near-end component of the send input signal is absent, the predictive filter  14  adapts to the output of the adder  15  by updating internal tap coefficients so as to reduce the adder output to zero. After the coefficient values have converged, during a time when the far-end signal is present and the near-end component of the send signal is absent, the gain controller  12  gradually increases the gain of the amplifier  11  from −8 dB toward a maximum gain value of 0 dB. If the speaker amplifier  4  is set to maximum gain (14 dB), then when the gain of amplifier  11  reaches −2 dB, the output level of the speaker amplifier  4  reaches the saturation level of 6 dBm (−6+(−2)+14=6). Beyond this point, echo prediction becomes increasingly inaccurate, and the level of the output signal of the adder  15  increases because of incomplete echo cancellation. When the level of the output signal of the adder  15  exceeds a predetermined value, the gain controller  12  detects the deterioration of echo cancellation performance, temporarily stops increasing the gain of the amplifier  11 , detects the level (substantially −8 dBm) of the amplified signal at this point, and changes the clipping threshold value of the clipping circuit  13  to this level (substantially −8 dBm). Then the gain controller  12  increases the gain of the amplifier  11  from −2 dB to 0 dB. 
   After the completion of this setting procedure, when the speaker amplifier  4  saturates and its output waveform is clipped, the waveform of the signal input to the predictive filter  14  is similarly clipped by the clipping circuit  13 , so predictive error is avoided, and a high level of echo cancellation performance is maintained despite the saturation of the speaker amplifier. 
   If the speaker amplifier  4  is set to a lower gain and does not saturate during the gradual increase of the gain of amplifier  11  from −8 dB to the maximum value of 0 dB, the output of the adder  15  remains at substantially zero. The gain controller  12  does not detect a deterioration in echo cancellation performance and leaves the clipping threshold value of the clipping circuit  13  fixed at the initial value of 0 dBm after setting the gain of the amplifier  11  to 0 dB. 
   Compared with the conventional echo canceler  1  in  FIG. 4 , the echo canceler  10  in the first embodiment inserts two additional elements on the input side of the predictive filter  14 : a receive input amplifier  11  for determining the input level at which the speaker amplifier saturates, and a clipping circuit  13  having a clipping threshold set at this input level. As a result, even when the speaker amplifier  4  saturates, echo cancellation performance does not deteriorate. 
     FIG. 2  shows the structure of a hands-free communication device having an echo canceler according to a second embodiment of the present invention. The echo canceler  20  in the second embodiment comprises a predictive filter  21 , an amplifier  22 , a gain controller  23 , another amplifier  24 , an adder  25 , and a further amplifier  26 . 
   The receiving input terminal Rin of the echo canceler  20  is connected to one input terminal of the predictive filter  21  and the receiving output terminal Rout of the echo canceler  20 . The output terminal of the predictive filter  21  is connected to one input terminal of amplifier  22 , and the output terminal of amplifier  22  is connected to the negative input terminal of the adder  25 . The positive input terminal of the adder  25  is connected to the output terminal of amplifier  24 . The sending input terminal Sin is connected to one input terminal of amplifier  24 . The output terminal of the adder  25  is connected to one input terminal of amplifier  26 , the other input terminal of the predictive filter  21 , and the input terminal of the gain controller  23 . The output terminal of amplifier  26  is connected to the sending output terminal Sout. The gain controller  23  has three gain control output terminals, which are connected to the other input terminals of the amplifiers  22 ,  24 , and  26 , respectively. 
   The operation of the hands-free communication device having the structure in  FIG. 2  will now be described. It is again assumed that the maximum permitted level (amplitude) of the signal input to the echo canceler  20  from the receiving input terminal Rin is 0 dBm, the maximum gain and maximum output level of the speaker amplifier  4  are 14 dB and 6 dBm, respectively, and the attenuation on the path between the loudspeaker  5  and microphone  6  is −10 dB. The level of the far-end signal input from the receiving input terminal Rin is now assumed to be −10 dBm. The gain controller  23  initially sets the gain of amplifier  22  to zero (0 dB), the gain of amplifier  24  to −14 dB, and the gain of amplifier  26  to 14 dB. During the subsequent operations, the gain controller  23  keeps amplifiers  24  and  26  set to gains of equal magnitude and opposite sign, so that the near-end component of the send output signal has the same level at the sending output terminal Sout as at the sending input terminal Sin. 
   When a far-end signal having a level of −10 dBm is input from the receiving input terminal Rin, the signal output from the receiving output terminal Rout has the same level of −10 dBm. If this signal is amplified by 14 dB by the speaker amplifier  4 , a signal having a level of 4 dBm is output from the loudspeaker  5 . Since the attenuation on the path between the loudspeaker and the microphone is −10 dB, an echo signal having a level of −6 dBm is input to the sending input terminal Sin of the echo canceler. As the initial gain of amplifier  24  is set to −14 dB, the level of the echo signal output from amplifier  24  is −20 dBm. Accordingly, the gain of the echo path as seen from the adder  25  is −10 dB. Since the echo path provides an attenuation of −10 dB, echo cancellation performance, as measured at the output terminal of the adder  25 , does not deteriorate. 
   Since the gain of amplifier  26  is initially set to 14 dB, however, any residual echo or other error present in the output of the adder  25  is amplified, degrading the quality of the send output signal. To avoid this, the gain controller  23  proceeds as follows in accordance with a built-in control program. 
   As in the conventional echo canceler, the predictive filter  21  adapts to the output of the adder  25  by updating internal tap coefficients so as to reduce the adder output to zero when the near-end component of the send input signal is absent. After the tap coefficient values have converged, during a time when the far-end signal is present and the near-end component is absent, the gain controller  23  gradually increases the gain of amplifier  24  from −14 dB toward 0 dB and gradually decreases the gain of amplifier  26  from 14 dB toward 0 dB. The echo path as seen from the adder  25  continues to provide attenuation until the gain of amplifier  24  has reached −4 dB and the gain of amplifier  26  has reached 4 dB. At this point, the error in the output of the adder  25  begins to increase because of echo amplification. By comparing the level of the output of the adder  25  with a predetermined value, the gain controller  23  detects the deterioration in echo cancellation performance and temporarily halts the increase in the gain of amplifier  24  and decrease in the gain of amplifier  26 . 
   The gain controller  23  now increases the gain of amplifier  22  from an initial value of 0 dB to the present value of the gain of amplifier  26 , or 4 dB. Next, the gain controller  23  increases the gain of amplifier  24  from −4 dB to 0 dB, and decreases the gain of amplifier  26  from 4 dB to 0 dB. In the resulting state, the output of the predictive filter  21  is amplified by amplifier  22  by an amount that compensates for the amplification on the echo path as seen from the adder  25 , so the predictive filter  21  and adder  25  operate as if there were no echo amplification, and deterioration of echo cancellation performance is avoided. 
   If the output of the adder  25  does not exceed the predetermined level while the gain of amplifier  24  is increasing from −14 dB to 0 dB, indicating that the echo path does not provide amplification, the gain controller  23  terminates the above procedure by leaving the gains of amplifiers  22 ,  24 , and  26  set at zero (0 dB). 
   In the second embodiment, amplifiers  24  and  26  are inserted in the path of the send signal in order to detect positive gain on the echo path, and the output of the predictive filter  21  is amplified by amplifier  22  to compensate for the positive gain, if detected. Thus, even if the echo path provides a positive echo amplification from the receiving output terminal Rout to the sending input terminal Sin, echo cancellation performance does not deteriorate. 
     FIG. 3  shows the structure of a hands-free communication device having an echo canceler according to a third embodiment of the present invention. The echo canceler  30  comprises an amplifier  31 , a first gain controller  32 , a clipping circuit  33 , a predictive filter  34 , another amplifier  35 , a second gain controller  36 , another amplifier  37 , an adder  38 , and yet another amplifier  39 . 
   The receiving input terminal Rin of the echo canceler  30  is connected to one input terminal of amplifier  31 . The output terminal of amplifier  31  is connected to the receiving output terminal Rout, one input terminal of the first gain controller  32 , and one input terminal of the clipping circuit  33 . The output terminal of the clipping circuit  33  is connected to one input terminal of the predictive filter  34 . The first gain controller  32  has a gain control output terminal connected to the other input terminal of amplifier  31 , and a clipping control output terminal connected to the other input terminal of the clipping circuit  33 . 
   The output terminal of the predictive filter  34  is connected to one input terminal of amplifier  35 ; the output terminal of amplifier  35  is connected to the negative input terminal of the adder  38 . The positive input terminal of the adder  38  is connected to the output terminal of amplifier  37 . The sending input terminal Sin is connected to one input terminal of amplifier  37 . The output terminal of the adder  38  is connected to one input terminal of amplifier  39 , the other input terminal of the predictive filter  34 , and input terminals of the two gain controllers  32 ,  36 . The sending output terminal Sout is connected to the output terminal of amplifier  39 . The second gain controller  36  has three gain control output terminals, which are connected to the other input terminals of amplifiers  35 ,  37 , and  39 , respectively. 
   The receiving output terminal Rout of the echo canceler  30  is connected to the input terminal of the speaker amplifier  4 , and the output terminal of the speaker amplifier  4  is connected to the input terminal of the loudspeaker  5 . The output terminal of the microphone  6  is connected to the sending input terminal Sin of the echo canceler  30 . 
   The operation of the hands-free communication device having the structure in  FIG. 3  will now be described. It is once more assumed that the maximum permitted input level (amplitude) of the far-end signal input to the echo canceler  30  from the receiving input terminal Rin is 0 dBm, the maximum gain and the maximum output level of the speaker amplifier  4  are 14 dB and 6 dBm, respectively, and the attenuation factor of the echo path is −10 dB. The level of the far-end signal at the receiving input terminal Rin is assumed to be −6 dBm. 
   Initially, the first gain controller  32  sets the gain of amplifier  31  to −8 dB in accordance with a built-in control program, so that the speaker amplifier  4  does not saturate, even if set to the maximum gain of 14 dB. The value of −8 dB is obtained by subtracting the maximum gain value of 14 dB from the saturation output level of 6 dBm. Since the maximum gain of the speaker amplifier  4  is 14 dB, the first gain controller  32  also sets the gain of amplifier  37  to −14 dB to ensure that the echo path does not provide positive amplification, and sets the gain of amplifier  39  to 14 dB to compensate for the attenuation by amplifier  37 . The first gain controller  32  sets the initial clipping threshold value of the clipping circuit  33  to 0 dBm. Since the maximum possible level of the receive input signal is 0 dBm, with these settings, the level of the signal input to the clipping circuit  33  is below the clipping threshold value, so the clipping circuit  33  does not clip the receive input signal. The gain of amplifier  35  is initially set to zero. 
   In this state, as in the conventional echo canceler, the predictive filter  34  adapts to the output of the adder  38  by updating its tap coefficients so as to reduce the output of the adder  38  to zero when the near-end component of the send input signal is absent. After the coefficient values have converged, during a time when the far-end signal is present and the near-end component is absent, the first gain controller  32  gradually increases the gain of amplifier  31  from −8 dB toward the maximum value of 0 dB. If the speaker amplifier  4  is set to maximum gain (14 dB), then when the gain of amplifier  31  reaches −2 dB, the output of the speaker amplifier  4  reaches the saturation level of 6 dBm (−6+(−2)+14=6). Beyond this point, the predictive filter  34  makes increasing prediction errors, and the output signal level of the adder  38  increases. By comparing the output level of the adder  38  with a predetermined value, the first gain controller  32  detects deterioration in the echo cancellation performance, stops increasing the gain of amplifier  31 , detects the level (substantially −8 dBm) of the signal output from amplifier  31  to the clipping circuit  33  at this point, and changes the clipping threshold value to this level (substantially −8 dBm). Then the gain controller  12  increases the gain of amplifier  31  from −2 dB to 0 dB. 
   As a result of the above settings, even though the speaker amplifier  4  is saturated and its output waveform is clipped, the waveform of the signal input to the predictive filter  34  is likewise clipped by the clipping circuit  33 , so predictive error is avoided, and echo cancellation performance does not deteriorate. 
   If the output of the adder  38  does not exceed the predetermined value during the increase in the gain of amplifier  31  from −8 dB to the maximum value of 0 dB, indicating that the speaker amplifier  4  has not reached the saturation output level, the first gain controller  32  leaves the clipping threshold value of the clipping circuit  33  fixed at the initial value of 0 dB. 
   Next, a description will be given of the operation when the echo path provides positive amplification. 
   The above operations by the first gain controller  32  leave amplifier  31  set to zero gain. Thus, when a signal having a level of −10 dB is input from the receiving input terminal Rin, the signal output from the receiving output terminal Rout also has a level of −10 dBm. If this signal is amplified by the speaker amplifier  4  with a gain of 14 dB, a signal having a level of 4 dBm is output from the loudspeaker  5 . Since the attenuation factor of the path between the loudspeaker and the microphone is −10 dB at this point, an echo signal having a level of −6 dBm is input to the sending input terminal Sin of the echo canceler. As the initial gain of amplifier  37  is −14 dB, the level of the echo signal output from amplifier  37  is −20 dBm. The gain of the echo path as seen from the adder  38  is −10 dB, because amplifier  37  provides an attenuation that compensates for the amplification by the speaker amplifier  4 . Since the echo path provides an attenuation of −10 dB, echo cancellation performance is not degraded by positive echo amplification. 
   Since the gain of amplifier  39  is set to 14 dB, however, so as not to change the level of the near-end component of the send signal, any error present in the output from the adder  38  is amplified. In order to avoid this unwanted error amplification, after completion of the operation of the first gain controller  32 , the second gain controller  36  makes the following settings in accordance with a built-in control program. 
   First, the second gain controller  36  gradually increases the gain of amplifier  37  from −14 dB toward 0 dB, and decreases the gain of amplifier  39  from 14 dB toward 0 dB. If the speaker amplifier  4  is set to its maximum gain (14 dB), then when the gain of amplifier  37  reaches −4 dB and the gain of amplifier  39  reaches 4 dB, the echo path as seen from the adder  38  begins to provide positive amplification and the error in the output of the adder  38  increases. By comparing the output level of the adder  38  with another predetermined value, the second gain controller  36  detects the resulting deterioration in echo cancellation performance, temporarily stops increasing the gain of amplifier  37  and decreasing the gain of amplifier  39 , and sets the gain of amplifier  35  to the current value of the gain of amplifier  39  (4 dB). Then the second gain controller  36  increases the gain of amplifier  37  from −4 dB to 0 dB, and decreases the gain of amplifier  39  from 4 dB to 0 dB. 
   The above settings cause amplifier  35  to amplify the output of the predictive filter  34  by an amount that compensates for the positive amplification on the echo path, so that the predictive filter  34  and adder  38  operate as if the echo were not amplified and deterioration in echo cancellation performance is prevented. If the echo cancellation performance does not deteriorate during the increase in the gain of amplifier  37  from −14 dB to 0 dB, indicating that the echo path does not provide amplification, the second gain controller  36  leaves the gains of amplifiers  35 ,  37 , and  39  set at respective values of 0 dB. 
   In the third embodiment, an amplifier  31  for determining the level at which the speaker amplifier saturates and a clipping circuit  33  that has a clipping threshold set according to the result of this determination are provided on the input side of the predictive filter  34 . Accordingly, even if the output of the speaker amplifier saturates, echo cancellation performance does not deteriorate. Further amplifiers  37  and  39  are inserted in the path of the send signal in order to detect positive gain on the echo path, and the output of the predictive filter  34  is amplified by amplifier  35  to compensate for any positive gain that is detected. As a result, even if the echo path provides positive amplification, echo cancellation performance does not deteriorate. 
   Accordingly, the present invention provides an echo canceler that can maintain high echo cancellation performance even if the echo signal input to the echo canceler via the echo path is distorted due to amplifier saturation and its waveform is deformed. The invention also provides an echo canceler that can maintain high echo cancellation performance even if attenuation on the acoustic coupling link or echo path is so small that the echo component of the send input signal has a higher level than the receive output signal from which the echo derives. The invention further provides an echo canceler that can maintain high echo cancellation performance even if the echo component of the send input signal is both distorted due to amplifier saturation and has a higher level than the receive output signal, due to positive amplification on the echo path. 
   Those skilled in the art will recognize that various modifications in the embodiments above are possible within the scope of the appended claims.