Abstract:
An echo canceller device includes a band adjuster including a filter for performing band adjustment by applying a gain to a signal in a specific frequency band of an input signal, an adaptive filter for inputting a speech reception signal, updating a filter coefficient, and generating a pseudo echo signal using the updated filter coefficient, a band corrector including a filter for correcting a frequency characteristic of a signal obtained by subtracting the pseudo echo signal from an input signal, to a frequency characteristic of the input signal prior to the band adjustment, and a residual echo suppressor for suppressing an echo component that remains in a signal, by a suppression amount, and the residual echo suppressor sets the suppression amount for a signal in a specific frequency band that has been subjected to the band adjustment.

Description:
TECHNICAL FIELD 
       [0001]    The present invention relates to a technology of cancelling an echo of a speech reception signal that is mixed in a signal input to a microphone. 
       BACKGROUND ART 
       [0002]    An echo canceller device is used in a loudspeaker calling device or the like, for the purpose of cancelling an echo of a speech reception signal that is mixed in a signal input to a microphone (hereinafter, referred to as an input signal), from the input signal. As described in Non-patent Literature 1, in many cases, the echo canceller device used for this purpose each is constituted of an adaptive filtering unit and a residual echo suppression unit.  FIG. 5  illustrates a schematic diagram of the echo canceller device having this constitution. 
         [0003]    An adaptive filtering unit  300  estimates (learns) an echo path h, generates a filter coefficient ĥ and a pseudo echo signal (echo replica), and cancels from an input signal input to a microphone. 
         [0004]    Nevertheless, an echo that can be cancelled by the adaptive filtering unit  300  is approximately 30 dB, and all echo signals cannot be cancelled. Thus, a residual echo suppression unit  400  is often included in order to suppress residual echoes. 
         [0005]    Nevertheless, residual echo suppressing processing simultaneously suppresses a speech transmission signal, for example, a signal of voice spoken to a microphone. Thus, a large suppression amount may have a negative influence on the quality of bidirectional simultaneous telephone call (double-talk) performed by a near-end speaker and a far-end speaker. For the purpose of enhancing the performance of the double-talk, it is effective to increase an amount of echoes cancelled by an adaptive filter and decrease the suppression amount of residual echo suppression as little as possible. 
         [0006]    On the other hand, in handsfree telephone call, the power of an echo signal in a low-frequency band is often strong. This is attributed to the fact that the low-frequency band of a speech signal has strong power, and to the design of speakers and echo characteristics in a vehicle. Residual echo signals, which are residual signals remaining after echoes having the aforementioned feature have been cancelled with the adaptive filter, are likely to remain in the low-frequency band.  FIG. 6  illustrates an example of input signals recorded in a vehicle. All of the input signals illustrated in  FIG. 6  are assumed to be echo signals.  FIG. 7  illustrates average powers at respective frequencies of an echo signal (input signal) illustrated in  FIG. 6 , and a residual signal remaining after the echo signal illustrated in  FIG. 6  has been cancelled with an adaptive filter, i.e., a residual echo signal. 
         [0007]    As illustrated in  FIG. 7 , it can be seen that a speech transmission signal has a peak of power in the vicinity of 500 Hz, and a residual echo signal similarly has large power in the vicinity of 500 Hz. If double-talk occurs in this vehicle, it is necessary to apply strong residual echo suppression on the vicinity of 500 Hz, for reducing the residual echo signal to an inaudible level for users. However, in the case where the residual echo suppression is applied, strong suppression is applied also to the vicinity of 500 Hz of the speech transmission signal, leading to deterioration in quality of double-talk. 
         [0008]    In the speech transmission signal, as for a high-frequency band having small power, if a large scale of echo reduction is not performed with the adaptive filter, but echo reduction is performed by subsequent residual echo suppression, telephone transmission voice is not negatively affected. Nevertheless, as for the low-frequency band of the speech transmission signal, double-talk quality can be enhanced by reducing more echoes with the adaptive filter. 
         [0009]    Thus, in an acoustic echo canceller disclosed in Patent Literature 1, the speech reception signal and the speech transmission signal are divided into a plurality of bands, and influence of an echo in each band is estimated, thereby determining a priority order. In each band, the adaptive filter determines processing based on the priority order. The estimation of influence of the echo is performed in such a manner that a higher priority order is allocated to a band in which echo feeling is considered to be acoustically felt the most. An adaptive filter in a high-priority band performs complicated processing with a large echo suppression effect, and an adaptive filter in a low-priority band performs simple processing. By determining a priority order in accordance with a band as described above, echo reduction processing, in which an acoustic characteristic is considered, is performed, and telephone call of high quality is provided. 
       CITATION LIST 
     Patent Literatures 
       [0010]    Patent Literature 1: JP 10-41859 A 
       Non-Patent Literatures 
       [0011]    Non-patent Literature 1: Emanuel A. P. Habets, Sharon Gannot, Israel Cohen and Piet C. W. Sommen, “Joint Dereverberation and Residual Echo Suppression of Speech Signals in Noisy Environments, ”IEEE Trans. Audio, Speech Lang. Process., vol. 16, no. 8, pp. 1433-1451, November 2008. 
       SUMMARY OF INVENTION 
     Technical Problem 
       [0012]    Nevertheless, in the technology disclosed in the above-described Non-patent Literature 1, there is a problem in that an input signal is divided using a filter bank to perform band restriction, and signals of the respective divided bands are synthesized using a synthesis filter bank and output as transmission voice, and filtering is performed also on transmission voice, so that the voice quality of the transmission voice may cannot be maintained. In addition, there has been a problem in that delay is caused by the filter bank and the synthesis filter bank. Furthermore, there has been another problem in that a calculation amount increases due to use of a plurality of adaptive filters in accordance with the priority order. 
         [0013]    The present invention has been devised for solving the above-described problems, and it is an object of the present invention to suppress a calculation amount without performing band division of an input signal, and to increase a cancellation amount of echo signals in a specific frequency band of an adaptive filter. 
       Solution to Problem 
       [0014]    An echo canceller device according to the present invention includes a band adjustment unit including a filter for performing band adjustment by applying a gain to a signal in a specific frequency band of the input signal, an adaptive filter for inputting the speech reception signal, updating a filter coefficient, and generating a pseudo echo signal using the updated filter coefficient, a band correction unit including a filter for correcting a frequency characteristic of a signal obtained by subtracting the pseudo echo signal generated by the adaptive filter from a signal that has been subjected to band adjustment performed by the band adjustment unit, to a frequency characteristic of the input signal prior to the band adjustment performed by the band adjustment unit, and 
         [0015]    a residual echo suppression unit for suppressing an echo component that remains in a signal of which the frequency characteristic has been corrected by the band correction unit, by a suppression amount set based on filter information of the band adjustment unit, and the residual echo suppression unit sets the suppression amount for a signal in a specific frequency band that has been subjected to the band adjustment performed by the band adjustment unit in such a manner as to be lower than suppression amounts for signals in other frequency bands. 
       Advantageous Effects of Invention 
       [0016]    According to the present invention, a calculation amount can be suppressed without performing band division of an input signal, and a cancellation amount of echo signals in a specific frequency band of an adaptive filter can be increased. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0017]      FIG. 1  is a block diagram illustrating a configuration of an echo canceller device according to a first embodiment. 
           [0018]      FIG. 2  is a diagram illustrating a speech signal waveform of double-talk. 
           [0019]      FIG. 3  is a diagram illustrating an effect of a band adjustment unit of the echo canceller device according to the first embodiment. 
           [0020]      FIG. 4  is a block diagram illustrating a configuration of an echo canceller device according to a second embodiment. 
           [0021]      FIG. 5  is a schematic diagram of a conventional echo canceller device. 
           [0022]      FIG. 6  is a diagram illustrating an example of an echo signal. 
           [0023]      FIG. 7  is a diagram illustrating average powers at each frequency of an echo signal and a residual echo signal. 
       
    
    
     DESCRIPTION OF EMBODIMENTS 
       [0024]    In order to describe the present invention in more detail, a mode for carrying out the present invention will be described below with reference to the attached drawings. 
       First Embodiment 
       [0025]      FIG. 1  is a block diagram illustrating a configuration of an echo canceller device  100  according to a first embodiment. 
         [0026]    The echo canceller device  100  includes an adaptive filter  101 , a band adjustment unit  102 , a subtractor  103 , a band correction unit  104 , and a residual echo suppression unit  105 . A speech reception signal is a signal by which voice of a far-end speaker is transmitted via a transmission path (not illustrated) to a near-end speaker side. A speech transmission signal is voice uttered by a near-end speaker. An input signal is a signal obtained by superimposing an echo component, which is generated from an output terminal (e.g., microphone, etc.) of the speech transmission signal via an echo path on the near-end speaker side, on the voice uttered by the near-end speaker. 
         [0027]    A frequency-domain adaptive filter is used as the adaptive filter  101  for the simplicity of description, and a filter coefficient thereof is represented by a capital Ĥ, and a transfer function representing an echo learned by the adaptive filter is similarly represented by a capital H. 
         [0028]    The frequency-domain adaptive filter will be described using a Fast Least Mean Square (FLMS) algorithm. Nevertheless, the present invention according to the first embodiment has no restriction on an adaptive filter algorithm, and can be applied to all adaptive filters. 
         [0029]    A coefficient update formula of the FLMS algorithm can be realized by a series of formulae represented by the following formulae (1) to (8). In addition, the calculation of the following formulae (1) to (8) is performed for each new block of M sets of input samples. 
         [0000]        X   2M×2M ( k )=diag{FFT[ x ( kM−M ), . . . ,  x ( kM ), . . . ,  x ( kM+M− 1)] T }  (1)
 
         [0000]      {circumflex over ( y )}( k )=The last  M  elements of IFFT[ X   2M×2M ( k ){circumflex over ( H )} 2M ( k )]  (2)
 
         [0000]        e ( k )= y ( k )− ŷ ( k )   (3)
 
         [0000]        E   2M ( k )=FFT[0 M   T    e   T ( k )] T    (4)
 
         [0000]        P   i ( k )=γ P   i ( k− 1)+(1−γ)| X   i ( k )| 2   , i= 0, . . . , 2 M− 1   (5)
 
         [0000]        D   2M×2M ( k )=diag[ P   0 ( k ) −1   , . . . , P   2M−1 ( k ) −1 ]  (6)
 
         [0000]        J   2M ( k )=The first  M  elements of IFFT[ D   2M×2M ( k ) X   2M×2M   H ( k ) E   2M ( k )]  (7)
 
         [0000]        Ĥ   2M ( k+ 1)= H   2M ( k )+FFT[ J   2M   T ( k )0 T ] T    (8)
 
         [0030]    In the above-described formulae (1) to (8), FFT denotes fast Fourier transformation, IFFT denotes inverse fast Fourier transform, k denotes an index of a block, x(n) denotes a speech reception signal, and y(n) denotes an input signal (here, echo signal). ŷ(n) denotes a pseudo echo generated by the adaptive filter  101 . 
         [0031]    In addition, a bold small letter denotes a vector with a length M, and denotes a signal sequence. 
         [0032]    For example, y(n)=[y(n), y(n−1), . . . , y(n−M+1)] T  is defined in Formula (3). 
         [0033]    In addition, a bold capital letter denotes a vector or a matrix, and a size is described at the lower right of the letter, and “diag” denotes a diagonal matrix. 
         [0034]    A H  denotes an Hermitian matrix for a matrix A. In addition, γ denotes a forgetting coefficient. 
         [0035]    X(k) in Formula (1) denotes a diagonal matrix in which results of the FFT are arranged as diagonal components. Formulae (2) and (3) respectively denote a pseudo echo and a residual signal generated by the adaptive filter  101 . Formula (4) denotes frequency conversion of the residual signal, and the purpose thereof is linear convolution. Formula (5) represents calculation of average power of the speech reception signal, and the average power is calculated using moving average. Formula (6) diagonally arranges an inverse number of an average power value obtained by Formula (5). Formula (7) is for calculating a coefficient update amount, and linear correlation processing is performed by frequency conversion on the right side. Formula (8) updates a coefficient. 
         [0036]    The present invention according to the first embodiment is characterized in that the band adjustment unit  102  and the band correction unit  104  are included. The band adjustment unit  102  is a block for adjusting a frequency characteristic of an input signal, so as to enhance the function of echo cancellation in a specific band of the adaptive filter  101 . In the band adjustment unit  102 , a bandpass filter for applying a gain to a low-frequency signal is formed using, for example, an infinite impulse response (IIR) filter, and adjustment for increasing power in the low-frequency band of the input signal is performed. Accordingly, an echo cancellation characteristic of the adaptive filter  101  is varied, thereby improving the learning speed of low-frequency echo signals. 
         [0037]    In the band adjustment unit  102 , increasing the power in the low-frequency band of the input signal, i.e., providing a gain means an increase in the power in the low-frequency band in terms of y(k) in Formula (3). The same applies to E 2M  (k) in Formula (4) and J 2M  (k) in Formula (7), and the power in the low-frequency band similarly increases. The filter coefficients of the adaptive filter  101  in Formula (8) can be obtained in such a manner that the power in the low-frequency band becomes stronger. By using this filter coefficient, a pseudo echo with larger low-frequency power can be generated. As described above, the band adjustment unit  102  can change the characteristics of the adaptive filter  101 . 
         [0038]    Subsequently, a change in performance characteristic of the adaptive filter  101  due to the band adjustment of the band adjustment unit  102  will be described with reference to  FIGS. 2 and 3 . 
         [0039]      FIG. 2  is a diagram illustrating an example of a speech signal waveform at the time when double-talk occurs. The data illustrated in  FIG. 2  indicates a speech signal waveform of an input signal, for example, in a case in which handsfree telephone call is performed in a vehicle, and indicates a case in which voice of a far-end speaker is output from audio speakers and observed by a microphone in the vehicle together with voice of a near-end speaker. The change in performance characteristic of the adaptive filter  101  that depends on whether the band adjustment of the band adjustment unit  102  is performed for an input signal having the speech signal waveform illustrated in  FIG. 2  will be described. In  FIG. 2 , an echo signal and a speech transmission signal mixedly exist. The description will now be given focusing on an echo signal indicated by an area A. 
         [0040]      FIG. 3  illustrates residual echo power obtained after processing is performed by the adaptive filter  101  without the band adjustment performed by the band adjustment unit  102 , and residual echo power obtained after the band adjustment performed by the band adjustment unit  102  and the processing implemented by the adaptive filter  101 . A bandpass filter with a resonance frequency of 750 Hz, a gain of 9.0 dB, and a Q value of 0.7 is used as the band adjustment unit  102 . As seen from  FIG. 3 , by applying the band adjustment performed by the band adjustment unit  102 , the residual echo power at frequencies up to 1000 Hz is reduced by approximately 6 dB at the maximum, and the effect of the band adjustment can be seen. 
         [0041]    The subtractor  103  subtracts a pseudo echo signal generated by the adaptive filter  101 , from a signal that has been subjected to the band adjustment performed by the band adjustment unit  102 . The band correction unit  104  performs filter processing in which a frequency characteristic of a signal, from which the pseudo echo signal has been subtracted by the subtractor  103 , is returned to a frequency characteristic of an input signal prior to the band adjustment implemented by the band adjustment unit  102 . The band adjustment unit  102  changes not only the frequency characteristics of echo signals, but also the frequency characteristics of speech transmission signals and noise signals. Thus, after the filter processing is performed by the adaptive filter  101 , the band correction unit  104  performs the processing of returning the frequency characteristic of the input signal prior to the band adjustment implemented by the band adjustment unit  102 , to the original frequency characteristic. In the example illustrated in  FIG. 3 , the band correction unit  104  uses a bandpass filter with a resonance frequency of 750 Hz, a gain of −9.0 dB, and a Q value of 0.7. 
         [0042]    The residual echo suppression unit  105  performs residual echo suppression on a signal of which the frequency characteristic has been corrected by the band correction unit  104 . The residual echo suppression unit  105  determines a suppression amount based on a resonance frequency, a gain, and a Q value, each of which corresponds to information on the filter generated by the band adjustment unit  102 . A method such as a method of reflecting an inverse characteristic with respect to the characteristic of the filter generated by the band adjustment unit  102 , in a residual echo suppression amount, and a method of simply reducing a suppression amount of a frequency in the vicinity of a resonance frequency in the band adjustment unit  102  can be applied. A suppression amount of transmission voice can be thereby reduced, and voice quality can be maintained. 
         [0043]    As described above, according to the first embodiment, the echo canceller device is configured to include the adaptive filter  101  that updates a filter coefficient using the speech reception signal and the signal from the subtractor  103  as inputs and generates a pseudo echo signal using the updated filter coefficient, the band adjustment unit  102  that adjusts a gain in a direction in which the power of an input signal in the low-frequency band becomes stronger, the band correction unit  104  that returns the frequency characteristic of a signal, which is obtained by subtracting a pseudo echo component from a signal that has been subjected to the band adjustment, to the frequency characteristic of the input signal prior to the band adjustment, and the residual echo suppression unit  105  that performs residual echo suppression based on the characteristic of a filter generated by the band adjustment unit  102 , so that the consumption amount of an echo signal in the low-frequency band in the adaptive filter  101  can be increased without performing band division of an input signal. In addition, the echo canceller device  100  having high double-talk performance can be obtained without deteriorating the echo cancellation performance of the adaptive filter  101 . In addition, as described above, in a case where IIR filters are applied to the band adjustment unit  102  and the band correction unit  104 , the calculation amount of the echo canceller device  100  can be reduced. 
         [0044]    In addition, in the above-described first embodiment, the band adjustment unit  102  that increases the power in the low-frequency band has been described. Nevertheless, the frequency band is not limited to the low-frequency band, and the band adjustment unit  102  may be configured to increase power in a specific band. 
       Second Embodiment 
       [0045]    In handsfree telephone call, a signal band of a speech reception signal varies, depending on a communication network. In Europe, services of wide-bandwidth telephone call (50 to 7000 Hz) have been already started. Nevertheless, in some areas, only narrow-bandwidth telephone call (300 to 3400 Hz) is operated. Thus, voice quality over the telephone needs to be maintained in accordance with each communication situation. Thus, in the second embodiment, the configuration of the echo canceller device is described that realizes adequate echo cancellation performance and high double-talk performance even in a situation in which the voice band of a speech reception signal varies. 
         [0046]      FIG. 4  is a block diagram illustrating a configuration of an echo canceller device  200  according to the second embodiment. 
         [0047]    The echo canceller device  200  according to the second embodiment includes a signal analyzing unit  201  in addition to the components of the echo canceller device  100  described in the first embodiment. In addition, a band adjustment unit  202  and a band correction unit  204  are provided in place of the band adjustment unit  102  and the band correction unit  104 . 
         [0048]    In the following description, the same signs as the signs used in the first embodiment are assigned to the same or corresponding parts as the components of the echo canceller device  100  according to the first embodiment, and therefore the description will be omitted or simplified. 
         [0049]    The signal analyzing unit  201  analyzes a voice band of a speech reception signal and determines whether it is narrow-bandwidth telephone call or wide-bandwidth telephone call. Various methods of analyzing a communication band can be applied. For example, a speech reception signal passes through a high-pass filter that can extract a signal of 4 kHz or higher, and if the power of a signal in the high-frequency band falls below a set threshold value, the telephone call is determined to be narrow-bandwidth telephone call, and if the power exceeds the threshold value, the telephone call is determined to be wide-bandwidth telephone call. 
         [0050]    The band adjustment unit  202  sets a filter to be used for band adjustment, based on the analysis result of the signal analyzing unit  201 . If the analysis result indicates the narrow-bandwidth telephone call, for example, a resonance frequency is set to 500 Hz to 1000 Hz. If the analysis result indicates the wide-bandwidth telephone call, for example, a resonance frequency is set to 200 to 300 Hz. Thus, by setting the resonance frequency in accordance with the voice band at the time of the wide-bandwidth telephone call, the echo cancellation performance of the adaptive filter in response to an echo signal with  50  to  300  Hz that has strong power can be enhanced. 
         [0051]    The band correction unit  204  similarly uses a filter corresponding to the filter used by the band adjustment unit  202 , based on the analysis result of the signal analyzing unit  201 . The residual echo suppression unit  105  performs residual echo suppressing processing. Similarly to the first embodiment, in the residual echo suppressing processing, the suppression of residual echoes is relieved in a band in the vicinity of a resonance frequency of the band adjustment unit  202 , and the suppression of the residual echoes is performed without impairing an input signal. 
         [0052]    As described above, according to the second embodiment, the echo canceller device is configured to include the signal analyzing unit  201  that analyzes a voice band of a speech reception signal, the band adjustment unit  202  that sets a filter to be used for band adjustment, based on an analysis result of the signal analyzing unit  201 , and the band correction unit  204  that sets a filter to be used for band modification, based on the analysis result of the signal analyzing unit  201 , so that high echo cancellation performance and high double-talk performance can be obtained in both narrow-bandwidth telephone call and high-bandwidth telephone call. 
         [0053]    In addition, the above-described second embodiment represents the case in which the signal analyzing unit  201  is provided as the internal element of the echo canceller device  200 . Alternatively, the analysis result of a voice band may be acquired from the outside. 
         [0054]    Aside from the foregoing, in the present invention, the embodiments can be freely combined, an arbitrary constituent element in the embodiments can be corrected, or an arbitrary constituent element in the embodiments can be omitted, without departing from the scope of the invention. 
       INDUSTRIAL APPLICABILITY 
       [0055]    The echo canceller device according to the present invention can increase an echo consumption amount of an echo signal in a specific band. Thus, the echo canceller device is suitable for being applied to a loudspeaker calling device or the like in which handsfree telephone call is performed, and for being used for enhancing double-talk quality. 
       REFERENCE SIGNS LIST 
       [0056]      100 ,  200  echo canceller device;  101  adaptive filter;  102 ,  202  band adjustment unit;  103  subtractor;  104 ,  204  band correction unit;  105  residual echo suppression unit;  201  signal analyzing unit.