Patent Publication Number: US-8996365-B2

Title: Howling canceller

Description:
CROSS-REFERENCE TO PRIOR FILED APPLICATIONS: 
     This is a National Phase Application filed under 35 U.S.C. §371 as a national stage of International Application No. PCT/JP2010/002004, filed on Mar. 19, 2010, claiming the benefit from Japanese Application 2009-068683, filed on Mar. 19, 2009, and claiming the benefit from Japanese Patent Application 2009-209298, filed on Sep. 10, 2009, the content of each of which is hereby incorporated by reference in its entirety. 
     TECHNICAL FIELD 
     The present invention relates to a howling canceller for suppressing howling of a speech signal using an adaptive filter. 
     BACKGROUND ART 
     Conventionally, there has been disclosed various howling cancellers capable of suppressing howling of a speech signal input from a microphone and emitting the resultant through a speaker and so forth. 
     A conventional howling canceller based on analog processing without using an adaptive filter has been disclosed in Patent literature 1. Patent literature 1 discloses a technology that assumes transmission characteristics of an acoustic system between a microphone of a hearing aid and a speaker are constant, and prevents the occurrence of howling by setting transmission characteristics of a feedback circuit to be equal to transmission characteristics of the acoustic system, which has been measured in advance, by using the feedback circuit with fixed characteristics. However, if a change occurs in the transmission characteristics of the acoustic system between the microphone and the speaker, it is difficult to suppress the howling with the technology disclosed in Patent literature 1. 
     In order to cope with the problem of the change in the transmission characteristics of the acoustic system, there has been disclosed a system capable of suppressing howling through digital processing using an adaptive filter in a loudspeaker. The system has a structure in which positive feedback is applied from the output to the input of an adaptive system having the same configuration as the system. In addition, a delay circuit is inserted into a feedback loop. The delay circuit improves convergence characteristics of the adaptive filter by reducing correlation between the output signal and the input signal of the adaptive system, which is caused by feedback. 
     If delay of the delay circuit is larger than an impulse response length of the system to be identified and disposed between the input of a D/A converter and the output of an A/D converter, and than an impulse response length of the adaptive filter, no increase of the correlation due to the feedback occur in principle. 
     In this system, even when howling occurs, if the adaptive filter can accurately estimate transmission characteristics between the input of the D/A converter and the output of the A/D converter, the howling can be suppressed. 
     However, when a gain of a loudspeaker system exceeds “1” in a wide frequency range, convergence of the adaptive filter does not catch up the rapid growth of the amplitude of howling sound caused by positive feedback, the amplitude of the howling sound increases beyond the linear region of any one of a D/A converter, a power amplifier, a speaker, a microphone, a microphone amplifier, and an A/D converter, and the waveform of the howling sound is saturated, resulting in the occurrence of non-linear distortion. 
     Since the adaptive filter performs a process based on the assumption of the linearity of a system, if non-linear distortion occurs in the progress of generating a desired signal to be estimated from an input signal, bias occurs in the operation of the adaptive filter and thus good convergence characteristics are not obtained. Therefore, if howling occurs in the loudspeaker system with a gain exceeding “1” over a wide frequency range and reaches a saturation state once, it is difficult to suppress the howling by the adaptive filter. 
     In order to solve the problem, Patent literature 2 discloses a technology capable of preventing saturation of an A/D converter and a D/A converter by using a limiter circuit in an active noise canceller using an adaptive filter. 
     Furthermore, Patent literature 3 discloses a technology capable of correcting and removing non-linear distortion by using the Volterra filter in order to prevent an adverse influence of the non-linear distortion occurring in a speaker, on the convergence characteristics of a howling canceller. 
     Furthermore, Patent literature 4 discloses a technology capable of achieving an effect similar to a change in transmission characteristics through a conversion process of a non-linear signal, and suppressing the rapid growth of howling. 
     CITATION LIST 
     Patent Literature 
     
         
         PTL 1 
         Japanese Patent Application Laid-Open No. 9-168195 
         PTL 2 
         Japanese Patent Application Laid-Open No. 8-129388 
         PTL 3 
         Japanese Patent Application Laid-Open No. 2001-86585 
         PTL 4 
         Japanese Patent Application Laid-Open No. 2006-261967 
       
    
     SUMMARY OF INVENTION 
     Technical Problem 
     However, in Patent literature 2, the limiter circuit is used only in order to prevent the saturation of the A/D converter and the D/A converter, and it is not ensured that a speaker and a microphone operates in a linear region without being saturated. 
     Furthermore, in the Volterra filter disclosed in Patent literature 3, rapid saturation characteristics of the D/A converter and the A/D converter are not improved, and an operation with linearity is not ensured regardless of values of the amplitude of an input signal. 
     Furthermore, the technique disclosed in Patent literature 4 does not ensure that all of the D/A converter, the power amplifier, the speaker, the microphone, the microphone amplifier, and the A/D converter operate in a linear region although grown howling reaches a saturation state. 
     As described above, in the conventional technologies using the adaptive filter, it is difficult to suppress howling grown in the system with an open loop gain exceeding “1” in the wide frequency range and reached the saturation state. 
     Furthermore, in the conventional technologies, if the howling occurs and the amplitude of the howling sound is rapidly grown, the non-linear distortion occurs beyond the linear region of any one of the D/A converter, the power amplifier, the speaker, the microphone, the microphone amplifier, and the A/D converter. 
     Therefore, since the convergence characteristics of the adaptive filter performing a process based on the assumption of the linearity of a system are distorted, it is difficult to suppress the howling having reached the saturation state once by using the adaptive filter. 
     As described above, in the conventional technologies, a howling suppression effect is achieved only in the state in which the growth of the amplitude of the howling sound is delayed to the extent that the open loop gain has slightly exceeded “1,” or when the open loop gain changes before and after the value 1, and it is difficult to suppress howling having occurred due to the fact that the open loop gain exceeds “1” in the whole playback band of the loudspeaker, and reached the saturation state. 
     In view of the above-described problems, it is therefore an object of the present invention to provide a howling canceller using an adaptive filter, which is capable of suppressing the occurrence of howling even when an open loop gain exceeds “1” in the whole playback band. 
     Solution to Problem 
     The howling canceller according to the present invention, which is mounted in an apparatus including a D/A converter that converts a digital received speech signal into an analog received speech signal, a power amplifier that amplifies the analog received speech signal output from the D/A converter, a speaker that plays back the analog received speech signal amplified by the power amplifier and outputs sound, a microphone that converts sound including playback sound output from the speaker into an analog transmitted speech signal, a microphone amplifier that amplifies the analog transmitted speech signal output from the microphone, and an A/D converter that converts the analog transmitted speech signal amplified by the microphone amplifier into a digital transmitted speech signal, includes: an adaptive filter that operates the digital received speech signal with a tap coefficient to generate a pseudo echo, and updates the tap coefficient such that a residual signal is an optimal value; a subtractor that subtracts the pseudo echo from the digital transmitted speech signal to generate the residual signal; and an amplitude limiting circuit that limits an absolute value of an amplitude of the digital received speech signal to be equal to or smaller than a predetermined threshold value, and outputs the amplitude-limited digital received speech signal to the D/A converter and the adaptive filter, wherein the threshold value is a minimum value of a first threshold value set in a linear region of the D/A converter, a second threshold value set in a linear region of the power amplifier, a third threshold value set in a linear region of the speaker, a fourth threshold value set in a linear region of the microphone, a fifth threshold value set in a linear region of the microphone amplifier, and a sixth threshold value set in a linear region of the A/D converter. 
     Advantageous Effects of Invention 
     According to the present invention, an amplitude limiting circuit for limiting the amplitude of an input signal of an adaptive filter to be equal to or smaller than a predetermined threshold value is inserted into a feedback loop from the output to the input of a system to be identified. Furthermore, even when howling is grown in the state which an open loop gain of a loudspeaker system is equal to or larger than “1,” the threshold value of the amplitude limiting circuit is set such that all of a A/D converter, a power amplifier, a speaker, a microphone, a microphone amplifier, and an A/D converter operate in a linear region without being saturated. 
     In addition, it is possible to prevent the occurrence of non-linear distortion in an adaptive system having the same configuration as the system, and to also prevent bias of the adaptive system, which is caused by the non-linear distortion. Consequently, even when an open loop gain exceeds “1” in the whole playback band, it is possible to suppress howling. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a block diagram showing a configuration of a loudspeaker having a howling canceller therein according to Embodiment 1 of the present invention; 
         FIG. 2  is a diagram showing open loop frequency characteristics of a loudspeaker system according to Embodiment 1 of the invention; 
         FIG. 3  is diagram showing the level of an output signal of a microphone during the operation of a loudspeaker having a howling canceller therein according to Embodiment 1 of the invention; 
         FIG. 4  is a diagram showing in detail the characteristics of each element of the loudspeaker of  FIG. 1 ; 
         FIG. 5  is a diagram showing input/output characteristics of a non-linear system NL and a linear system L; 
         FIG. 6  is a diagram showing an example in which a tolerable input signal level and a gain parameter of each unit shown in  FIG. 4  are set according to predetermined conditions; 
         FIG. 7  is a diagram showing a tolerable input signal level and a maximum input signal level of each unit according to a conventional art and Embodiment 1 of the invention under condition A of  FIG. 6 ; 
         FIG. 8  is a diagram showing a tolerable input signal level and a maximum input signal level of each unit according to a conventional art under condition B of  FIG. 6 ; 
         FIG. 9  is a diagram showing a tolerable input signal level and a maximum input signal level according to Embodiment 1 of the invention under condition B of  FIG. 6 ; 
         FIG. 10  is a diagram showing a tolerable input signal level and a maximum input signal level of each unit according to a conventional art under the condition C of  FIG. 6 ; 
         FIG. 11  is a diagram showing a tolerable input signal level and a maximum input signal level according to Embodiment 1 of the invention under the condition C of  FIG. 6 ; 
         FIG. 12  is a block diagram showing a configuration of a loudspeaker having a howling canceller therein according to Embodiment 3 of the invention; 
         FIG. 13  is a diagram showing a circuit configuration of an amplitude limiting circuit of a howling canceller according to Embodiment 4 of the invention; 
         FIG. 14  is a diagram showing a circuit configuration when an amplitude limiting circuit is formed of a limiter circuit having a magnitude comparator and a multiplexer therein; 
         FIG. 15  is a diagram plotting an input signal of an adaptive filter, which is obtained by performing a simulation with respect to the circuit of  FIG. 13  and the circuit of  FIG. 14 ; 
         FIG. 16  is a diagram showing a circuit configuration of an amplitude limiting circuit of a howling canceller according to Embodiment 5 of the invention; 
         FIG. 17  is a diagram showing a simulation result when continuously controlling a threshold value with respect to the circuit of  FIG. 16 ; 
         FIG. 18  is a block diagram showing a configuration of a loudspeaker having a howling canceller therein according to Embodiment 6 of the invention; 
         FIG. 19  is a diagram showing a change in threshold value k[n] when a value of a parameter is set in detail with respect to the circuit of  FIG. 18 ; 
         FIG. 20  is a diagram showing a waveform of an output signal (playback sound from a speaker) of a howling canceller; 
         FIG. 21  is a block diagram showing a configuration of a communication apparatus when a howling canceller according to each Embodiment of the invention is applied to an echo canceller of a bi-directional communication system; 
         FIG. 22  is a block diagram showing a configuration of a hearing aid having a howling canceller therein according to Embodiment 7 of the invention; and 
         FIG. 23  is a diagram showing a result of a simulation for checking effectiveness of a howling canceller according to Embodiment 7 of the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Now, embodiments of the present invention will be described in detail with reference to the accompanying drawings. 
     (Embodiment 1) 
       FIG. 1  is a block diagram showing a configuration of a loudspeaker according to Embodiment 1 of the present invention. 
     As shown in  FIG. 1 , the loudspeaker includes digital-to-analog (D/A) converter  101 , power amplifier  102 , speaker  103 , microphone  104 , microphone amplifier  105 , analog-to-digital (A/D) converter  106 , adaptive filter  107 , subtractor  108 , delay circuit  109 , and amplitude limiting circuit  110 . 
     D/A converter  101  converts a digital received speech signal x[n] at the discrete time n into an analog received speech signal. The analog received speech signal output from D/A converter  101  is amplified by power amplifier  102 . 
     Speaker  103  plays back the analog received speech signal output from power amplifier  102  and outputs sound. The playback sound output from speaker  103  is input to microphone  104 . 
     Microphone  104  converts sound including the playback sound output from speaker  103  into an analog transmitted speech signal. The analog transmitted speech signal output from microphone  104  is amplified by microphone amplifier  105 , and is input to A/D converter  106 . In addition, when describing the operation of a howling canceller, since a speech signal may be considered to be the same as noise in an amplifier (power amplifier  102  and microphone amplifier  105 ) and indoor background noise, which become an excitation signal of howling in a silent time,  FIG. 1  does not show a speech signal of a person which is input from microphone  104 . 
     A/D converter  106  converts the analog transmitted speech signal into a digital transmitted speech signal d[n]. Digital transmitted speech signal d[n] is input to subtractor  108 . 
     Adaptive filter  107  computes a digital received speech signal x [n] by tap coefficient H[n] so as to generate pseudo echo y[n]. Furthermore, adaptive filter  107  updates tap coefficient H[n] such that residual signal e[n] output from subtractor  108  is an optimal value. In general, adaptive filter  107  has a finite impulse response (FIR) configuration. However, adaptive filter  107  may have an infinite impulse response (IIR) configuration. In the case of using adaptive filter  107  with the IIR configuration, the entire system between digital received speech signal x[n], which is an input signal of an adaptive system, and residual signal e[n], which is an output signal of an adaptive system, may operate as an adaptive notch filter. A system with such an adaptive notch is effective to suppress howling of a system with a gain significantly exceeding “1” at a specific frequency. Furthermore, as an adaptive algorithm of adaptive filter  107 , an Least Mean Square (LMS) algorithm, an Normalized LMS (NLMS) algorithm, a projection method, an Recursive Least Square (RLS) algorithm and so forth are generally used. These algorithms are adaptive algorithms that allow an iterative operation to be performed whenever a sampled value of a new signal is input and a tap coefficient to be gradually converged to an optimal value. 
     Subtractor  108  subtracts pseudo echo y[n] from digital transmitted speech signal d[n] to generate echo-suppressed residual signal e[n]. 
     Delay circuit  109  delays residual signal e[n], which is the output signal of the adaptive system due to feedback, for a predetermined time, and outputs a delayed signal. The output signal of delay circuit  109  is digital received speech signal x[n] which is the input signal of the adaptive system. A delay time in delay circuit  109  is allowed to be the same as an impulse response length of an acoustic system between speaker  103  and microphone  104 , so that it is possible to improve the convergence characteristics of adaptive filter  107 . 
     Amplitude limiting circuit  110  limits an absolute value of the amplitude of input signal x[n] of the adaptive system to be equal to or smaller than a predetermined threshold value K. In detail, if the absolute value of the amplitude of input signal x[n] is equal to or smaller than threshold value k, amplitude limiting circuit  110  operates in a linear region to output input signal x[n] as is. If the absolute value of input signal x[n] is larger than threshold value k, a non-linear movement is performed so as to restrict the amplitude of input signal x[n] to be −K or K and then to output input signal x[n]. 
     In addition, as amplitude limiting circuit  110 , a simple limiter circuit may be used or a compressor circuit with a time constant may be used. The compressor circuit is an amplifier that calculates short-time mean power (or short-time mean of an absolute value of an amplitude) of an input signal, and controls a gain using the calculated value. The compressor circuit adjusts an output amplitude according to the short-time mean power or the short-time mean of the absolute value of the amplitude of the input signal, thereby reducing the distortion of a waveform caused by amplitude control, as compared with a limiter circuit that instantaneously saturates a waveform. 
     Threshold value k of amplitude limiting circuit  110  ensures that all of D/A converter  101 , power amplifier  102 , speaker  103 , microphone  104 , microphone amplifier  105 , and A/D converter  106  operate in the linear region without being saturated. 
     In this way, if all of D/A converter  101 , power amplifier  102 , speaker  103 , microphone  104 , microphone amplifier  105 , and A/D converter  106  operate in the linear region, even when howling occurs once, adaptive filter  107  converges and the howling is suppressed. 
     According to the invention, a howling suppression experiment was performed in an anechoic room in order to verify that howling suppression is actually possible. In this experiment, the distance between speaker  103  and microphone  104  is 2 m.  FIG. 2  shows open loop frequency characteristics of a loudspeaker system between the input of D/A converter  101  including transmission characteristics of an acoustic system and the output of A/D converter  106 . As apparent from  FIG. 2 , a loudspeaker has a gain of 0 dB or more, about an average 10 dB, in the whole speech band of about 300 Hz to about 3200 Hz. In addition, in this experiment, a sampling frequency of a howling canceller was set to 8 kHz and the NLMS algorithm was used as the adaptive algorithm. 
       FIG. 3  is diagram showing the level of an output signal of a microphone during the operation of a loudspeaker having a howling canceller therein. In  FIG. 3 , the horizontal axis denotes time (unit: second) and the vertical axis denotes amplitude. 
     The loudspeaker system and the howling canceller start to operate from the time of two seconds on the time axis of  FIG. 3 . At this time, sound is not input from microphone  104 . However, noise in power amplifier  102  and microphone amplifier  105  or background noise in an anechoic room becomes an excitation signal, resulting in the immediate occurrence of howling. 
     In a start period (for a little while after the system starts operating), since the convergence of the adaptive algorithm does not catch up the growth of the amplitude of howling sound, the howling sound is directly saturated. In such a state, since the amplitude of a signal is limited by amplitude limiting circuit  110 , even when howling occurs, all of D/A converter  101 , power amplifier  102 , speaker  103 , microphone  104 , microphone amplifier  105 , and A/D converter  106  operate in the linear region. 
     Adaptive filter  107  gradually converges even while howling with a saturated amplitude in amplitude limiting circuit  110  is being continued, and the howling is suppressed at the time of five seconds. 
     Voice is input to microphone  104  from the time of 12 seconds. However, a loudspeaking operation is performed in a stable state. 
     At the time of 34 seconds, the output signal y[n] of adaptive filter  107  is forcedly set to 0 and the howling suppression process is forcedly stopped. 
     Therefore, howling occurs again. The forced stop of the howling suppression process is for simulating the state, in which the howling suppressed once has occurred again, because the transmission characteristics of the loudspeaker system rapidly change and the convergence of the adaptive filter  107  does not catch up. 
     Even while the howling is occurring, a coefficient update operation of adaptive filter  107  is continued. However, since the y[n] is forcedly set to 0, residual signal e[n] is equal to d[n] and the coefficient of adaptive filter  107  is diverged to a random value. 
     At the time of 41 seconds, the operation of forcedly setting the output signal y[n] of adaptive filter  107  to 0 is stopped, and the howling canceller normally operates. 
     Since the y[n] is forcedly set to 0 and thus the coefficient of adaptive filter  107  is diverged, howling is continued for a little while. However, adaptive filter  107  gradually converges, and the howling is suppressed at the time of 47 seconds. After the howling is suppressed, the loudspeaking operation is normally continued. 
     The operations of the loudspeaker system and the howling canceller are stopped at the time of 70 seconds, and no output is generated from the speaker of the loudspeaker system. Therefore, after 70 seconds, the amplitude of the speech signal output from microphone  104  is reduced. Consequently, it is possible to check that the loudspeaker system has a loudspeaking gain of 0 dB or more. 
     As described above, according to the present invention, through the anechoic room experiment, it is verified that it is possible to suppress howling having occurred from the loudspeaker having a gain of 0 dB or more, about an average 10 dB, in the whole speech band of 300 Hz to 3200 Hz. 
     Furthermore, after suppressing howling having occurred in the start period, robustness of an operation capable of suppressing howling having occurred again due to a change and so forth in the transmission characteristics of the acoustic system is also verified. 
     (Embodiment 2) 
     In Embodiment 2, a method for setting threshold value k of amplitude limiting circuit  110  shown in  FIG. 1  will be described. 
       FIG. 4  is a diagram showing in detail the characteristics of each element of the loudspeaker of  FIG. 1 . In addition,  FIG. 4  shows a model in which each of power amplifier  102 , speaker  103 , the acoustic system between speaker  103  and microphone  104 , microphone  104 , and microphone amplifier  105  has flat frequency characteristics. 
     In the model of  FIG. 4 , each of D/A converter  101  with non-linearity, power amplifier  102 , speaker  103 , microphone  104 , microphone amplifier  105 , and A/D converter  106  connects a non-linear system NL to a linear system L. 
     The non-linear system NL has input/output characteristics shown in  FIG. 5A . In a linear region where an absolute value of the amplitude of an input signal is equal to or smaller than threshold value k, a gain of the non-linear system NL is “1” and output of the non-linear system NL is not saturated. In a non-linear region where the absolute value of the amplitude of the input signal is equal to or higher than threshold value k, the output of the non-linear system NL is saturated. Furthermore, the linear system L has input/output characteristics shown in  FIG. 5B  and a gain G. 
     In  FIG. 4 , NL DA , NL PA , NL SP , NL MIC , NL MA , and NL AD  represent the characteristics of non-linear system parts of D/A converter  101 , power amplifier  102 , speaker  103 , microphone  104 , microphone amplifier  105 , and A/D converter  106 , respectively. Furthermore, in  FIG. 4 , L DA , L PA , L SP , L AC , L MIC , L MA , and L AD  represent the characteristics of linear system parts of D/A converter  101 , power amplifier  102 , speaker  103 , the acoustic system between speaker  103  and microphone  104 , microphone  104 , microphone amplifier  105 , and A/D converter  106 , respectively. In addition, the acoustic system is linear and does not have non-linear characteristics. 
     Here, K DA , K PA , K SP , K MIC , K MA , and K AD  are set as tolerable input signal levels of the non-linear system parts of D/A converter  101 , power amplifier  102 , speaker  103 , microphone  104 , microphone amplifier  105 , and A/D converter  106 , respectively. 
     Furthermore, G DA , G PA , G SP , G AC , G MIC , G MA , and G AD  are set as gains of the linear system parts of D/A converter  101 , power amplifier  102 , speaker  103 , the acoustic system between speaker  103  and microphone  104 , microphone  104 , microphone amplifier  105 , and A/D converter  106 , respectively. 
     At this time, an open loop gain G ALL  of the entire loudspeaker is expressed by equation 1 below.
 
[1]
 
 G   ALL   =G   DA   ·G   PA   ·G   SP   ·G   AC   ·G   MIC   ·G   MA   ·G   AD   Equation 1
 
     In equation 1, if 1&lt;G ALL , even when no speech signal is input from microphone  104 , howling occurs immediately after the system starts operating because indoor background noise or noise occurring in power amplifier  102  and microphone amplifier  105  becomes an excitation signal. 
     In order to suppress howling using adaptive filter  107  based on the assumption of the linearity of a system, all elements of the loudspeaker should be linear. In order to ensure that all of D/A converter  101 , power amplifier  102 , speaker  103 , microphone  104 , microphone amplifier  105 , and A/D converter  106  always operate in the linear region without being saturated, threshold value k of amplitude limiting circuit  110  should satisfy all of conditional equation 2 below.
 
[2]
 
 K≦K   DA  
 
 K·G   DA   ≦K   PA  
 
 K·G   DA   ·G   PA   ≦K   SP  
 
 K·G   DA   ·G   PA   ·G   SP   ·G   AC   ≦K   MIC  
 
 K·G   DA   ·G   PA   ·G   SP   ·G   AC   ·G   MIC   ≦K   MA  
 
 K·G   DA   ·G   PA   ·G   SP   ·G   AC   ·G   MIC   ·G   MA   ≦K   AD   Equation 2
 
     Threshold value k of amplitude limiting circuit  110  satisfying all of the above-mentioned limiting conditions and ensuring that all of D/A converter  101 , power amplifier  102 , speaker  103 , microphone  104 , microphone amplifier  105 , and A/D converter  106  operate in the linear region is calculated by equation 3 below. In equation 3, a function min( ) is for calculating a minimum value of arguments. Furthermore, K 1  denotes a threshold value (e.g. a maximum value of a linear region of D/A converter  101 ) set in the linear region of D/A converter  101 , K 2  denotes a threshold value (e.g. a maximum value of a linear region of power amplifier  102 ) set in the linear region of power amplifier  102 , K 3  denotes a threshold value (e.g. a maximum value of a linear region of speaker  103 ) set in the linear region of speaker  103 , K 4  denotes a threshold value (e.g. a maximum value of a linear region of microphone  104 ) set in the linear region of microphone  104 , K 5  denotes a threshold value (e.g. a maximum value of a linear region of microphone amplifier  105 ) set in the linear region of microphone amplifier  105 , and K 6  denotes a threshold value (e.g. a maximum value of a linear region of A/D converter  106 ) set in the linear region of A/D converter  106 . 
     
       
         
           
             
               
                 
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     The tolerable input signal levels K DA , K PA , K SP , K MIC , K MA , and K AD  and the gains G DA , G PA , G SP , G AC , G MIC , G MA , and G AD  of equation 3 can be obtained from parameters and actually measured data which are written in specifications, an instruction manual and so forth of an apparatus. 
     Now, a method for calculating these values will be described in detail. 
     Tolerable input signal level K DA  of D/A converter  101  can be obtained from a resolution thereof. For example, if an input signal format of D/A converter  101  is a complement of 2 and a resolution is 65536 steps, since an input signal range is −32768 to 32768, K DA  is 32767. 
     The conversion gain G DA  of D/A converter  101  is defined as a variation of an output voltage when an input signal of D/A converter  101  is changed by 1 step, and can be obtained by the resolution and the output voltage range of D/A converter  101 . For example, the conversion gain G DA  of D/A converter  101  with the resolution of 65536 steps and the output voltage range of −5 V to 5 V is 0.000152587 V (=(5−(−5))/65536). 
     Tolerable input signal level K PA  expressed by a peak value of power amplifier  102  can be obtained from gain G PA  and an effective maximum output power P PA  [W] of power amplifier  102 , and impedance Z SP  [Ω] of speaker  103 , which is connected to power amplifier  102 , by equation 4 below. In addition, a unit [V pk ] of equation 4 represents that a voltage of the K PA  is a peak value. When the gain of a power amplifier is not written in specifications or a gain is changed, gain G PA  in a use state may be obtained by actual measurement. 
     
       
         
           
             
               
                 
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                   = 
                   
                     
                       
                         
                           2 
                           · 
                           
                             P 
                             PA 
                           
                           · 
                           
                             Z 
                             SP 
                           
                         
                       
                       
                         G 
                         PA 
                       
                     
                     ⁡ 
                     
                       [ 
                       
                         V 
                         pk 
                       
                       ] 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   4 
                 
               
             
           
         
       
     
     Tolerable input signal level K SP  represented by a peak value of speaker  103  can be obtained from an effective tolerable input power P SP  [W] and impedance Z SP  [Q] of speaker  103  by equation 5 below.
 
[5]
 
 K   SP =√{square root over (2 ·P   SP   ·Z   SP )}[ V   pk ]  Equation 5
 
     Gain G SP  of speaker  103  is defined as sound pressure occurring at the position of a distance 1 m when a signal with a peak value 1 [V pk ] is input to speaker  103 . Gain G SP  can obtained from sensitivity S SP  [dB SPL ] and impedance Z SP  [Ω] of speaker  103  by equation 6 below. 
     
       
         
           
             
               
                 
                   [ 
                   6 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     G 
                     SP 
                   
                   = 
                   
                     0.00002 
                     · 
                     
                       10 
                       
                         
                           S 
                           SP 
                         
                         20 
                       
                     
                     · 
                     
                       
                         1 
                         
                           
                             2 
                             · 
                             
                               Z 
                               SP 
                             
                           
                         
                       
                       ⁡ 
                       
                         [ 
                         
                           Pa 
                           / 
                           
                             V 
                             pk 
                           
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   6 
                 
               
             
           
         
       
     
     Since sensitivity S SP  of speaker is represented by the level of sound pressure occurring at the position of the distance 1 m when a signal with an effective power of 1 W is input to speaker  103 , sensitivity S SP  is written in the specifications of speaker  103 . In the specifications, a catalog and so forth, the S SP  may not be written as sensitivity, but an index representing efficiency. In addition, a sound pressure level S [dB SPL ] and sound pressure P [Pa] have relationship of equation 7. 
     
       
         
           
             
               
                 
                   [ 
                   7 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   P 
                   = 
                   
                     0.00002 
                     · 
                     
                       10 
                       
                         S 
                         20 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   7 
                 
               
             
           
         
       
     
     In an open outdoor loudspeaker system with a small echo, an attenuation amount G AC  of sound pressure of the acoustic system between speaker  103  and microphone  104  can be obtained from distance D AC  [m] between speaker  103  and microphone  104 . In the case of a general speaker system, the attenuation amount G AC  can be obtained by equation 8 below under the assumption that sound pressure is attenuated by an inverse square law. 
     
       
         
           
             
               
                 
                   [ 
                   8 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     G 
                     AC 
                   
                   = 
                   
                     1 
                     
                       D 
                       AC 
                       2 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   8 
                 
               
             
           
         
       
     
     In the case of using an array-type speaker system, since sound pressure is generally attenuated in proportional to the distance, the attenuation amount G AC  can be obtained by equation 9. 
     
       
         
           
             
               
                 
                   [ 
                   9 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     G 
                     AC 
                   
                   = 
                   
                     1 
                     
                       D 
                       AC 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   9 
                 
               
             
           
         
       
     
     In the case of using a large-sized flat speaker system having a large scale diaphragm with respect to the distance between speaker  103  and microphone  104 , since distance attenuation is very small, the G AC  may be 1. 
     In the indoor environment with an echo, due to an echo component, actual distance attenuation is small as compared with the attenuation amount G AC  obtained by distance D AC  between speaker  103  and microphone  104  as described above. In this regard, in the indoor environment where the influence of echo cannot be ignored, it is preferable to obtain the attenuation amount G AC  by actual measurement. 
     A sound pressure level in the nearest position to speaker  103  and a sound pressure level at the position of a diaphragm of microphone  104  are measured using a noise level meter, thereby directly measuring the attenuation amount G AC . Otherwise, a gain between an input terminal of power amplifier  102  and an output terminal of microphone amplifier  105  is actually measured, and is divided by G PA ·G SP ·G MIC ·G MA , thereby calculating the attenuation amount G AC . 
     Tolerable input signal level K MIC  represented by a peak value of microphone  104  can be obtained from maximum input sound pressure level A MIC  [dB SPL ], which is written in the specifications of microphone  104 , by equation 10. 
     
       
         
           
             
               
                 
                   [ 
                   10 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     K 
                     MIC 
                   
                   = 
                   
                     0.00002 
                     · 
                     
                       
                         10 
                         
                           
                             A 
                             MIC 
                           
                           20 
                         
                       
                       ⁡ 
                       
                         [ 
                         PA 
                         ] 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   10 
                 
               
             
           
         
       
     
     In addition, since it is considered that a dynamic-type microphone and so forth have the maximum input sound pressure level exceeding about 120 dB SPL , which is the maximum audible field of a person, and are not saturated in a realistic use state, the maximum input sound pressure level may not be written in the specifications. In such a case, tolerable input signal level K MIC  may be infinite. 
     Gain G MIC  of microphone  104  is defined as a value obtained by expressing an output voltage when an input sound pressure is 1 [Pa] as a peak value. Gain G MIC  can be obtained from sensitivity S MIC  [dB], which is written in the specifications of microphone  104 , by equation 11. 
     
       
         
           
             
               
                 
                   [ 
                   11 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     G 
                     MIC 
                   
                   = 
                   
                     
                       2 
                     
                     · 
                     
                       
                         10 
                         
                           
                             SENS 
                             MIC 
                           
                           20 
                         
                       
                       ⁡ 
                       
                         [ 
                         
                           
                             V 
                             pk 
                           
                           / 
                           Pa 
                         
                         ] 
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   11 
                 
               
             
           
         
       
     
     Sensitivity S MIC  [dB] of microphone  104  is obtained by expressing the output voltage when the input sound pressure is 1 Pa as an effective value in the case in which a reference level 0 dB is 1 Vrms. 
     Tolerable input signal level K MA  represented by a peak value of microphone amplifier  105  can be obtained from gain G MA  and the effective maximum output voltage A MA  [Vrms] of microphone amplifier  105  by equation 12. Gain G MA  and the effective maximum output voltage A MA  are written in the specifications of microphone amplifier  105 . In addition, when gain G MA  of microphone amplifier  105  is not written in the specifications or gain G MA  is changed, gain G MA  in a use state may be obtained by actual measurement. 
     
       
         
           
             
               
                 
                   [ 
                   12 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     K 
                     MA 
                   
                   = 
                   
                     
                       
                         
                           2 
                         
                         · 
                         
                           A 
                           MA 
                         
                       
                       
                         G 
                         MA 
                       
                     
                     ⁡ 
                     
                       [ 
                       
                         V 
                         pk 
                       
                       ] 
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   12 
                 
               
             
           
         
       
     
     Tolerable input signal level K AD  of A/D converter  106  is obtained from a convertible input voltage range written in specifications. For example, tolerable input signal level K AD  of A/D converter  106 , in which the convertible input voltage range is −5V to 5V, is 5 V. 
     The conversion gain G AD  of A/D converter  106  is represented by a variation of an output signal when an input signal of A/D converter has been changed by 1 V. The conversion gain G AD  can be obtained from the resolution and the convertible input voltage range of A/D converter  106 . For example, the conversion gain G AD  of A/D converter  106  with the resolution of 65536 steps and the input voltage range of −5 V to 5 V is 6553.6 [1/V](=65536/(5−(−5))). 
     In calculating the above-mentioned parameters, parameters such as sensitivity written in the specifications or the instruction manual of speaker  103  and microphone  104  are used. In general, sensitivity characteristics and so forth of speaker  103  and microphone  104  are defined at the frequency of 1 kHz. However, when frequency characteristics are not flat, sensitivity at a frequency, at which a correction value based on a graph of frequency characteristics written in the specifications or the instruction manual is maximal, is obtained, and the parameters are calculated based on the value. Even when frequency characteristics of power amplifier  102  and microphone amplifier  105  are not flat, the frequency characteristics may be corrected in the same manner, and calculation at a frequency, at which a gain is maximal, may be performed. 
     Next, the operation of the system when the tolerable input signal levels and the gain parameters of the respective units shown in  FIG. 4  are set as shown in  FIG. 6  will be described. In  FIG. 6 , the open loop gain G ALL  of the loudspeaker system is the same under the conditions A to C. 
     In the parameter setting under condition A in  FIG. 6 , a howling canceller having a limiter circuit for preventing the saturation of a D/A converter according to the conventional art is considered. In the conventional art, threshold value k of the limiter circuit (corresponds to amplitude limiting circuit  110  of  FIG. 4 ) for preventing the saturation of the D/A converter is “1.” 
     At this time, the tolerable input signal levels and the maximum input signal levels of the respective units are shown in  FIG. 7 . In  FIG. 7 , a stepped graph denotes the tolerable input signal levels and a broken line graph with black circles denotes the maximum input signal levels. 
     In such a case, in the limiter circuit for preventing the saturation of the D/A converter, since the input signal level of the D/A converter is limited to be equal to or smaller than an absolute value “1,” other units also operate in a linear region without being saturated. 
     Consequently, the howling canceller using an adaptive filter with the same configuration as the system also operates in the linear region, thereby suppressing howling. 
     However, in the case of condition B, when threshold value k of the amplitude limiting circuit for preventing the saturation of the D/A converter has been set to 1 as with the conventional art, the output signal level of a power amplifier exceeds the tolerable input signal level of a speaker as shown in  FIG. 8 , and the speaker is saturated, resulting in the occurrence of non-linear distortion. 
     Therefore, it is difficult to ensure the linear operation of the howling canceller, and hence difficult to ensure the convergence of the adaptive filter, so that it is difficult to suppress howling. 
     Meanwhile, threshold value k obtained by the scheme of the present embodiment is 0.1. In such a case, the tolerable input signal levels and the maximum input signal levels of the respective units are shown in  FIG. 9 , and it can be understood that it is possible to ensure the linear operation of the howling canceller without the saturation of all units. 
     Furthermore, in the case of condition B, when threshold value k of the amplitude limiting circuit for preventing the saturation of the D/A converter has been set to 1 as with the conventional art, the output signal level of a microphone exceeds the tolerable input signal level of a microphone amplifier as shown in  FIG. 10 , and the microphone amplifier is saturated, resulting in the occurrence of non-linear distortion. Therefore, it is difficult to ensure the linear operation of the howling canceller, and hence difficult to assure the convergence of the adaptive filter, so that it is difficult to suppress howling. 
     Meanwhile, threshold value k obtained by the scheme of the present embodiment is 0.1. In such a case, the tolerable input signal levels and the maximum input signal levels of the respective units are shown in  FIG. 11 , and it can be understood that it is possible to ensure the linear operation of the howling canceller without the saturation of all units. 
     In  FIGS. 7 to 11 , the [D/A] denotes a D/A converter, the [PA] denotes a power amplifier, the [SP] denotes a speaker, the [MIC] denotes a microphone, the [MA] denotes a microphone amplifier, the [A/D] denotes an A/D converter. 
     As described above, according to the present embodiment, it is possible to more accurately obtain threshold value k of amplitude limiting circuit  110  achieving a howling suppression effect. 
     (Embodiment 3) 
     In Embodiment 3, a case of automatically setting threshold value k while operating the loudspeaker in the state in which howling has actually occurred will be described. 
       FIG. 12  is a block diagram showing a configuration of the loudspeaker having the howling canceller therein according to the present embodiment.  FIG. 12  shows a configuration in which threshold value setting circuit  200  is further added to the configuration of  FIG. 1 . Amplitude limiting circuit  110  limits the amplitude of input signal x[n] to be equal to or smaller than threshold value k set by threshold value setting circuit  200 . 
     Threshold value setting circuit  200  includes absolute value circuit  201 , Low Pass Filter (LPF)  202 , constant generation circuit  203 , multiplier  204 , magnitude comparator  205 , clock generation circuit  206 , constant generation circuit  207 , multiplier  208 , and register  209 . 
     Absolute value circuit  201  full-wave rectifies input signal x[n]. Low Pass Filter (LPF)  202  smoothes the output of absolute value circuit  201 . 
     Constant generation circuit  203  generates a constant P (0&lt;P&lt;1) for detecting howling. Usually, the value of constant P may be set to about 0.2 to about 0.5. Multiplier  204  multiplies threshold value k by constant P to calculate value k·P. 
     Magnitude comparator  205  outputs “0” if a relationship in magnitude between the amplitude of an input signal (an output signal of multiplier  204 ) of terminal A thereof and the amplitude of an input signal (an output signal of LPF  202 ) of terminal B thereof satisfies A≧B while outputting “1” if the magnitude relation satisfies A&lt;B. Thus, the output of magnitude comparator  205  is “0” in a howling suppression state, but is “1” in the state in which the amplitude of the output signal of LPF  202  has exceeded the product K·P due to the occurrence of howling. 
     Clock generation circuit  206  generates a clock signal with a cycle of about 1 second to about 10 seconds to output the clock signal to register  209 . In addition, “E” and “O” of clock generation circuit  206  denote a control signal input terminal and an output terminal, respectively. Clock generation circuit  206  generates the clock signal when E=1, stops the output of the clock signal when E=0, and completes the update of threshold value k. 
     Constant generation circuit  207  generates a constant Q (0&lt;Q&lt;1) for detecting howling. Usually, the value of the constant Q may be set to about 0.7 to about 0.5 which correspond to a variation of −3 dB to −6 dB. Multiplier  208  multiplies threshold value k by the constant Q to calculate value k·Q. 
     Register  209  holds an initial value of threshold value k. If value k·Q is input from multiplier  208 , register  209  holds the input value K·Q as a new threshold value K. Then, register  209  outputs the held threshold value K in synchronization with the clock signal input to “CK.” In addition, “D,” “Q” and “CK” of register  209  denote an input terminal, an output terminal, and a clock input terminal, respectively. 
     In this way, threshold value k of register  209  is updated in synchronization with the clock signal, so that howling suppression is possible and an optimal threshold value K for allowing the achievement of the maximum output sound pressure level can be automatically set. 
     In addition, the initial value of threshold value k is set to be the same as the tolerable input signal level of D/A converter  101 . For example, if the resolution of D/A converter  101  is 65536 steps and the input signal range is −32768 to 32768, the initial value of threshold value k may be set to 32767. 
     Now, the operation sequence of threshold value setting circuit  200  will be described. 
     If the loudspeaker starts operating, when an open loop gain of a loudspeaker system is equal to or higher than “1,” since indoor background noise or noise occurring in power amplifier  102  and microphone amplifier  105  becomes an excitation signal and howling immediately occurs, the amplitude of an input signal of amplitude limiting circuit  110  exceeds threshold value k. 
     A peak value of input signal x[n] of amplitude limiting circuit  110  is detected by absolute value circuit  201  and LPF  202 , and is input to magnitude comparator  205 . 
     Magnitude comparator  205  outputs a result obtained by comparing the peak value of input signal x[n] of amplitude limiting circuit  110  with value k·P. If howling occurs, since the peak value of input signal x[n] of amplitude limiting circuit  110  is larger than value k·P, “1” is output from magnitude comparator  205 . 
     An output signal of magnitude comparator  205  is input to the control signal input terminal of clock generation circuit  206 , and clock generation circuit  206  continues to output a clock signal while the howling is occurring. 
     Register  209  continues to update a held threshold value K to a value Q·K while the clock signal is being supplied due to the occurrence of howling. 
     As described above, during the continuance of howling having occurred directly after the system starts operating, threshold value k of amplitude limiting circuit  110  is gradually reduced. If the value reaches threshold value k at which all of D/A converter  101 , power amplifier  102 , speaker  103 , microphone  104 , microphone amplifier  105 , and A/D converter  106  operate in a linear region, adaptive filter  107  can converge regardless of the influence of the non-linearity of the loudspeaker system, resulting in the suppression of howling. 
     Furthermore, if adaptive filter  107  converges and the howling is suppressed, since the input signal of amplitude limiting circuit  110  is “0,” the output of magnitude comparator  205  is also “0.” As a consequence, since the control signal of clock generation circuit  206  is also “0,” the output of the clock signal is stopped and threshold value k held in register  209  is not updated. 
     As described above, according to the present embodiment, it is possible to suppress howling and automatically obtain threshold value k for allowing the achievement of the maximum sound pressure level of reinforced sound. Furthermore, threshold value k is constant at a value at the time of howling suppression, and even when person&#39;s speech is input from microphone  104  later, it is possible to continue a loudspeaking operation while maintaining a howling suppression state. Then, although the transmission characteristics of an acoustic system is suddenly changed, the convergence of adaptive filter  107  does not catch up, and howling occurs again, adaptive filter  107  continues a convergence operation without being affected by non-linearity by amplitude limiting circuit  110 , resulting in the suppression of howling. 
     (Embodiment 4) 
     When an input signal is a colored signal, convergence characteristics of adaptive filter  107  generally deteriorate as compared with the case in which an input signal is a white signal. Since a person&#39;s speech signal is a colored signal, adaptive filter  107  does not achieve ideal convergence characteristics in a howling canceller that processes speech. 
     In Embodiment 4, the case for solving the above-mentioned problem will be described. In detail, amplitude limiting circuit  110  uses a circuit that replaces input signal x[n] equal to or higher than threshold value k with a signal having an absolute amplitude value equal to threshold value k and random characteristics. 
       FIG. 13  is a diagram showing a circuit configuration of amplitude limiting circuit  110  according to the present embodiment. Meanwhile,  FIG. 14  is a diagram showing a circuit configuration when amplitude limiting circuit  110  is formed of a limiter circuit having a magnitude comparator and a multiplexer therein. 
     In  FIGS. 13 and 14 , a block having input terminals A and B and an output terminal A&lt;B denotes the magnitude comparator. 
     Furthermore, in  FIG. 13 , a block having input terminals S, I 0  and I 1  and an output terminal Y denotes the multiplexer, wherein the S denotes a control signal and the I 0  and I 1  denote a signal to be selected. The multiplexer outputs a signal input to the terminal I 0  when the S is 0 from terminal Y, and outputs a signal input to the terminal I 1  when the S is 1 from terminal Y. In addition, in  FIG. 13 , a block represented by a mark “OR” denotes an OR circuit, and a block having input terminals A and B and an output terminal Y denotes a multiplier. Moreover, in  FIG. 13 , a block represented by a mark “RAND” denotes a binary pseudo random number generator and generates a pseudo random number with a value “1” or “−1.” 
     Furthermore, in  FIG. 14 , a block having input terminals S 0 , S 1 , and I 0  to I 3  (in addition, input I 3  is non-connection) and an output terminal Y denotes the multiplexer, wherein the S 0  and S 1  denote a control signal and I 0  to I 3  denote a signal to be selected. 
     The circuit of  FIG. 13  outputs binary white noise having an absolute amplitude value equal to threshold value k and random characteristics when an absolute value of the amplitude of input signal x[n] exceeds threshold value k. Consequently, in the circuit of  FIG. 13 , the convergence of adaptive filter  107  is fast and howling is also quickly suppressed, as compared with the circuit of  FIG. 14 . 
     In order to verify the above-mentioned fact, a computer simulation of a howling canceller has been performed with respect to the case of using the simple circuit of  FIG. 14  and the case of using the circuit of  FIG. 13 .  FIG. 15  shows a simulation result and is a diagram plotting input signal x[n] of adaptive filter  107 , which is obtained by the simulation. In  FIG. 15 , the horizontal axis denotes time (unit: sample) and the vertical axis denotes amplitude.  FIG. 15A  shows the case of using the circuit of  FIG. 14  and  FIG. 15B  shows the case of using the circuit of  FIG. 13 . 
     In addition, in the computer simulation, as a speech signal to be input from microphone  104 , a signal which is recorded at a sampling frequency of 8 kHz and has an absolute amplitude value normalized to be equal to or smaller than 1. 
       15 A, the saturation of howling sound at the time of 8000 samples is made, but oscillating sound is continuously generated after that and howling is completely suppressed after the time of 20000 samples. Meanwhile, in  FIG. 15B , howling is completely suppressed by the time of 8000 samples. 
     As described above, according to the present embodiment, when an absolute value of the amplitude of input signal x[n] exceeds threshold value k, since the output signal of the circuit becomes binary white noise, the convergence characteristics of adaptive filter  107  during the occurrence of howling can be improved and howling can be suppressed at a high speed. 
     (Embodiment 5) 
     In Embodiment 5, a case in which the amplitude of howling sound occurring in a start period is decreased to reduce discomfort of auditory sensation in a howling canceller using an adaptive filter will be described. In detail, in the present embodiment, threshold values K different in a start period and a normal operation state (after the start period) are used, and the initial values of the threshold values K are set as a small value as compared with the normal operation state and are increased in a continuous manner or a step-by-step manner. 
       FIG. 16  is a diagram showing a circuit configuration of amplitude limiting circuit  110  according to the present embodiment. In  FIG. 16 , an initial value of a counter is 0 and an initial value of a latch is also 0. In this circuit, if a count value held in the counter is set as n and an output value of a third constant generation circuit is set as C, threshold value k is C·n. 
     The initial value “0” is held in the latch when the system starts operating, and a control signal S of a selector is “0.” When the control signal S is “0,” a clock signal is output from terminal Y of the selector. The counter is reset to the initial value 0 when the system starts operating, and then counts the number of input clock signals. If the value of the counter is equal to a value output from a second constant generation circuit, “1” is output from terminal Y of a comparator and a value “1” is held in the latch. As a consequence, the control signal S of the selector is “1,” and a logic value “0” generated by a first constant generation circuit is permanently output from terminal Y of the selector. Thus, the counting operation of the counter is stopped. 
     Consequently, in the circuit of  FIG. 16 , threshold value k after the system starts operating is increased from “0,” and is constant if a time determined by the second constant generation circuit is reached. 
       FIG. 17  is a diagram showing a simulation result according to the present embodiment when continuously controlling threshold value k, which is a diagram plotting input signal x[n]. In addition, in the present simulation, threshold value k of amplitude limiting circuit  110  is controlled as expressed by equation 13 below. In detail, threshold value k is controlled to be gradually increased from “0” in the range of n≦10000. In the range of 10000&lt;n, threshold value k is controlled to be constant. In addition, the n of equation 13 denotes a variable in which a time is expressed in units of samples. 
     The process of equation 13 corresponds to the case in which an output value C of the third constant generation circuit is set to 0.0002 and an output value of the second constant generation circuit is set to 10000 in  FIG. 16 . 
     
       
         
           
             
               
                 
                   [ 
                   13 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   K 
                   = 
                   
                     { 
                     
                       
                         
                           
                             
                               2 
                               · 
                               
                                 n 
                                 10000 
                               
                             
                             = 
                             
                               0.0002 
                               · 
                               n 
                             
                           
                         
                         
                           
                             ( 
                             
                               n 
                               ≤ 
                               10000 
                             
                             ) 
                           
                         
                       
                       
                         
                           2 
                         
                         
                           
                             ( 
                             
                               10000 
                               &lt; 
                               n 
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   13 
                 
               
             
           
         
       
     
     As compared with the example of  FIG. 15 , in the example of  FIG. 17 , threshold value k is gradually increased from 0, so that the growth of howling sound having occurred at the time of start is gently suppressed to be small. 
     As described above, according to the present embodiment, threshold value k in the start period is set to be small as compared with the normal operation state and is increased in a continuous manner or a step-by-step manner, so that the growth of the amplitude of howling having occurred in the start period can be gently limited, thereby reducing user&#39;s discomfort. 
     ( Embodiment 6) 
     In Embodiment 6, a case in which the amplitude of howling sound occurring in a start period is decreased to reduce discomfort of auditory sensation in a howling canceller using an adaptive filter will be described. In detail, in the present embodiment, an initial value k[0] of threshold value k[n] is set as a very small value, threshold value k[n] is exponentially increased at the time of start until threshold value k[n] is constant K, and a threshold value is set as constant K after threshold value k[n] reaches constant K. In addition, as described in Embodiment 1, constant K is a value for ensuring that all of D/A converter  101 , power amplifier  102 , speaker  103 , microphone  104 , microphone amplifier  105 , and A/D converter  106  operate in the linear region without being saturated. 
       FIG. 18  is a block diagram showing a configuration of a loudspeaker having a howling canceller therein according to the present embodiment.  FIG. 18  shows a configuration in which threshold value control circuit  300  is further added to the configuration of  FIG. 1 . Amplitude limiting circuit  110  limits the amplitude of input signal x[n] to be equal to or smaller than threshold value k[n], which has been set by threshold value control circuit  300 . 
     As expressed by equation 14 below, threshold value control circuit  300  sets an initial value k[0] of threshold value k[n] as a very small value (about 0.001 to about 0.01), exponentially increases threshold value k[n] at the time of start until threshold value k[n] is constant K, and sets a threshold value as constant K after threshold value k[n] reaches constant K. In equation 14, α denotes a constant for controlling the degree of an increase of k[n], wherein 1&lt;α. 
     
       
         
           
             
               
                 
                   [ 
                   14 
                   ] 
                 
               
               
                 
                     
                 
               
             
             
               
                 
                   
                     k 
                     ⁡ 
                     
                       [ 
                       n 
                       ] 
                     
                   
                   = 
                   
                     { 
                     
                       
                         
                           
                             α 
                             · 
                             
                               k 
                               ⁡ 
                               
                                 [ 
                                 
                                   n 
                                   - 
                                   1 
                                 
                                 ] 
                               
                             
                           
                         
                         
                           
                             ( 
                             
                               
                                 α 
                                 · 
                                 
                                   k 
                                   ⁡ 
                                   
                                     [ 
                                     
                                       n 
                                       - 
                                       1 
                                     
                                     ] 
                                   
                                 
                               
                               &lt; 
                               K 
                             
                             ) 
                           
                         
                       
                       
                         
                           K 
                         
                         
                           
                             ( 
                             
                               K 
                               ≤ 
                               
                                 α 
                                 · 
                                 
                                   k 
                                   ⁡ 
                                   
                                     [ 
                                     
                                       n 
                                       - 
                                       1 
                                     
                                     ] 
                                   
                                 
                               
                             
                             ) 
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                   ⁢ 
                   
                       
                   
                   ⁢ 
                   14 
                 
               
             
           
         
       
     
     Threshold value control circuit  300  includes clock generation circuit  301 , counter  302 , constant generation circuit  303 , selector  304 , register  305 , constant generation circuit  306 , multiplier  307 , constant generation circuit  308 , magnitude comparator  309 , and selector  310 . 
     Clock generation circuit  301  generates a clock signal with a sampling frequency at which the entire system operates, and outputs the clock signal to counter  302 . 
     Counter  302  is reset to an initial value 0 when the system starts operating and then counts the number of input clock signals. Then, counter  302  outputs a count value n of the clock signal to selector  304 . 
     Constant generation circuit  303  generates the initial value k[0] of threshold value k[n]. 
     Selector  304  selects a signal (an initial value k[0] of constant generation circuit  303 ) input to a terminal I 0  thereof when a control signal S (=the count value n) is “0,” and outputs the selected signal to register  305 , magnitude comparator  309 , and selector  310  from terminal Y thereof. Meanwhile, selector  304  selects a signal (the output of multiplier  307 ) input to terminal I 1  thereof when the control signal S (=the count value n) is not “0,” and outputs the selected signal to register  305 , magnitude comparator  309 , and selector  310  from terminal Y thereof. 
     Register  305  delays a signal (the output of selector  304 ), which is input to a terminal D thereof, by one sample, and outputs a delayed signal to multiplier  307  from a terminal Q thereof. 
     Constant generation circuit  306  generates constant α. Multiplier  307  multiplies the output of register  305  by constant α, and outputs a multiplication result to selector  304 . 
     Constant generation circuit  308  generates constant K (the maximum value of threshold value k[n]). 
     Magnitude comparator  309  outputs “0” if a relationship in magnitude between an input signal (an output signal of selector  304 ) of terminal A thereof and an input signal (constant K) of terminal B thereof satisfies A≧B while outputting “1” if the magnitude relation satisfies A&lt;B. 
     Selector  310  selects a signal (constant K) input to a terminal I 0  thereof when a control signal S (an output signal of magnitude comparator  309 ) is “0,” and outputs the selected signal to amplitude limiting circuit  110  from terminal Y thereof. Meanwhile, selector  310  selects a signal (the output signal of selector  304 ) input to terminal I 1  thereof when the control signal S is “1,” and outputs the selected signal to amplitude limiting circuit  110  from terminal Y thereof. The output signal of selector  310  is k[n] of equation 14 above. 
     Now, the operation sequence of threshold value control circuit  300  will be described. 
     At the time of start, since the count value n is 0, selector  304  outputs the initial value k[0] of threshold value k[n], which is input to the terminal I 0  thereof, from terminal 
     Y thereof. Since value k[0] is smaller than constant K, the output signal of magnitude comparator  309  is “1” and k[n] output from selector  310  is k[0]. 
     Threshold value k[n] (=α×k[n−1]), which is obtained by an operation of register  305 , constant generation circuit  306 , and multiplier  307 , is input to the terminal I 1  of selector  304 . 
     Then, if a clock signal is output from clock generation circuit  301 , since the count value n is not 0, selector  304  outputs threshold value k[n] (=α×k[n−1]), which is input to the terminal I 1  thereof, from terminal Y thereof. 
     When value k[n] is smaller than constant K, the output signal of magnitude comparator  309  is “1” and k[n] output from selector  310  is α×k[n−1]. That is, when value k[n] is smaller than constant K, it is exponentially increased. 
     Then, if value k[n] is equal to or higher than constant K, the output signal of magnitude comparator  309  is “0” and k[n] output from selector  310  is constant K. 
     As described above, according to the present embodiment, in the start period, the threshold value of amplitude limiting circuit  110  is exponentially increased from a very small value. In this way, the amplitude of howling sound having occurred once in the start period is suppressed to a small value according to threshold value k[n], and the convergence of an adaptive filter is made while the threshold value is very small and howling is suppressed, so that excessive howling sound is prevented from occurring. 
     In addition, in the start period, if threshold value k[n] is controlled to be increased, the above-mentioned effects can be achieved. However, threshold value k[n] is exponentially increased, so that it is possible to achieve an additional effect that a feeling of strangeness of a change in volume is reduced because a person feels that volume is very naturally linearly increased, due to auditory characteristics (Weber-Fechner law) of a person who feels that the magnitude of sound is proportional to a logarithm of sound pressure. 
       FIG. 19  is a diagram showing a change in threshold value k[n] when a value of a parameter is set in detail with respect to the circuit of  FIG. 18 . In  FIG. 19 , the horizontal axis denotes time and the vertical axis denotes threshold value k[n]. In  FIG. 19 , K is set to 2, k[0] is set to 0.01, and a is set to 1.002. As is apparent from  FIG. 19 , threshold value k[n] is exponentially increased at the time of start, and is maintained as a constant value after reaching constant K (=2). 
       FIG. 20  is a diagram showing a waveform of an output signal (playback sound from a speaker) of a howling canceller.  FIG. 20A  shows the case in which threshold value k[n] is fixed to constant K at the time of start, and  FIG. 20B  shows the case in which the threshold value has been controlled as shown in  FIG. 19 . 
     As shown in  FIG. 20A , if threshold value k[n] is fixed to constant K at the time of start, howling with a large amplitude occurs once in the start period. 
     Meanwhile, as shown in  FIG. 20B , if threshold value k[n] is exponentially increased in the start period, since the amplitude of howling occurring in the start period is controlled at the same level as a speech signal after howling suppression, auditory discomfort of a person is significantly reduced. 
     So far, in the above-mentioned embodiments, the howling canceller of the loudspeaker system has been described. However, the present invention can also be applied to an echo canceller (a howling canceller) of a bi-directional communication system shown in  FIG. 21 . If the input and the output of the echo canceller of  FIG. 21  are short-circuited to each other, the echo canceller has the same configuration as that of the howling canceller of the loudspeaker system of  FIG. 1 . 
     The purpose of adaptive filter  107  of  FIG. 21  is to cancel echo first of all. However, if the present invention is employed, it is possible to obtain the function of a howling canceller that suppresses howling having occurred due to insufficient suppression of echo. 
     ( Embodiment 7) 
     Embodiment 7 describes a method for removing processing delay while maintaining the quality of playback sound when the howling canceller of the present invention is applied to a hearing aid. 
     If processing delay (propagation delay) of the hearing aid is large, a user feels discomfort by a time lag between motion of the mouth of a communication partner and sound actually heard. In this regard, in the hearing aid, it is necessary to reduce the processing delay to the greatest extent possible. 
     However, when delay circuit  209  is simply removed from the loudspeaker shown in  FIG. 1 , abnormal noise is introduced to playback sound, resulting in the deterioration of sound quality. First, this problem will be described. The following description will be given on the assumption that adaptive filter  107  is completely converged and howling has been suppressed. 
     If the howling is suppressed, residual signal e[n] includes only a component of a speech signal s[n] input to microphone  104 . 
     Residual signal e[n] is fed back to an input side of the system, and becomes input signal x[n] of adaptive filter  107  via amplitude limiting circuit  110 . 
     Since arithmetic processing of a discrete time system is sequentially performed, input signal x[n] at the time n becomes residual signal e[n−1] at the time n−1 prior to one sample. That is, x[n]=e[n−1]=s[n−1]. 
     Speech signal s[n] input to microphone  104  theoretically becomes additive noise added to an adaptive system with the same configuration as that of the system. 
     Thus, in the case of performing an arithmetic operation of adaptive filter  107  using input signal x[n]=s[n−1] at the time n, a signal component s[n]=x[n+1], which has a time difference corresponding to one sample, is input to the adaptive system via microphone  104  as additive noise. 
     That is, in the analysis based on the assumption that adaptive filter  107  converges, due to the presence of a feedback of a residual signal from the output side to the input side of the adaptive system, a signal component (hereinafter referred to as “correlation component”), which has a time difference corresponding to one sample with respect to a current signal and a large correlation with the current signal, is always introduced to the system as noise. 
     Furthermore, in the analysis based on the assumption that adaptive filter  107  is not converged, a noise component with a large correlation with the current signal circulates a closed signal path lots of times via a feedback path, thereby having an adverse influence on the convergence of adaptive filter  107 . 
     Through the above analysis, it can be understood that it is difficult to achieve good convergence characteristics of the adaptive filter in a howling canceller in which additive noise corresponding to a correlation component exists. 
     Therefore, in the howling cancellers described in the above embodiments, if the delay circuit is simply removed, it is possible to suppress howling in a saturated state, but abnormal noise is introduced to playback sound due to the fluctuation of an adaptive filter coefficient, resulting in the deterioration of sound quality. 
     Present inventor(s) predicts a correlation component and reduces in advance a correlation component predicted from an input signal of an adaptive filter and a desired signal, together with correlation characteristics of sound itself, thereby recognizing that it is possible to improve the convergence characteristics of the adaptive filter and solve the above-mentioned problem. 
       FIG. 22  is a block diagram showing a configuration of a hearing aid having a howling canceller therein according to Embodiment 7 of the present invention.  FIG. 22  shows a configuration in which adaptive filter  107 , subtractor  108 , and delay circuit  109  are removed, but FIR filter  401 , subtractor  402 , predictor  403 , filter circuit  404 , adaptive filter  405 , and subtractor  406  are further added with respect to the configuration of  FIG. 1 . 
     FIR filter  401  operates input signal x[n] with tap coefficient H[n] to generate replica y 0 [n] of a playback sound component (a howling sound component/an echo component) output from speaker  103 . Tap coefficient H[n] of FIR filter  401  is obtained by copying tap coefficient H[n] of adaptive filter  405 . Furthermore, a tap length of FIR filter  401  is the same as adaptive filter  405 . 
     Subtractor  402  subtracts the replica y 0 [n] of the playback sound component output from FIR filter  401  from speech signal d[n] output from A/D converter  106 , thereby generating residual signal e 0 [n]. Residual signal e 0 [n] is obtained by removing a reinforced sound component played back by speaker  103  from signals input to microphone  104 . 
     Predictor  403  predicts a correlation component of input signal x[n] and removes the correlation component from input signal x[n]. Predictor  403  includes delay circuit (z −1 )  411 , adaptive filter  412 , and subtractor  413 . 
     Delay circuit  411  delays input signal x[n] by one sample to obtain input signal x[n−1]. 
     Adaptive filter  412  operates input signal x[n−1] with tap coefficient H′[n] to generate prediction value (correlation component) y 2 [n] next to one sample. Furthermore, adaptive filter  412  updates tap coefficient H′[n] such that residual signal e 2 [n] output from subtractor  413  is an optimal value. In addition, adaptive filter  412  has a FIR configuration and uses existing LMS algorithm, projection algorithm, RLS algorithm and so forth as adaptive algorithm thereof. Even when the tap length of adaptive filter  412  is about 1 tap to about 3 taps, it is possible to sufficiently achieve the effects of the present invention. 
     Subtractor  413  subtracts prediction value y 2 [n] output from adaptive filter  412  from input signal x[n] to generate residual signal e 2 [n]. Residual signal e 2 [n], which is an output signal of predictor  403 , is obtained by subtracting the correlation component from input signal x[n], and becomes an input signal of adaptive filter  405  of the next stage. 
     Filter circuit  404  removes the correlation component from speech signal d[n] output from A/D converter  106 . Filter circuit  404  includes delay circuit (z −1 )  421 , FIR filter  422 , and subtractor  423 . 
     Delay circuit  421  delays speech signal d[n] by one sample to obtain speech signal d[n−1]. 
     FIR filter  422  operates to speech signal d[n−1] with tap coefficient H′[n] to generate prediction value y 3 [n] next to one sample. Tap coefficient H′[n] of FIR filter  422  is obtained by copying tap coefficient H′[n] of adaptive filter  412 . Furthermore, a tap length of FIR filter  422  is the same as adaptive filter  412 . 
     Subtractor  423  subtracts prediction value y 3 [n] output from FIR filter  422  from speech signal d[n] to generate a desired signal d 1 [n] of adaptive filter  405 . 
     Adaptive filter  405  operates to residual signal e 2 [n] with tap coefficient H[n] to generate pseudo echo y 1 [n]. 
     Subtractor  406  subtracts pseudo echo y 1 [n] from desired signal d 1 [n] of adaptive filter  405  to generate echo-suppressed residual signal e 1 [n]. 
     If adaptive filter  405  converges and energy of residual signal e 1 [n] is minimum, since tap coefficient H[n] of adaptive filter  405  becomes an estimation value of impulse response of the acoustic system between speaker  103  and microphone  104 , tap coefficient H[n] is copied to FIR filter  401  to perform a process of removing a howling component. 
     As described above, according to the present embodiment, a correlation component generated by additive noise to the system is predicted and removed from residual signal e 2 [n], which is the input signal of adaptive filter  405 , and desired signal d 1 [n], so that adaptive filter  405  can perform a stable adaptive operation regardless of the influence of a noise component with a large correlation. 
     Consequently, in accordance with the howling canceller according to the present invention, the delay circuit is removed from the feedback path to realize low processing delay, and a filter operation is performed using a signal obtained by removing a predicted correlation component, thereby preventing the deterioration of the convergence characteristics of the adaptive filter due to the removal of the delay circuit and the generation of abnormal noise. 
       FIG. 23  shows a result of a simulation for checking the effectiveness of the howling canceller according to the present embodiment. 
       FIG. 23A  shows the waveform of input sound to a microphone.  FIG. 23B  shows the waveform of playback sound emitted from a speaker after excluding a predictor and a filter circuit from the howling canceller of  FIG. 22  and performing a simulation.  FIG. 23C  shows the waveform of playback sound emitted from the speaker after performing a simulation in the howling canceller of  FIG. 22 . 
     In  FIG. 23B , although howling in a saturated state after having occurred at the time of start of the system is suppressed and 10 seconds or more pass, abnormal noise is generated in the playback sound from the speaker. However, in  FIG. 23C , abnormal noise is not significantly generated. From these simulation results, it can be understood that the howling canceller of  FIG. 22  stably operates. 
     In addition, in the howling canceller of  FIG. 22 , the frequency characteristics of residual signal e 0 [n] having no howling component is whitened with spectrum envelope characteristics input to microphone  104 . 
     Since residual signal e 0 [n] is fed back to an input side of speaker  103  and played back from speaker  103 , spectral characteristics of playback sound from speaker  103  have been whitened. 
     Since a long-term average spectrum of person&#39;s speech has high frequency drop characteristics, whitened playback sound output from speaker  103  has been subject to high frequency emphasis in terms of auditory sensation. 
     The high frequency emphasis can be reduced by adding a filter having high frequency drop characteristics equivalent to the average spectral characteristics of person&#39;s speech. When the filter having the high frequency drop characteristics is realized by a digital filter, the filter is inserted just prior to D/A converter  101 . When the filter having the high frequency drop characteristics is realized by an analog filter, the filter is inserted immediately after D/A converter  101 . 
     The disclosures of Japanese Patent Application No. 2009-068683, filed on Mar. 19, 2009, and Japanese Patent Application No. 2009-209298, filed on Sep. 10, 2009, including the specifications, drawings and abstracts, are incorporated herein by reference in their entirety. 
     INDUSTRIAL APPLICABILITY 
     The present invention is useful for a howling canceller of a loudspeaker, a howling canceller of a hearing aid, an echo canceller of a bi-directional communication system (a radio telephone, a wire telephone, an interphone, a TV conference system and so forth), and so forth. 
     REFERENCE SIGNS LIST 
     
         
           101  Digital-to-analog converter 
           102  Power amplifier 
           103  Speaker 
           104  Microphone 
           105  Microphone amplifier 
           106  Analog-to-digital converter 
           107 ,  405  Adaptive filter 
           108 ,  402 ,  406  Subtractor 
           109  Delay circuit 
           110  Amplitude limiting circuit 
           200  Threshold value setting circuit 
           300  Threshold value control circuit 
           401  FIR filter 
           403  Predictor 
           404  Filter circuit