Abstract:
An input/output apparatus includes a speaker, a microphone, an adaptive filter circuit and a control circuit. The speaker is configured to output audio based on a voice-receiving signal. The microphone is configured to produce a voice-transmitting signal based on gathered audio. The adaptive filter circuit is configured to produce a residual signal based on a pseudo echo signal and a amplified voice-transmitting signal. The pseudo echo signal is produced from the voice-receiving signal and a transfer coefficient which expresses audio transfer properties between the speaker and the microphone. The amplified voice-transmitting signal is produced by causing a microphone amplification circuit to amplify the voice-transmitting signal. The control circuit is configured to stop output of the residual signal from the adaptive filter circuit during at least one of the speaker and the microphone is muted.

Description:
This application claims priority under 35 U.S.C. §119 to Japanese Patent Application No. 2011-206917, filed on Sep. 22, 2011. The entire disclosure of Japanese Patent Application No. 2011-206917 is hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The technique disclosed herein relates to an input/output apparatus comprising a speaker and a microphone. 
     2. Description of the Related Art 
     A method for stopping a coefficient series in an adaptive filter circuit from being updated has been recently proposed in order to inhibit the occurrence of an echo and/or howling created by a situation in which a voice-receiving signal outputted from a speaker picks up sound in a microphone when double-talk occurs in a teleconferencing system (for example, in Japanese Laid-Open Patent Publication No. 5-48547). 
     According to the method in Japanese Laid-Open Patent Publication No. 5-48547, changing of a learned coefficient series can be avoided in cases in which the echo path has been changed by the muting of the speaker and/or the microphone. 
     SUMMARY OF THE INVENTION 
     However, in Japanese Laid-Open Patent Publication No. 5-48547, there remains a path for deducting the pseudo echo signal, which is produced by a convolution operation with the voice-receiving signal series and the coefficient series, from the voice-receiving signal and outputting the signal even when the coefficient series update in the adaptive filter circuit is stopped. Accordingly, an echo and/or howling is liable to occur due to the fact that the pseudo echo signal is transmitted to a counterpart. 
     An object of the technique disclosed herein is to provide an input/output apparatus capable of suppressing the occurrence of an echo and/or howling. 
     An input/output apparatus includes a speaker, a microphone, an adaptive filter circuit and a control circuit. The speaker is configured to output audio based on a voice-receiving signal. The microphone is configured to produce a voice-transmitting signal based on gathered audio. The adaptive filter circuit is configured to produce a residual signal based on a pseudo echo signal and a amplified voice-transmitting signal. The pseudo echo signal is produced from the voice-receiving signal and a transfer coefficient which expresses audio transfer properties between the speaker and the microphone. The amplified voice-transmitting signal is produced by causing a microphone amplification circuit to amplify the voice-transmitting signal. The control circuit is configured to stop output of the residual signal from the adaptive filter circuit during at least one of the speaker and the microphone is muted. 
     According to the technique disclosed herein, it is possible to provide an input/output apparatus capable of suppressing the occurrence of an echo and/or howling. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of the input/output apparatus according to a first embodiment; 
         FIG. 2  is a block diagram of the input/output apparatus according to a second embodiment; 
         FIG. 3  is a block diagram of the input/output apparatus according to a third embodiment; 
         FIG. 4  is a block diagram of the input/output apparatus according to a fourth embodiment; and 
         FIG. 5  is a block diagram of the input/output apparatus according to a fifth embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     1. First Embodiment 
     (Configuration Of Input/Output Apparatus  1 ) 
       FIG. 1  is a block diagram of an input/output apparatus  1  according to a first embodiment. The input/output apparatus  1  comprises a speaker and a microphone, and is suitable for use in a communication terminal capable of bi-directionally exchanging audio signals with a counterpart terminal Such a communication terminal may comprise a receiver for receiving a voice-receiving signal X(t) from the counterpart terminal, and a transmitter for transmitting, to the counterpart terminal, a voice-transmitting signal Y(t) outputted from the input/output apparatus  1 , although neither are shown in the drawings. 
     The input/output apparatus  1  comprises an input terminal  100 , a speaker amplification circuit  101 , a speaker  102 , a microphone  103 , a microphone amplification circuit  104 , an adaptive filter circuit  120 , an output terminal  108 , a mute signal input part  114 , and a control circuit  115 . 
     The voice-receiving signal X(t) is inputted to the input terminal  100 . The voice-receiving signal X(t) is then outputted from the input terminal  100  to the speaker amplification circuit  101  and the adaptive filter circuit  120 , respectively. 
     The speaker amplification circuit  101  amplifies the voice-receiving signal X(t) outputted from the input terminal  100 . The speaker amplification circuit  101  outputs the amplified voice-receiving signal X(t) to the speaker  102 . However, the speaker amplification circuit  101  stops amplification and output of the voice-receiving signal X(t) in cases in which a stop signal has been inputted from the control circuit  115 . Input of the voice-receiving signal X(t) to the speaker  102  is thereby stopped while the speaker  102  is muted. 
     The speaker  102  is connected to the speaker amplification circuit  101 . The speaker  102  outputs audio based on the voice-receiving signal X(t) amplified by the speaker amplification circuit  101 . 
     The microphone  103  produces the voice-transmitting signal Y(t) based on the audio gathered from the surrounding area. The microphone  103  outputs the voice-transmitting signal Y(t) to the microphone amplification circuit  104 . 
     The microphone amplification circuit  104  amplifies the voice-transmitting signal Y(t) inputted from the microphone  103 . The microphone amplification circuit  104  outputs the amplified voice-transmitting signal Y(t) to the adaptive filter circuit  120 . However, the microphone amplification circuit  104  stops production and output of the voice-transmitting signal Y(t) in cases in which a stop signal has been inputted from the control circuit  115 . Input of the voice-transmitting signal Y(t) to the adaptive filter circuit  120  is thereby stopped while the microphone  103  is muted. 
     The voice-receiving signal X(t) outputted from the input terminal  100  and the voice-transmitting signal Y(t) outputted from the microphone amplification circuit  104  are inputted to the adaptive filter circuit  120 . The adaptive filter circuit  120  has a function for removing, from the voice-transmitting signal Y(t), audio components outputted from the speaker  102  and gathered by the microphone  103 . The occurrence of an echo and/or howling in the voice-transmitting signal Y(t) is thereby suppressed. 
     The adaptive filter circuit  120  comprises a voice-receiving signal input terminal  109 , a voice-receiving signal memory circuit  110 , a coefficient memory circuit  113 , a product-summing circuit  112 , a voice-transmitting signal input terminal  105 , a subtraction circuit  106 , a coefficient correction circuit  111 , and a voice-transmitting signal output terminal  107 . 
     The voice-receiving signal X(t) outputted from the input terminal  100  is inputted to the voice-receiving signal input terminal  109 . The voice-receiving signal input terminal  109  outputs the voice-receiving signal X(t) to the voice-receiving signal memory circuit  110  and the coefficient correction circuit  111 . 
     The voice-receiving signal memory circuit  110  stores the voice-receiving signal X(t) inputted from the voice-receiving input terminal  109 . 
     The coefficient memory circuit  113  stores the transfer coefficient expressing the audio transfer properties in a space between the speaker  102  and the microphone  103 . The transfer coefficient stored in the coefficient memory circuit  113  is updated by the coefficient correction circuit  111 . 
     The product-summing circuit  112  produces a pseudo echo signal Z(t) by performing a convolution operation with the voice-receiving signal X(t) saved in the voice-receiving signal memory circuit  110  and the transfer coefficient saved in the coefficient memory circuit  113 . The product-summing circuit  112  outputs the produced pseudo echo signal Z(t) to the subtraction circuit  106 . However, the product-summing circuit  112  stops production of the pseudo echo signal Z(t) in accordance with a control signal outputted from the control circuit  115 . Input of the pseudo echo signal Z(t) to the subtraction circuit  106  is thereby stopped in cases in which the speaker  102  and/or the microphone  103  are muted. 
     The voice-transmitting signal Y(t) outputted from the microphone amplification circuit  104  is inputted to the voice-transmitting signal input terminal  105 . The voice-transmitting signal input terminal  105  outputs the voice-transmitting signal Y(t) to the subtraction circuit  106 . 
     The subtraction circuit  106  produces a residual signal E(t) by deducting the pseudo echo signal Z(t) from the voice-transmitting signal Y(t). Audio components outputted from the speaker  102  and gathered by the microphone  103  are thereby removed from the voice-transmitting signal Y(t). The subtraction circuit  106  outputs the residual signal E(t) to the coefficient correction circuit  111  and the voice-transmitting signal output terminal  107 , respectively. 
     However, input of the voice-transmitting signal Y(t) to the adaptive filter circuit  120  from the microphone amplification circuit  104  is stopped in cases in which the microphone  103  is muted, as described above. In addition, input of the pseudo echo signal Z(t) to the subtraction circuit  106  from the product-summing circuit  112  is stopped while the microphone  103  is muted. Accordingly, the voice-transmitting signal Y(t) and the pseudo echo signal Z(t) are not inputted to the subtraction circuit  106  while the microphone  103  is muted, and no signal is therefore outputted from the subtraction circuit  106 . 
     In addition, input of the pseudo echo signal Z(t) to the subtraction circuit  106  from the product-summing circuit  112  is stopped in cases in which the speaker  102  is muted, as described above. On the other hand, the voice-transmitting signal Y(t) is inputted to the subtraction circuit  106  when the microphone  103  is not muted. Accordingly, the subtraction circuit  106  outputs the voice-receiving signal Y(t) unchanged as the residual signal E(t) while only the speaker  102  is muted. 
     Output, from the adaptive filter circuit  120 , of the residual signal E(t) produced based on the pseudo echo signal Z(t) and the voice-transmitting signal Y(t) is thus stopped in cases in which either the speaker  102  or the microphone  103  is muted. 
     The coefficient correction circuit  111  computes the update value of the transfer coefficient between the speaker  102  and the microphone  103  based on the residual signal E(t) so that the output of the subtraction circuit  106  is minimized The coefficient correction circuit  111  stores the update value of the transfer coefficient in the coefficient memory circuit  113 . The occurrence of an echo and/or howling in the voice-transmitting signal Y(t) is thereby suppressed even in cases in which the transfer coefficient between the speaker  102  and the microphone  103  changes. However, the coefficient correction circuit  111  stops computation of the update value of the transfer coefficient in accordance with a stop signal outputted from the control circuit  115 . In this case, the coefficient correction circuit  111  does not store the update value of the transfer coefficient in the coefficient memory circuit  113 . 
     The voice-transmitting signal output terminal  107  outputs the voice-transmitting signal Y(t) and/or the residual signal E(t) produced from the voice-transmitting signal Y(t) using the adaptive filter circuit  120 . 
     The residual signal E(t) and/or the voice-transmitting signal Y(t) outputted from the adaptive filter circuit  120  is inputted to the output terminal  108 . The output terminal  108  outputs the residual signal E(t) and/or the voice-transmitting signal Y(t) to external equipment or the like. 
     A speaker mute signal expressing an operation for muting the speaker  102  is inputted to the mute signal input part  114 . The speaker mute signal is produced by the depression of a speaker mute button on a remote control (not shown) in cases in which a user desires to temporarily stop the output of audio from the speaker  102 . In addition, a microphone mute signal expressing an operation for muting the microphone  103  is inputted to the mute signal input part  114 . The microphone mute signal is produced by the depression of a microphone mute button on the remote control in cases in which the user desires to temporarily stop the gathering of audio using the microphone  103 . 
     The speaker mute signal and/or the microphone mute signal is outputted from the mute signal input part  114  to the control circuit  115 . The control circuit  115  outputs to the speaker amplification circuit  101  a stop signal for stopping the speaker amplification circuit  101  in accordance with the speaker mute signal. The speaker  102  is thereby muted. The control circuit  115  outputs to the microphone amplification circuit  104  a stop signal for stopping the microphone amplification circuit  104  in accordance with the microphone mute signal. The microphone  103  is thereby muted. In addition, the control circuit  115  outputs to the coefficient correction circuit  111  a control signal for stopping the update value operation for the coefficient series in the coefficient correction circuit  111  during the mute period of the speaker  102  and/or the microphone  103 . Furthermore, the control circuit  115  outputs to the product-summing circuit  112  a control signal for stopping the convolution operation in the product-summing circuit  112  during the mute period of the speaker  102  and/or the microphone  103 . 
     (Operation and Effect) 
     (1) In the input/output apparatus  1  according to the first embodiment, the adaptive filter circuit  120  stops output of the residual signal E(t) produced based on the pseudo echo signal Z(t) and the voice-transmitting signal Y(t) in cases in which either the speaker  102  or the microphone  103  is muted. Specifically, the control circuit  115  also stops the convolution operation in the product-summing circuit  112  in cases in which the speaker amplification circuit  101  and/or the microphone amplification circuit  104  has been stopped. 
     Accordingly, a path formed by the product-summing circuit  112  and the subtraction circuit  106  is blocked in cases in which an echo path has been blocked by the stoppage of the speaker amplification circuit  101  and/or the microphone amplification circuit  104 . Deduction of the pseudo echo signal Z(t) from the voice-transmitting signal Y(t) can therefore be suppressed despite the fact that the speaker  102  is muted, and production of a reverse-sign pseudo echo signal Z(t) can also be suppressed despite the fact that the microphone  103  is muted. As a result, the occurrence of an echo and/or howling can be suppressed. 
     (2) In the input/output apparatus  1  according to the first embodiment, the control circuit  115  also stops updating of the coefficient series in the coefficient correction circuit  111  in cases in which the speaker amplification circuit  101  and/or the microphone amplification circuit  104  has been stopped. 
     Accordingly, updating of the transfer coefficient can be suppressed based on the voice-transmitting signal Y(t) in cases in which audio components outputted from the speaker  102  are not included in the voice-transmitting signal Y(t) by, for example, muting only the speaker  102 . A proper pseudo echo signal Z(t) based on a learned transfer coefficient can therefore be produced when muting of the speaker  102  has been cancelled. As a result, an echo and/or howling can be quickly suppressed after cancellation of muting. 
     2. Second Embodiment 
     (Configuration Of Input/Output Apparatus  1 ) 
       FIG. 2  is a block diagram of the input/output apparatus  1  according to a second embodiment. The difference between the first embodiment and the second embodiment is that the control circuit  115  stops the process in the subtraction circuit  106  rather than stopping the convolution operation in the product-summing circuit  112 . This difference will be mainly described below. 
     The control circuit  115  stops the process in which the pseudo echo signal Z(t) is deducted from the voice-transmitting signal Y(t) in the subtraction circuit  106  in cases in which the speaker mute signal and/or the microphone mute signal is inputted from the mute signal input part  114 . The subtraction circuit  106  thereby allows the voice-transmitting signal Y(t) to pass through unchanged as a residual signal E(t) in cases in which the voice-transmitting signal Y(t) is inputted from the voice-transmitting signal input terminal  105 , and outputs no signal in cases in which the voice-transmitting signal Y(t) is not inputted from the voice-transmitting signal input terminal  105 . 
     (Operation and Effect) 
     (1) In the input/output apparatus  1  according to the second embodiment, the adaptive filter circuit  120  stops output of the residual signal E(t) produced based on the pseudo echo signal Z(t) and the voice-transmitting signal Y(t) in cases in which either the speaker  102  or the microphone  103  is muted. Specifically, the control circuit  115  stops the process in which the pseudo echo signal Z(t) is deducted from the voice-transmitting signal Y(t) in the subtraction circuit  106  in cases in which the speaker amplification circuit  101  and/or the microphone amplification circuit  104  has been stopped. 
     Accordingly, deduction of the pseudo echo signal Z(t) from the voice-transmitting signal Y(t) can be suppressed despite the fact that the speaker  102  is muted, and production of a reverse-sign pseudo echo signal Z(t) can also be suppressed despite the fact that the microphone  103  is muted, in the same manner as in the first embodiment. As a result, the occurrence of an echo and/or howling can be suppressed. 
     (2) In addition, an echo and/or howling can be quickly suppressed after cancellation of muting by stopping the updating of the coefficient series in the coefficient correction circuit  111  using the control circuit  115  in the same manner as in the first embodiment. 
     3. Third Embodiment 
     (Configuration Of Input/Output Apparatus  1 ) 
       FIG. 3  is a block diagram of the input/output apparatus  1  according to a third embodiment. The difference between the first embodiment and the third embodiment is that the adaptive filter circuit  120  comprises an output blocking circuit  117  subsequent to the subtraction circuit  106 . This difference will be mainly described below. In the present embodiment, a scene is envisioned in which the microphone  103  is muted. 
     The output blocking circuit  117  is disposed between the subtraction circuit  106 , and the voice-transmitting signal output terminal  107  and the coefficient correction circuit  111 . The output blocking circuit  117  stops output of a signal inputted from the subtraction circuit  106  in cases in which a stop signal has been inputted from the control circuit  115 . Output of the residual signal E(t) to the coefficient correction circuit  111  and the voice-transmitting signal output terminal  107  is thereby stopped while the microphone  103  is muted. 
     The control circuit  115  outputs to the output blocking circuit  117  a control signal for stopping output from the output blocking circuit  117  in cases in which the microphone mute signal has been inputted from the mute signal input part  114 . The signal outputted from the subtraction circuit  106  is thereby blocked by the output blocking circuit  117 . 
     In the present embodiment, the control circuit  115  does not stop the process of the microphone amplification circuit  104 , the product-summing circuit  112 , and the subtraction circuit  106 , and the residual signal E(t) is therefore outputted to the output blocking circuit  117  from the subtraction circuit  106 . 
     In the present embodiment, the control circuit  115  does not stop the process of the coefficient correction circuit  111 , but output from the output blocking circuit  117  is entirely stopped, and the coefficient correction circuit  111  does not therefore compute the update value of the transfer coefficient. As a result, the transfer coefficient stored in the coefficient memory circuit  113  is held while the microphone  103  is muted. 
     (Operation and Effect) 
     In the input/output apparatus  1  according to the third embodiment, the adaptive filter circuit  120  stops output of the residual signal E(t) produced based on the pseudo echo signal Z(t) and the voice-transmitting signal Y(t) in cases in which the microphone  103  is muted. Specifically, the control circuit  115  allows the signal outputted from the subtraction circuit  106  to be blocked by the output blocking circuit  117  in cases in which the microphone mute signal has been inputted from the mute signal input part  114 . 
     Accordingly, output of the residual signal E(t) can be suppressed despite the fact that the microphone  103  is muted, and the occurrence of an echo and/or howling can therefore be suppressed in the same manner as in the first embodiment. 
     In addition, output of the residual signal E(t) to the coefficient correction circuit  111  is blocked, and updating of the transfer coefficient is therefore stopped while the microphone  103  is muted. Accordingly, an echo and/or howling can be quickly suppressed after cancellation of muting. 
     4. Fourth Embodiment 
     (Configuration Of Input/Output Apparatus  1 ) 
       FIG. 4  is a block diagram of the input/output apparatus  1  according to a fourth embodiment. The difference between the third embodiment and the fourth embodiment is that the adaptive filter circuit  120  comprises an output blocking circuit  116  subsequent to the voice-transmitting signal output terminal  107 . This difference will be mainly described below. In the present embodiment, a scene is envisioned in which the microphone  103  is muted. 
     The output blocking circuit  116  is disposed between the voice-transmitting signal output terminal  107  and the output terminal  108 . The output blocking circuit  116  stops output of a signal inputted from the voice-transmitting output terminal  107  in cases in which a stop signal has been inputted from the control circuit  115 . Output of the residual signal E(t) to the output terminal  108  is thereby stopped while the microphone  103  is muted. 
     The control circuit  115  outputs to the output blocking circuit  116  a control signal for stopping output from the output blocking circuit  116  in cases in which the microphone mute signal has been inputted from the mute signal input part  114 . The signal outputted from the subtraction circuit  106  is thereby blocked by the output blocking circuit  116 . 
     In the present embodiment, the control circuit  115  does not stop the processes of the microphone amplification circuit  104 , the product-summing circuit  112 , and the subtraction circuit  106 . The residual signal E(t) is therefore outputted to the output blocking circuit  116  from the subtraction circuit  106 . 
     In the present embodiment, a path to the coefficient correction circuit  111  from the subtraction circuit  106  is maintained, and the process of the coefficient correction circuit  111  is not stopped by the control circuit  115 . The coefficient correction circuit  111  therefore continues to compute the update value of the transfer coefficient even while the microphone  103  is muted. 
     (Operation and Effect) 
     (1) In the input/output apparatus  1  according to the fourth embodiment, the adaptive filter circuit  120  stops output of the residual signal E(t) produced based on the pseudo echo signal Z(t) and the voice-transmitting signal Y(t) in cases in which the microphone  103  is muted. Specifically, the control circuit  115  uses the output blocking circuit  116  to block the signal outputted from the voice-transmitting signal output terminal  107  in cases in which the microphone mute signal is inputted from the mute signal input part  114 . 
     Accordingly, output of the residual signal E(t) can be suppressed despite the fact that the microphone  103  is muted in the same manner as in the third embodiment, and the occurrence of an echo and/or howling can therefore be suppressed. 
     (2) In addition, output of the residual signal E(t) to the coefficient correction circuit  111  is not blocked, and the coefficient correction circuit  111  is therefore allowed to continue computing the update value of the transfer coefficient even while the microphone  103  is muted. 
     Accordingly, the learning function of the transfer coefficient can be allowed to continue even during muting. An echo and/or howling can therefore be quickly suppressed after cancellation of muting even in cases in which the positions (namely, the echo paths) of the speaker  102  and the microphone  103  are changed during muting. 
     5. Fifth Embodiment 
     (Configuration Of Input/Output Apparatus  1 ) 
       FIG. 5  is a block diagram of the input/output apparatus  1  according to a fifth embodiment. The difference between the first embodiment and the fifth embodiment is that the adaptive filter circuit  120  comprises an input blocking circuit  119  preceding the voice-receiving signal input terminal  109 . This difference will be mainly described below. In the present embodiment, a scene is envisioned in which the speaker  102  is muted. 
     The input blocking circuit  119  is disposed between the input terminal  100  and the voice-receiving signal input terminal  109 . The input blocking circuit  119  stops output of the voice-receiving signal X(t) inputted from the input terminal  100  in cases in which a stop signal has been inputted from the control circuit  115 . Production of the pseudo echo signal Z(t) and the residual signal E(t) is thereby stopped while the speaker  102  is muted. 
     The control circuit  115  outputs to the input blocking circuit  119  a control signal for stopping output from the input blocking circuit  119  in cases in which the speaker mute signal has been inputted from the mute signal input part  114 . The voice-receiving signal X(t) inputted from the input terminal  100  is thereby blocked by the input blocking circuit  119 . 
     In the present embodiment, the control circuit  115  does not stop the processes of the microphone amplification circuit  104  and the subtraction circuit  106 , but the product-summing circuit  112  ceases to function due to the blocking of input of the voice-receiving signal X(t) to the voice-receiving signal memory circuit  110 . The voice-transmitting signal Y(t) is therefore output unchanged from the subtraction circuit  106  as the residual signal E(t). 
     In the present embodiment, the control circuit  115  does not stop the process of the coefficient correction circuit  111 , but the coefficient correction circuit  111  ceases to function due to the blocking of input of the voice-receiving signal X(t) to the coefficient correction circuit  111 . Updating of the transfer coefficient by the coefficient correction circuit  111  is therefore stopped. As a result, the transfer coefficient stored in the coefficient memory circuit  113  is held while the speaker  102  is muted. 
     (Operation and Effect) 
     In the input/output apparatus  1  according to the fifth embodiment, the adaptive filter circuit  120  stops output of the residual signal E(t) produced based on the pseudo echo signal Z(t) and the voice-transmitting signal Y(t) in cases in which the speaker  102  is muted. Specifically, the control circuit  115  causes the voice-receiving signal X(t) inputted from the input terminal  100  to be blocked by the input blocking circuit  119  in cases in which the speaker mute signal has been inputted from the mute signal input part  114 . 
     Accordingly, output of the residual signal E(t) can be suppressed despite the fact that the speaker  102  is muted, in the same manner as in the first embodiment. The occurrence of an echo and/or howling can therefore be suppressed. 
     In addition, input of the voice-receiving signal X(t) to the coefficient correction circuit  111  is blocked, whereby the transfer coefficient stored in the coefficient memory circuit  113  can be held. Accordingly, an echo and/or howling can be quickly suppressed after cancellation of muting. 
     Other Embodiments 
     The present invention is described according to the aforementioned embodiments, but it should not be understood that the present invention is limited to the statements and drawings constituting a part of this disclosure. Various alternative embodiments, examples, and operational techniques are apparent from this disclosure to a person skilled in the art. 
     For example, in the present embodiment, the adaptive filter circuit  120  comprises the coefficient correction circuit  111 , but no limitation is imposed thereby. The adaptive filter circuit  120  may not comprise the coefficient correction circuit  111 , and the coefficient memory circuit  113  may save a fixed value as the transfer coefficient. Even in this case, the influence on sound quality is small in the case of an environment in which the change in audio transfer properties between the speaker  102  and the microphone  103  is small.