Patent Publication Number: US-9847094-B2

Title: Voice processing device, voice processing method, and non-transitory computer readable recording medium having therein program for voice processing

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2014-173078, filed on Aug. 27, 2014, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a voice processing device, voice processing method, and a non-transitory computer readable recording medium having therein a program for voice processing. 
     BACKGROUND 
     When a user uses a telephone set such as a portable telephone to make conversation in a place in a noisy environment such as a bustling street, conversation voices collected by a microphone included in the telephone set may include surrounding noise. In this case, since the conversation voices include the voice of the user and the surrounding noise, it is difficult to listen to the voice of the user included in the conversation voices. Thus, technologies have been studied in which, based on a plurality of voice signals generated by a plurality of microphones, noise components included in these voice signals are suppressed (for example, refer to Japanese Laid-open Patent Publication No. 2010-54731 and Japanese Laid-open Patent Publication No. 2010-81495). 
     For example, a noise suppressing device disclosed in Japanese Laid-open Patent Publication No. 2010-54731 performs a noise suppressing process using use environment information in accordance with the use form on respective sound signals converted by a plurality of sound receiving units. 
     Also, a portable communication terminal disclosed in Japanese Laid-open Patent Publication No. 2010-81495 includes a first microphone and a second microphone, and a DSP included in a control unit operates as a dual microphone noise canceller or a single microphone noise canceller for removing noise components included in an output signal of the first microphone. This operation of the DSP is switched based on the detection result of a proximity sensor which detects an approach to the face of a caller, and is activated as a dual microphone noise canceller when an approach to the face of the caller is detected. 
     SUMMARY 
     According to an aspect of the invention, a voice processing device includes a first sound collecting unit configured to collect a voice propagating through space and generate a first voice signal representing the voice; a second sound collecting unit disposed at a position different from the first sound collecting unit and configured to collect a voice propagating through space and generate a second voice signal representing the voice; a human-body vibration obtaining unit configured to obtain vibration propagating through a human body and generate a human-body vibration signal representing the vibration; a storage unit configured to store distance estimation information representing a relation between a ratio in power or amplitude between the human-body vibration signal and the first voice signal and a distance from the first sound collecting unit to a sound source which produces the voice corresponding to the vibration; a first distance calculating unit configured to calculate, for each frame having a predetermined time length, the ratio in power or amplitude between the human-body vibration signal and the first voice signal and calculate a first distance from the first sound collecting unit to the sound source in accordance with the ratio and the distance estimation information; a time frequency converting unit configured to convert, for each frame, the first voice signal and the second voice signal to a first frequency signal and a second frequency signal, respectively, in a frequency domain; a second distance calculating unit configured to calculate, for each frame and for each of a plurality of frequencies, a second distance from the first sound collecting unit to a sound source which produces a component of the frequency of the first frequency signal, based on the first frequency signal and the second frequency signal; a gain determining unit configured to determine, for each frame and for each of the plurality of frequencies, a gain based on a comparison result between the first distance and the second distance; a signal correcting unit configured to correct the first frequency signal by correcting, for each frame, a component of each of the plurality of frequencies of the first frequency signal in accordance with the gain of the corresponding frequency; and a frequency time converting unit configured to convert, for each frame, the corrected first frequency signal to a corrected voice signal in a time domain. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is a schematic diagram of the structure of a voice processing device according to a first embodiment; 
         FIG. 2  is a diagram depicting an example of arrangement of sound collecting units and a human-body vibration obtaining unit; 
         FIG. 3  is a schematic diagram of the structure of a processing unit; 
         FIG. 4  is a diagram depicting a relation between a ratio of power of a first voice signal to power of a human-body vibration signal and a distance between the user&#39;s mouth and a sound collecting unit; 
         FIG. 5  is a diagram depicting an example of a relation between a power difference between a first frequency signal and a second frequency signal and an estimated value of the distance from the sound collecting unit to the user&#39;s mouth; 
         FIG. 6  is a diagram depicting an example of a relation between a sound source direction and a distance to a sound source and gains; 
         FIG. 7  is an operation flowchart of voice processing; 
         FIG. 8  is a schematic diagram of the structure of a voice processing device according to a second embodiment; and 
         FIG. 9  is a diagram depicting an example of arrangement of a pressure sensor, sound collecting units, and a human-body vibration obtaining unit according to the second embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     As described in the background, in a place in a noisy environment, in addition to the user&#39;s mouth, which is a sound source of the voice of the user to be collected, various sound sources producing noises are present around the microphone of the telephone set. Also, a distance between the user&#39;s mouth and the microphone and a distance between a sound source producing noises and the microphone are often varied. Thus, to suppress noise components, the degree of suppression of a voice signal may be set in accordance with a distance from a sound source to the microphone. 
     However, at the time of conversation, how to hold the telephone set is varied depending on the user. In accordance with the way of holding the telephone set, the distance from the microphone to the user&#39;s mouth, which is a sound source as a sound collection target, is fluctuated. Thus, from the use form or the distance between the portable telephone and the face, the distance from the microphone to the sound source as a sound collection target may not be correctly estimated. When the distance from the microphone to the sound source as a sound collection target is not correctly estimated, it is difficult to appropriately set the degree of suppression of a voice signal in accordance with the distance between the microphone and the sound source. 
     Accordingly, it is desired to provide a voice processing device capable of appropriately setting a gain of a voice signal in accordance with a distance to a sound source as a sound collection target. 
     Hereinafter, a voice processing device is described with reference to the drawings. This voice processing device calculates a distance from a sound collecting unit to a sound source as a sound collection target in accordance with a ratio between power of a voice signal generated by a sound collecting unit which collects a voice propagating through space and power of a human-body vibration signal generated by a human-body vibration obtaining unit which obtains vibration propagating through a human body. In accordance with the calculated distance from the sound collecting unit to the sound source, the voice processing device determines an upper limit of distance at which the voice signal is not suppressed. The voice processing device sets a gain for each frequency in accordance with the distance from the sound collecting unit to the sound source calculated for each frequency so as to suppress a component from a sound source positioned away from the sound collecting unit farther than the upper limit of the distance among components of each of a plurality of frequencies included in the voice signal. The voice processing device then multiplies the component of each frequency included in the voice signal by its corresponding gain to suppress the component included in the voice signal from the sound source other than the sound source as a sound collection target. 
       FIG. 1  is a schematic diagram of the structure of a voice processing device according to a first embodiment. In this example, a voice processing device  1  is mounted as a portable telephone. The voice processing device  1  includes sound collecting units  2 - 1  and  2 - 2 , a human-body vibration obtaining unit  3 , analog/digital converting units  4 - 1  to  4 - 3 , a user interface unit  5 , a storage unit  6 , a processing unit  7 , a communicating unit  8 , an output unit  9 , and a storage medium access device  10 . 
     The sound collecting units  2 - 1  and  2 - 2  each include, for example, a microphone, to collect a voice propagating through space around the sound collecting units  2 - 1  and  2 - 2  and generate an analog voice signal with an intensity in accordance with the sound pressure of the voice. The sound collecting unit  2 - 1  then outputs the generated analog voice signal to the analog/digital converting unit (hereinafter referred to as an A/D converting unit)  4 - 1 . Similarly, the sound collecting unit  2 - 2  outputs the generated analog voice signal to the A/D converting unit  4 - 2 . 
     The human-body vibration obtaining unit  3  is disposed at a position directly or indirectly in contact with a human body such as the body of a user to obtain vibration propagating through the human body and generate an analog human-body vibration signal with an intensity in accordance with the intensity of the vibration. This vibration represents vibration occurring at vocal cords and propagating through the human body when the user produces a speech, for example. Therefore, the user&#39;s mouth is a sound source which produces a voice in accordance with vibration occurring at the vocal cords and obtained at the human-body vibration obtaining unit  3 . The human-body vibration obtaining unit  3  then outputs the human-body vibration signal to the A/D converting unit  4 - 3 . In the present embodiment, the human-body vibration obtaining unit  3  is a bone-conduction microphone. Note that the human-body vibration obtaining unit  3  may be a flesh-conduction microphone or pharynx microphone. 
       FIG. 2  is a diagram depicting an example of arrangement of the sound collecting units and the human-body vibration obtaining unit. Note that in the following, for convenience, a surface on a side of a housing of the voice processing device  1  where the user interface unit  5  is disposed is referred to as a front surface, and a surface opposite thereto is referred to as a back surface. The sound collecting unit  2 - 1  and the sound collecting unit  2 - 2  are disposed, for example, away from each other with a predetermined space (for example, on the order of several cm to 10 cm) so that a voice arrival time is different for each sound collecting unit in accordance with the position of the sound source. In this example, the sound collecting unit  2 - 1  is a main sound collecting unit for collecting the voice of the user. Thus, the sound collecting unit  2 - 1  is disposed along a longitudinal direction of the housing of the voice processing device  1  on a front surface side near an end opposite to an end on a side where the output unit  9  which outputs voice is disposed, so as to be positioned closer to the user&#39;s mouth than the sound collecting unit  2 - 2  at the time of conversation. On the other hand, the sound collecting unit  2 - 2  is disposed along the longitudinal direction of the housing of the voice processing device  1  on a back surface side near the end on the side where the output unit  9  is disposed, in order to generate a voice signal for use in calculating a sound source direction and a distance from the voice processing device  1  to the sound source. Thus, a phase difference between voice signals obtained from the two sound collecting units  2 - 1  and  2 - 2  is also changed depending on the sound source direction. Therefore, by examining this phase difference, the voice processing device  1  calculates a sound source direction and a distance to the sound source. 
     Also, the human-body vibration obtaining unit  3  is desired to be directly or indirectly in contact with the user to obtain vibration propagating through the human body. In general, at the time of conversation in a non-hands-free state, the user brings the output unit  9  closer to the ear so as to easily listen to the voice outputted from the output unit  9 . Thus, in this example, the human-body vibration obtaining unit  3  is mounted on the front surface side of the housing of the voice processing device  1  so as to be adjacent to the output unit  9 . 
     With reference to  FIG. 1  again, the A/D converting units  4 - 1  to  4 - 3  each include, for example, an amplifier and an analog/digital converter. By using the amplifier, the A/D converting unit  4 - 1  amplifies the analog voice signal received from the sound collecting unit  2 - 1 . The A/D converting unit  4 - 1  then samples the amplified analog voice signal with a predetermined sampling period (for example, 8 kHz) by using the analog/digital converter, thereby generating a digitalized voice signal. Similarly, the A/D converting unit  4 - 2  amplifies the analog voice signal received from the sound collecting unit  2 - 2  and samples the amplified analog voice signal with a predetermined sampling period, thereby generating a digitalized voice signal. 
     The A/D converting unit  4 - 3  amplifies the analog human-body vibration signal received from the human-body vibration obtaining unit  3  by using the amplifier. The A/D converting unit  4 - 3  then samples the amplified human-body vibration signal with a predetermined sampling period by using the analog/digital converter, thereby generating a digitalized human-body vibration signal. Note that, for simplification of processing at the processing unit  7 , the sampling period of each A/D converting unit is preferably identical. 
     Note that, for convenience, the voice signal obtained by digitalizing the analog voice signal generated by the sound collecting unit  2 - 1  is hereinafter referred to as a first voice signal and the voice signal obtained by digitalizing the analog voice signal generated by the sound collecting unit  2 - 2  is hereinafter referred to as a second voice signal. Also, the digitalized human-body vibration signal is hereinafter simply referred to a human-body vibration signal. The A/D converting units  4 - 1  and  4 - 2  output the first and second voice signals, respectively, to the processing unit  7 . Also, the A/D converting unit  4 - 3  outputs the human-body vibration signal to the processing unit  7 . 
     The user interface unit  5  includes, for example, a touch panel. The user interface unit  5  generates an operation signal in response to an operation by the user, for example, a signal for making an instruction for starting conversation, and outputs the operation signal to the processing unit  7 . The user interface unit  5  also causes an icon, image, text, or the like to be displayed in accordance with a signal for display received from the processing unit  7 . The user interface unit  5  may separately include a plurality of operation buttons for inputting operation signals and a display device such as a liquid-crystal display. 
     The storage unit  6  includes, for example, a readable/writable semiconductor memory and a read-only semiconductor memory. The storage unit  6  stores various computer programs and various data for use in the voice processing device  1 . 
     The storage unit  6  also stores various information for use in voice processing. For example, the storage unit  6  stores distance estimation information representing a relation between a ratio of power between the first voice signal and the human-body vibration signal and a distance from the voice processing device  1  to the sound source of a target sound as a sound collection target. Note that details of the distance estimation information will be described further below. 
     The storage medium access device  10  is, for example, a device which accesses a storage medium  10   a  such as a semiconductor memory card. For example, the storage medium access device  10  reads a computer program stored in the storage medium  10   a  and to be executed on the processing unit  7 , and passes the read computer program to the processing unit  7 . 
     The processing unit  7  includes one or a plurality of processors, a memory circuit, and a peripheral circuit. The processing unit  7  controls the entire voice processing device  1 . 
     With an operation of the user via the user interface unit  5  included in the voice processing device  1 , for example, when a conversation is started, the processing unit  7  performs a calling control process, such as calling, responding, or disconnection. 
     The processing unit  7  also corrects the first voice signal so as to suppress noise other than the target sound as a sound collection target included in the first and second voice signals, thereby making the target sound easily listened to. The processing unit  7  then performs high-efficiency coding and further performs channel coding on the corrected voice signal, and then outputs the coded voice signal via the communicating unit  8 . The processing unit  7  also decodes a coded voice signal received from another device via the communicating unit  8 , and outputs the decoded voice signal to the output unit  9 . 
     In the present embodiment, the target sound is the voice of the user using the voice processing device  1  for conversation, and the sound source of the target sound as a sound collection target is the user&#39;s mouth. 
     Note that details of voice processing by the processing unit  7  will be described further below. 
     The communicating unit  8  transmits the corrected and coded first voice signal to another device. For this process, the communicating unit  8  includes, for example, a baseband processing unit (not depicted), a wireless processing unit (not depicted), and an antenna (not depicted). The baseband processing unit of the communicating unit  8  generates an uplink signal by modulating the voice signal coded by the processing unit  7  by following a modulation scheme conforming to wireless communication standards the communicating unit  8  conforms to. The wireless processing unit of the communicating unit  8  superposes the uplink signal on a carrier wave with radio frequency. The uplink signal is then transmitted to another device via the antenna. The wireless processing unit of the communicating unit  8  also receives a downlink signal including a voice signal from another device via the antenna, converts the downlink signal to a signal with a baseband frequency, and outputs the resultant signal to the baseband processing unit. The baseband processing unit demodulates the signal received from the wireless processing unit, extracts various signals such as the voice signal included therein, or information, and passes the extracted signal or information to the processing unit  7 . 
     The output unit  9  includes, for example, a digital/analog converter for converting the voice signal received from the processing unit  7  to analog and a loudspeaker, and reproduces the voice signal received from the processing unit  7  as a sound wave. 
     The details of voice processing by the processing unit  7  are described below. 
       FIG. 3  is a schematic diagram of the structure of the processing unit  7 . The processing unit  7  includes a target sound source distance calculating unit  11 , a time frequency converting unit  12 , a sound source direction estimating unit  13 , an individual sound source distance calculating unit  14 , a gain determining unit  15 , a signal correcting unit  16 , and a frequency time converting unit  17 . 
     Each unit included in the processing unit  7  is mounted, for example, as a functional module achieved by a computer program executed on a processor included in the processing unit  7 . Alternatively, each unit included in the processing unit  7  may be mounted on the voice processing device  1  as one integrated circuit achieving the function of each unit separately from the processor included in the processing unit  7 . 
     The target sound source distance calculating unit  11  is an example of a first distance calculating unit, and calculates a distance between the user&#39;s mouth, which is a sound source of the target sound as a sound collection target, and the sound collecting unit  2 - 1  based on a ratio in power or amplitude between the first voice signal and the human-body vibration signal. 
     Since the sound collecting unit  2 - 1  generates a voice signal in accordance with the voice propagating through space, as the distance between the user&#39;s mouth and the sound collecting unit  2 - 1  is increased, the power and amplitude of the first voice signal are decreased. On the other hand, since the human-body vibration obtaining unit  3  generates a human-body vibration signal in accordance with vibration propagating from the vocal cords via the human body, the power and amplitude of the human-body vibration signal do not depend on the distance between the user&#39;s mouth and the sound collecting unit  2 - 1 . Therefore, in accordance with the distance between the user&#39;s mouth and the sound collecting unit  2 - 1 , the ratios in power and amplitude between the first voice signal and the human-body vibration signal are changed. 
       FIG. 4  is a diagram depicting a relation between the ratio of the power of the first voice signal to the power of the human-body vibration signal and the distance between the user&#39;s mouth and the sound collecting unit  2 - 1 . In  FIG. 4 , the horizontal axis represents the ratio of the power of the first voice signal to the power of the human-body vibration signal. On the other hand, the vertical axis represents the distance between the user&#39;s mouth and the sound collecting unit  2 - 1 . A graph  400  represents the relation between the ratio of the power of the first voice signal to the power of the human-body vibration signal and the distance between the user&#39;s mouth and the sound collecting unit  2 - 1 . As depicted in the graph  400 , as the ratio of the power of the first voice signal to the power of the human-body vibration signal is increased, the distance between the user&#39;s mouth and the sound collecting unit  2 - 1  is shortened. Similarly, as the ratio of the amplitude of the first voice signal to the amplitude of the human-body vibration signal is increased, the distance between the user&#39;s mouth and the sound collecting unit  2 - 1  is shortened. 
     Thus, for example, for each distance between the user&#39;s mouth and the sound collecting unit  2 - 1 , the ratio of the power of the first voice signal to the power of the human-body vibration signal is measured in advance. Then, in accordance with the measurement results, distance estimation information is generated which represents the relation between the ratio of the power of the first voice signal to the power of the human-body vibration signal and the distance between the user&#39;s mouth and the sound collecting unit  2 - 1 . For example, by applying a least squares method to the measurement results, a distance estimation function is found as distance estimation information, the distance estimation function representing the relation between the ratio of the power of the first voice signal to the power of the human-body vibration signal and the distance between the user&#39;s mouth and the sound collecting unit  2 - 1 . Alternatively, based on the measurement results, a reference table may be generated as distance estimation information, the reference table having stored therein the ratio of the power of the first voice signal to the power of the human-body vibration signal for each distance between the user&#39;s mouth and the sound collecting unit  2 - 1 . Note that a distance estimation function or reference table may be generated as distance estimation information, representing the relation between the ratio of the amplitude of the first voice signal to the amplitude of the human-body vibration signal and the distance between the user&#39;s mouth and the sound collecting unit  2 - 1 . Still alternatively, a distance estimation function or reference table may be generated as distance estimation information, representing the relation between the ratio of the power or amplitude of the human-body vibration signal to the first voice signal and the distance between the user&#39;s mouth and the sound collecting unit  2 - 1 . In this case, conversely, as the ratio of the power or amplitude of the human-body vibration signal to the first voice signal is increased, the distance between the user&#39;s mouth and the sound collecting unit  2 - 1  is increased. The distance estimation information is then stored in advance in the storage unit  6 . 
     The target sound source distance calculating unit  11  divides each of the first voice signal and the human-body vibration signal in units of frames each having a predetermined time length (for example, several tens of msec) at the time of performing voice processing. The target sound source distance calculating unit  11  then calculates, for each frame, a power P 1 ( t ) of the first voice signal (=10 log 10 (Σx(t) 2 ), where x(t) is the first voice signal at each sampling point included in a t-th frame). Furthermore, the target sound source distance calculating unit  11  calculates, for each frame, a power B(t) of the human-body vibration signal (=10 log 10 (Σy(t) 2 ), where y(t) is the human-body vibration signal at each sampling point included in a t-th frame). When the power B(t) of the human-body vibration signal is equal to or larger than a lower-limit value (for example, 3 dB) of power allowing distance estimation, the target sound source distance calculating unit  11  calculates a ratio (P 1 ( t )/B(t)) of the power P 1 ( t ) of the first voice signal to the power B(t) of the human-body vibration signal. With reference to the distance estimation information stored in the storage unit  6 , the target sound source distance calculating unit  11  specifies the distance between the user&#39;s mouth and the sound collecting unit  2 - 1  corresponding to the power ratio (P 1 ( t )/B(t)). The target sound source distance calculating unit  11  then sets the specified distance as an estimated value LP of the distance between the user&#39;s mouth and the sound collecting unit  2 - 1 . On the other hand, when the power B(t) of the human-body vibration signal is smaller than the lower-limit value of the power allowing distance estimation, the target sound source distance calculating unit  11  does not calculate the distance between the user&#39;s mouth and the sound collecting unit  2 - 1 . 
     Note that when the distance estimation information represents the relation between the ratio of the amplitude of the first voice signal to the amplitude of the human-body vibration signal and the distance between the user&#39;s mouth and the sound collecting unit  2 - 1 , the target sound source distance calculating unit  11  finds, for each frame, a sum of absolute values of the amplitudes of the first voice signal and the human-body vibration signal. Then, with reference to the distance estimation information, the target sound source distance calculating unit  11  finds the distance between the user&#39;s mouth and the sound collecting unit  2 - 1  corresponding to a ratio of the sum of the absolute values of the amplitudes of the first voice signal to the sum of the absolute values of the amplitudes of the human-body vibration signal. 
     The target sound source distance calculating unit  11  outputs, for each frame, the distance between the user&#39;s mouth and the sound collecting unit  2 - 1  to the gain determining unit  15 . 
     The time frequency converting unit  12  divides the first voice signal into units of frames each having a predetermined time length (for example, several tens of msec) and performs time frequency conversion for each frame, thereby calculating a first frequency signal, which is a frequency-domain signal. Similarly, the time frequency converting unit  12  divides the second voice signal into units of frames and performs time frequency conversion for each frame, thereby calculating a second frequency signal, which is a frequency-domain signal. As time frequency conversion, for example, the time frequency converting unit  12  may use fast Fourier transform (FFT) or modified discrete cosine transform (MDCT). The first and second frequency signals each include, for example, components of frequencies half the total number of sampling points included in a frame. 
     For each frame, the time frequency converting unit  12  outputs the first frequency signal to the sound source direction estimating unit  13 , the individual sound source distance calculating unit  14 , and the signal correcting unit  16 , and outputs the second frequency signal to the sound source direction estimating unit  13  and the individual sound source distance calculating unit  14 . 
     For each frame, the sound source direction estimating unit  13  estimates, for each of a plurality of frequencies, a direction of the sound source producing components of that frequency included in the first and second frequency signals. In the present embodiment, the sound source direction estimating unit  13  estimates a direction of the sound source based on a phase difference between the first frequency signal and the second frequency signal for each frequency. For example, the sound source direction estimating unit  13  finds a phase difference Δφ(f) for each frequency in accordance with the following equation (1). 
     
       
         
           
             
               
                 
                   
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     Here, S 1 (f) represents a component at a frequency f in the first frequency signal, and S 2 (f) represents a component at the frequency f in the second frequency signal. Also, fs represents a sampling frequency of the A/D converting units  4 - 1  and  4 - 2 . In this case, a difference ΔT(f) in voice arrival time between the sound collecting unit  2 - 1  and the sound collecting unit  2 - 2  for the component of the frequency f is represented by the following equation (2). 
     
       
         
           
             
               
                 
                   
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     Therefore, an estimated direction θ(f) of the sound source for the component of the frequency f at the midpoint of a line connecting the sound collecting unit  2 - 1  and the sound collecting unit  2 - 2  when the direction of the normal of that line is taken as 0 is represented by the following equation (3).
 
θ( f )=sin −1 (Δ T ( f )/Δ T   max )
 
Δ T   max   =d/V   s    (3)
 
     Here, d is a distance between the sound collecting unit  2 - 1  and the sound collecting unit  2 - 2 , and V s  is a velocity of sound. 
     For each frame, the sound source direction estimating unit  13  outputs the estimated direction θ(f) of the sound source for each frequency to the individual sound source distance calculating unit  14  and the gain determining unit  15 . 
     The individual sound source distance calculating unit  14  is an example of a second distance calculating unit, and calculates, for each frame and for each plurality of frequencies, a distance between the sound source and the sound collecting unit  2 - 1  for the component of that frequency included in the first frequency signal. 
     In the present embodiment, the individual sound source distance calculating unit  14  calculates a distance in accordance with the following equations (4-1) and (4-2) based on the estimated direction θ(f) of the sound source for each frequency and a difference between the power of the first frequency signal and the power of the second frequency signal. 
     
       
         
           
             
               
                 
                   
                       
                   
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                               ⁢ 
                               
                                   
                               
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                             + 
                             
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                               ⁢ 
                               
                                   
                               
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                                   ⁡ 
                                   
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                                         ⁢ 
                                         
                                             
                                         
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                               ⁡ 
                               
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                               Th 
                               ⁢ 
                               
                                   
                               
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                   ( 
                   
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                     ⁢ 
                     
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     Here, R(f) is an estimated distance from the sound collecting unit  2 - 1  to the sound source for the component of the frequency f included in the first and second frequency signals. Pdiff(f) is a difference (P 1 ( f )-P 2 ( f )) between the power P 1 ( f ) of the component of the frequency f of the first frequency signal and the power P 2 ( f ) of the component of the frequency f of the second frequency signal. Thθ is a minimum angle with respect to the normal to the line connecting the two sound collecting units  2 - 1  and  2 - 2 , the minimum angle at which the difference between the distance from the sound source to the sound collecting unit  2 - 1  and the distance from the sound source to the sound collecting unit  2 - 2  is too small to calculate a distance to the sound source based on the power difference between the first frequency signal and the second frequency signal. For example, Thθ is set at 0.3 radians. Also, L is a maximum value of the estimated distance from the sound collecting unit  2 - 1  to the user&#39;s mouth, for example, 20 cm. Furthermore, ΔL is a difference obtained by subtracting a distance at which the gain is minimum from an upper-limit value of the distance at which the gain is 1, and is set at, for example, 5 cm. Still further, Th 2  is an upper-limit value of the power difference at which the distance from the sound collecting unit  2 - 1  to the sound source is estimated to be away enough to be able to ignore a difference between the distance from the sound collecting unit  2 - 1  to the sound source and the distance from the sound collecting unit  2 - 2  to the sound source, and is set at, for example, 3 dB. On the other hand, Th 3  is a lower-limit value of a power difference at which the distance from the sound collecting unit  2 - 1  to the sound source is estimated to be shorter than the distance from the sound collecting unit  2 - 2  to the sound source, and is set at, for example, 6 dB. 
       FIG. 5  is a diagram depicting an example of a relation between the power difference between the first frequency signal and the second frequency signal and the estimated value R(f) of the distance from the sound collecting unit  2 - 1  to the user&#39;s mouth. In  FIG. 5 , the horizontal axis represents the power difference Pdiff(f) between the first frequency signal and the second frequency signal, and the vertical axis represents the estimated value R(f) of the distance. A graph  500  represents the relation between the power difference Pdiff(f) between the first frequency signal and the second frequency signal and the estimated value R(f) of the distance. 
     As represented by equations (4-1) and (4-2) and  FIG. 5 , when the power difference Pdiff(f) between the first frequency signal and the second frequency signal is smaller than Th 2 , the estimated value R(f) of the distance between the sound collecting unit  2 - 1  and the user&#39;s mouth is set at (L+ΔL). On the other hand, when the power difference Pdiff(f) is equal to or larger than Th 3 , the estimated value R(f) of the distance is set at L. When the power difference Pdiff(f) is equal to or larger than Th 2  and smaller than Th 3 , the estimated value R(f) of the distance is decreased as the power difference Pdiff(f) is increased. 
     The individual sound source distance calculating unit  14  outputs, for each frame, the estimated value R(f) of the distance between the user&#39;s mouth and the sound collecting unit  2 - 1  for each frequency to the gain determining unit  15 . 
     The gain determining unit  15  determines, for each frame and for each of a plurality of frequencies, a gain for the component of that frequency of the first frequency signal based on the estimated value θ(f) of the sound source direction and the estimated value R(f) of the distance. In the present embodiment, a gain G(f) for the frequency f is any value in a range of 0 to 1. As the gain G(f) is decreased, the component of the corresponding frequency is suppressed. On the other hand, when the gain G(f) is 1, the component of the corresponding frequency is not suppressed. 
     Also, in the present embodiment, when the estimated value of the distance from the sound collecting unit  2 - 1  to the user&#39;s mouth has been found, the gain determining unit  15  sets the estimated value LP of the distance at an upper-limit value LE of the distance from the sound collecting unit  2 - 1  to the sound source, the distance at which the component of each frequency included in the first frequency signal is not suppressed. That is, when the estimated value R(f) of the distance between the sound collecting unit  2 - 1  and the sound source at the frequency f is larger than the estimated value LP of the distance between the sound collecting unit  2 - 1  and the user&#39;s mouth, the gain determining unit  15  determines the gain G(f) so that the gain G(f) is smaller than that when the estimated distance R(f) is equal to or smaller than the estimated value LP of the distance. With this, the gain determining unit  15  determines the gain G(f) so that the component corresponding to noise from another sound source farther away from the sound collecting unit  2 - 1  than the user&#39;s mouth is suppressed, while suppression of the component corresponding to the sound produced from the user&#39;s mouth, which is a sound source as a sound collection target, is reduced. 
     Note that an error may be present in the estimated value LP of the distance from the sound collecting unit  2 - 1  to the user&#39;s mouth, and an actual distance from the sound collecting unit  2 - 1  to the user&#39;s mouth may be longer than the estimated value LP of the distance. In this case, to reduce suppression of components corresponding to the sound produced from the user&#39;s mouth, the upper-limit value LE of the distance from the sound collecting unit  2 - 1  to the sound source, the distance at which the component of each frequency included in the first frequency signal is not suppressed, may be set at a value obtained by adding a predetermined margin (for example, 1 cm to several cm) to the estimated value LP of the distance. 
     The gain determining unit  15  determines the gain G(f) with respect to the component of the frequency f in accordance with the following equation (5), for example. 
     
       
         
           
             
               
                 
                   
                       
                   
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                         G 
                         ⁡ 
                         
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                               ⁢ 
                               
                                   
                               
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                                       + 
                                       
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                   ) 
                 
               
             
           
         
       
     
     Here, Gθ is a gain defined based on the sound source direction, and GL is a gain defined based on the distance between the sound collecting unit  2 - 1  and the and the sound source. PL and PH are a lower-limit value and an upper-limit value, respectively, of the sound source direction in which the user&#39;s mouth is assumed to be present, and are set in accordance with the arrangement of the sound collecting unit  2 - 1  and the sound collecting unit  2 - 2 . For example, when the direction of the normal to the line connecting the sound collecting unit  2 - 1  and the sound collecting unit  2 - 2  is taken as 0 and a direction from the direction of the normal toward a sound collecting unit  2 - 1  side is taken as positive, PL=0 and PH=π. Also, Δθ is an offset, and is set at, for example, 0.1 radians to 0.3 radians. Furthermore, LE is the upper-limit value of the distance at which the frequency component is not suppressed and, as described above, is set at the distance from the sound collecting unit  2 - 1  to the user&#39;s mouth calculated by the target sound source distance calculating unit  11 . As described regarding equations (4-1) and (4-2), ΔL is the difference obtained by subtracting the distance at which the gain is minimum from the upper-limit value of the distance at which the gain is 1, and is set at, for example, 5 cm. A function min(x,y) is a function for outputting a minimum value of variables x and y. Note that, according to a modification example, the gain G(f) may be calculated as the product of the gain Gθ and the gain GL. 
       FIG. 6  is a diagram depicting an example of a relation between the sound source direction θ(f) and the distance R(f) to the sound source and gains. On a left side of  FIG. 6 , a relation between the sound source direction θ(f) and the gain Gθ is depicted. In this relation diagram, the horizontal axis represents the sound source direction θ(f) and the vertical axis represents the gain Gθ. A graph  601  depicts the relation between the sound source direction θ(f) and the gain Gθ. On the other hand, on a right side of  FIG. 6 , a relation between the distance R(f) from the sound collecting unit  2 - 1  to the sound source and the gain GL is depicted. In this relation diagram, the horizontal axis represents the distance R(f) and the vertical axis represents the gain GL. A graph  602  depicts the relation between the distance R(f) and the gain GL. 
     As represented by  FIG. 6  and equation (5), the gain Gθ is decreased as the estimated sound source direction θ(f) for the component of the frequency f in the first frequency signal falls more out of a range of the sound source direction in which the user&#39;s mouth is estimated to be present. Also, the gain GL is decreased as the distance R(f) from the sound collecting unit  2 - 1  to the sound source calculated for the component of the frequency f in the first frequency signal is farther away from the estimated value LP of the distance from the sound collecting unit  2 - 1  to the user&#39;s mouth. And, the gain G(f) takes a value which is a smaller one of Gθ and GL. 
     The gain determining unit  15  passes, for each frame, the gain G(f) of each frequency to the signal correcting unit  16 . 
     The signal correcting unit  16  corrects, for each frame, the component of each frequency of the first frequency signal in accordance with the gain G(f) and in accordance with, for example, the following equation (6).
 
 RE ( CS   1 ( f ))= G ( f )· RE ( S   1 ( f ))
 
 IM ( CS   1 ( f ))= G ( f )· IM ( S   1 ( f ))   (6)
 
     Here, S 1 (f) represents a component of the frequency f in the first frequency signal, and CS 1 (f) represents a component of the frequency f in the corrected first frequency signal. Also, RE(X) represents a real part of a variable X, and IM(X) represents an imaginary part of the variable X. As evident from equation (6), as the gain G(f) is decreased, CS 1 (f) is decreased. Therefore, the first frequency signal is attenuated more as the estimated sound source direction θ(f) falls more out of the range of the sound source direction in which the user&#39;s mouth is estimated to be present. Furthermore, the first frequency signal is attenuated more as the distance R(f) from the sound collecting unit  2 - 1  to the sound source is farther away from the estimated value LP of the distance from the sound collecting unit  2 - 1  to the user&#39;s mouth. 
     The signal correcting unit  16  passes, for each frame, the corrected first frequency signal to the frequency time converting unit  17 . 
     The frequency time converting unit  17  converts, for each frame, the corrected first frequency signal to a time-domain signal by using inverse conversion of the time frequency conversion used by the time frequency converting unit  12 , thereby obtaining the corrected voice signal. 
     With this, noise other than the voice produced from the user&#39;s mouth is attenuated, and therefore the corrected first voice signal is a voice signal in which the user&#39;s voice is made easily listened to. 
       FIG. 7  is an operation flowchart of voice processing to be performed by the processing unit  7 . The processing unit  7  performs the following process for each frame. 
     The target sound source distance calculating unit  11  calculates the power B(t) of the human-body vibration signal obtained from the human-body vibration obtaining unit  3  (step S 101 ). The target sound source distance calculating unit  11  then determines whether the power B(t) of the human-body vibration signal is larger than a predetermined threshold ThB (step S 102 ). When the power B(t) of the human-body vibration signal is equal to or smaller than the predetermined threshold ThB (No in step S 102 ), the target sound source distance calculating unit  11  sets the estimated value LP of the distance between the sound collecting unit  2 - 1  and the user&#39;s mouth at a value representing that the distance is unknown (for example, a negative value) (step S 103 ). 
     On the other hand, when the power B(t) of the human-body vibration signal is larger than the predetermined threshold ThB (Yes in step S 102 ), the target sound source distance calculating unit  11  is able to calculate the distance from the sound collecting unit  2 - 1  to the user&#39;s mouth in accordance with the power B(t) and the power P 1 ( t ) of the first voice signal obtained from the sound collecting unit  2 - 1 . Thus, the target sound source distance calculating unit  11  calculates the estimated value LP of the distance from the sound collecting unit  2 - 1  to the user&#39;s mouth based on the ratio of the power P 1 ( t ) to the power B(t) (P 1 ( t )/B(t)) and distance estimation information (step S 104 ). After step S 103  or S 104 , the target sound source distance calculating unit  11  outputs the estimated value LP of the distance to the gain determining unit  15 . 
     The time frequency converting unit  12  performs time-frequency conversion on the first and second voice signals to calculate first and second frequency signals in a frequency domain (step S 105 ). The time frequency converting unit  12  then outputs the first frequency signal to the sound source direction estimating unit  13 , the individual sound source distance calculating unit  14 , and the signal correcting unit  16 , and outputs the second frequency signal to the sound source direction estimating unit  13  and the individual sound source distance calculating unit  14 . 
     The sound source direction estimating unit  13  estimates the sound source direction θ(f) for each of the plurality of frequencies, based on the first and second frequency signals (step S 106 ). The sound source direction estimating unit  13  then outputs the estimated sound source direction θ(f) of each frequency to the individual sound source distance calculating unit  14  and the gain determining unit  15 . 
     The individual sound source distance calculating unit  14  finds the estimated value R(f) of the distance from the sound collecting unit  2 - 1  to the sound source for each of the plurality of frequencies, based on the power difference of frequency components between the first frequency signal and the second frequency signal and the estimated sound source direction θ(f) (step S 107 ). The individual sound source distance calculating unit  14  then outputs the estimated distance R(f) of each frequency to the gain determining unit  15 . 
     The gain determining unit  15  determines whether the estimated value LP of the distance has been calculated (step S 108 ). When the estimated value LP of the distance has been calculated (Yes in step S 108 ), the gain determining unit  15  sets the upper-limit value LE of a distance in which the frequency component included in the first frequency signal is not attenuated at the estimated value LP of the distance (step S 109 ). On the other hand, when the estimated value LP of the distance has not been calculated (No in step S 108 ), the gain determining unit  15  sets, for each frequency, the upper-limit value LE at the distance R(f) calculated based on the first frequency signal and the second frequency signal (step S 110 ). Alternatively, the gain determining unit  15  may set the upper-limit value LE at an assumed maximum distance L from the sound collecting unit  2 - 1  to the user&#39;s mouth. In this case, as evident from equation (5), a frequency component from a sound source farther away from the sound collecting unit  2 - 1  than the assumed maximum distance L is suppressed. 
     After step  5109  or S 110 , the gain determining unit  15  calculates, for each frequency, the gain G(f) so that a frequency component is attenuated when the distance R(f) exceeds the upper-limit value LE or when the sound source direction θ(f) falls out of an assumed range of the user&#39;s mouth direction (step S 111 ). The gain determining unit  15  then outputs the gain G(f) for each frequency to the signal correcting unit  16 . 
     The signal correcting unit  16  corrects the first frequency signal for each frequency by multiplying the first frequency signal by the gain G(f) of that frequency (step S 112 ). The signal correcting unit  16  then outputs the corrected first frequency signal to the frequency time converting unit  17 . 
     The frequency time converting unit  17  calculates a corrected first voice signal in a time domain by performing frequency time conversion on the corrected first frequency signal (step S 113 ). The frequency time converting unit  17  then outputs the corrected voice signal, and then the voice processing ends. 
     Note that the sequence of the processes in steps S 101  to S 104  and the sequence of the processes in steps S 105  to S 107  may be transposed. 
     As has been described above, the voice processing device accurately estimates the distance from the sound collecting unit to the user&#39;s mouth, which is a sound source as a sound collection target, based on the ratio between the power of the human-body vibration signal and the power of the first voice signal. Thus, the voice processing device appropriately sets a gain for the component of each frequency included in the first frequency signal in accordance with the estimated distance from the sound collecting unit to the sound source for that frequency. As a result, even when the user holds the voice processing device in any manner, the voice processing device appropriately suppresses noise components included in the voice signal other than the voice of the user. 
     Note that the pressure with which the human body is pressed onto the voice processing device is varied depending on the user. In accordance with the pressure, the relation between the ratio between the power of the human-body vibration signal and the power of the first voice signal and the distance from the sound collecting unit to the user&#39;s mouth is also varied. 
     Thus, according to a second embodiment, a voice processing device has stored therein, for each pressure value of the pressure of the human body onto the voice processing device, distance estimation information representing the relation between the ratio between the power of the human-body vibration signal and the power of the first voice signal and the distance from the sound collecting unit to the user&#39;s mouth found in advance. At the time of conversation, by using a pressure sensor, the voice processing device detects the pressure of the human body onto the voice processing device and selects the distance estimation information in accordance with the detected pressure, thereby calculating the distance from the sound collecting unit to the user&#39;s mouth. 
       FIG. 8  is a schematic diagram of the structure of a voice processing device according to a second embodiment. A voice processing device  21  includes the sound collecting units  2 - 1  and  2 - 2 , the human-body vibration obtaining unit  3 , the analog/digital converting units  4 - 1  to  4 - 3 , the user interface unit  5 , the storage unit  6 , the processing unit  7 , the communicating unit  8 , the output unit  9 , the storage medium access device  10 , and a pressure sensor  22 . Compared with the voice processing device  1  according to the first embodiment, the voice processing device  21  according to the second embodiment is different in that the pressure sensor  22  is provided and the process in the target sound source distance calculating unit  11  of the processing unit  7  is different. Thus, in the following, the pressure sensor  22 , the target sound source distance calculating unit  11 , and their related portions are described. Regarding the other components of the voice processing device  21 , refer to the description of the components corresponding to those of the voice processing device according to the first embodiment. 
       FIG. 9  is a diagram depicting an example of arrangement of the pressure sensor, the sound collecting units, and the human-body vibration obtaining unit according to the second embodiment. Also in this embodiment, the sound collecting unit  2 - 2  is disposed along a longitudinal direction of a housing of the voice processing device  21  on the back surface side near the end on the side where the output unit  9  which outputs voice is disposed, and the sound collecting unit  2 - 1  is disposed on the front surface side near the end opposite to the output unit  9 . 
     Also, the human-body vibration obtaining unit  3  is mounted on the front surface side of the housing of the voice processing device  21  so as to be adjacent to the output unit  9 . Furthermore, to detect a pressure by pressing from the ear of the user or its proximity at the time of conversation, the pressure sensor  22  is also mounted on the front surface side of the housing of the voice processing device  21  so as to be adjacent to the output unit  9 . 
     The storage unit  6  stores, for each of a plurality of different pressure values, distance estimation information in associated with that pressure value. When voice processing is performed, the target sound source distance calculating unit  11  reads, from the storage unit  6 , distance estimation information associated with a pressure value closest to the pressure value obtained from the pressure sensor  22 . Then, with reference to the read distance estimation information, the target sound source distance calculating unit  11  calculates the distance from the sound collecting unit  2 - 1  to the user&#39;s mouth corresponding to the ratio of the power of the human-body vibration signal and the power of the first voice signal. 
     According to the present embodiment, the voice processing device uses appropriate distance estimation information in accordance with the pressure by pressing. Therefore, the distance from the sound collecting unit  2 - 1  to the user&#39;s mouth is more correctly estimated. 
     Also, according to a modification example, the voice processing device may estimate the distance from the sound collecting unit  2 - 1  to the sound source in accordance with another distance estimation method capable of estimating the distance to the sound source for each frequency. For example, the voice processing device may include three or more microphones disposed at different positions. In this case, for each of a plurality of sets each including two microphones selected from these three or more microphones, the voice processing device estimates a sound source direction for each frequency based on a voice signal from each microphone included in that set. The voice processing device then estimates, from the estimated value of the sound source direction for each set of microphones and a positional relation among these microphones, a distance to the sound source for each frequency based on the principles of triangulation. 
     Furthermore, according to another modification example, the voice processing device may correct both of the first and second voice signals. In this case, the signal correcting unit  16  corrects not only the first frequency signal but also the second frequency signal in accordance with equation (6). The frequency time converting unit  17  then converts each of the corrected first and second frequency signals to a time-domain signal. 
     According to still another modification example, the gain determining unit  15  may newly set a value obtained by adding 1 to the gain GL found in accordance with equation (5) as the gain GL. In this case, when the distance R(f) between the sound collecting unit  2 - 1  and the sound source is equal to or larger than (LE+ΔL), the component of the frequency f of the first frequency signal is not suppressed. Instead, when the distance R(f) is smaller than (LE+ΔL), the gain GL is determined so that the component of the frequency f of the first frequency signal is emphasized. Similarly, the gain determining unit  15  may newly set a value obtained by adding 1 to the gain Gθ found in accordance with equation (5) as the gain Gθ. 
     Still further, a computer program which causes a computer to achieve each function included in the processing unit of the voice processing device according to each of the above-described embodiments and modification examples may be provided in a form of being recorded on a medium readable by the computer, such as a magnetic recording medium or optical recording medium. 
     All examples and specific terms recited herein are intended for pedagogical purposes to aid the reader in understanding of the concepts contributed by the inventor to promote the disclosure and relevant technology, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification regarding the superiority and inferiority of the disclosure. While the embodiments have been described in detail, it is to be understood that various changes, substitutions, and alterations may be made to the embodiments without departing from the spirit and scope of the disclosure. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.