Patent Publication Number: US-8996367-B2

Title: Sound processing apparatus, sound processing method and program

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a sound processing apparatus, a sound processing method, and a program, and in particular, relates to a sound processing apparatus that remixes sounds separated based on input sound characteristics, a sound processing method, and a program. 
     2. Description of the Related Art 
     A call voice, sound of a shooting target and the like are generally recorded by a device equipped with a sound recording apparatus capable of recording sound such as a mobile phone and camcorder. Sound recorded in a sound recording apparatus has sounds originating from various sound sources including a voice uttered by a person and ambient noise mixed therein. If sounds originating from various sound sources are mixed and a sound originating from a desired sound source is recorded relatively lower than sounds originating from other sound sources, there is an issue that it is difficult to determine content of the desired sound. 
     Thus, technologies to separate a mixed sound in which sounds originating from various sound sources are mixed and then each separated sound is remixed at a desired sound volume are disclosed (for example, Japanese Patent Application Laid-Open No. 2003-131686 and Japanese Patent Application Laid-Open No. 5-56007). According to Japanese Patent Application Laid-Open No. 2003-131686, characteristic data representing a likeness of voice or that of music is learned in advance and a mixing ratio of a voice signal to a music signal is estimated for the music signal on which a narration signal is superimposed to be able to emphasize the desired voice. According to Japanese Patent Application Laid-Open No. 5-56007, a broadcast voice to which additional information is added in advance to separate the broadcast voice into a voice signal and background noise is separated into a voice signal and background noise after the broadcast voice is received so that the voice signal can be remixed at a desired sound volume. 
     SUMMARY OF THE INVENTION 
     However, Japanese Patent Application Laid-Open No. 2003-131686 has an issue that it is difficult to separate a mixed sound without learning in advance. Japanese Patent Application Laid-Open No. 5-56007 has an issue that it is difficult to remix the voice in a desired ratio without addition of information in advance. 
     The present invention has been made in view of the above issues and it is desirable to provide a novel and improved sound processing apparatus capable of separating a mixed sound originating from various sound sources without advance learning and remixing in a desired ratio, a sound processing method, and a program. 
     According to an embodiment of the present invention, there is provided a sound processing apparatus including a sound separation unit that separates an input sound into a plurality of sounds caused by a plurality of sound sources, a sound type estimation unit that estimates sound types of the plurality of sounds separated by the sound separation unit, a mixing ratio calculation unit that calculates a mixing ratio of each sound in accordance with the sound type estimated by the sound type estimation unit, and a sound mixing unit that mixes the plurality of sounds separated by the sound separation unit in the mixing ratio calculated by the mixing ratio calculation unit. 
     According to the above configuration, an input sound input into the sound processing apparatus is separated into sounds caused by a plurality of sound sources and a plurality of separated sound types is estimated. Then, a mixing ratio of each sound is calculated in accordance with the estimated sound type and each separated sound is remixed in the mixing ratio. Accordingly, it becomes possible to independently control sounds originating from different sound sources by separating a mixed sound originating from various sound sources and remixing each separated sound in a desired ratio. A desired sound can be prevented from being made difficult to hear by being masked by a sound whose volume is higher than that of the desired sound. Also, the volume originating from each sound source can be adjusted to a desired volume without the need to arrange a microphone or the like for each different sound source. 
     The sound separation unit may separate the input sound into the plurality of sounds in units of blocks of a predetermined length, comprising including an identity determination unit that determines whether the sounds separated by the sound separation unit are identical among a plurality of blocks, and a recording unit that records volume information of the sounds separated by the sound separation unit in units of the blocks. 
     The sound separation unit may separate the input sound into the plurality of sounds using statistical independence of sound and differences in spatial transfer characteristics. 
     The sound separation unit may separate the input sound into a sound originating from a specific sound source and other sounds using a paucity of overlapping between time-frequency components of sound sources. 
     The sound type estimation unit may estimate whether the input sound is a steady sound or non-steady sound using a distribution of amplitude information, direction, volume, zero crossing number and the like at discrete times of the input sound. 
     The sound type estimation unit may estimate whether the sound estimated to be a non-steady sound is a noise sound or a voice uttered by a person. 
     The mixing ratio calculation unit may calculate a mixing ratio that does not significantly change the volume of the sound estimated to be a steady sound by the sound type estimation unit. 
     The mixing ratio calculation unit may calculate a mixing ratio that lowers the volume of the sound estimated to be a noise sound by the sound type estimation unit and does not lower the volume of the sound estimated to be a voice uttered by a person. 
     According to another embodiment of the present invention, there is provided a sound processing method including the steps of separating a input sound input by a sound processing apparatus into a plurality of sounds, estimating sound types of the plurality of separated sounds, calculating a mixing ratio of each sound in accordance with the estimated sound type, and mixing the plurality of separated sounds in the calculated mixing ratio. 
     According to another embodiment of the present invention, there is provided a program for causing a computer to function as a sound processing apparatus including a sound separation unit that separates an input sound into a plurality of sounds, a sound type estimation unit that estimates sound types of the plurality of sounds separated by the sound separation unit, a mixing ratio calculation unit that calculates a mixing ratio of each sound in accordance with the sound type estimated by the sound type estimation unit, and a sound mixing unit that mixes the plurality of sounds separated by the sound separation unit in the mixing ratio calculated by the mixing ratio calculation unit. 
     According to the present invention, as described above, a mixed sound originating from various sound sources can be separated and then remixed in a desired ratio without performing preprocessing. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing a functional configuration of a sound processing apparatus according to an embodiment of the present invention; 
         FIG. 2  is a functional block diagram showing the configuration of a sound type estimation unit according to the embodiment; 
         FIG. 3  is an explanatory view showing a state that a sound source position of input sound is estimated based on a phase difference of two input sounds; 
         FIG. 4  is an explanatory view showing a state that a sound source position of input sound is estimated based on a phase difference of three input sounds; 
         FIG. 5  is an explanatory view showing a state that a sound source position of input sound is estimated based on a volume of two input sounds; 
         FIG. 6  is an explanatory view showing a state that a sound source position of input sound is estimated based on a volume of three input sounds; 
         FIG. 7  is an explanatory view illustrating a method of fine-tuning a reduction rate according to the embodiment; and 
         FIG. 8  is flow chart showing the flow of processing of a sound processing method executed by the sound processing apparatus according to the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENT 
     Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the appended drawings. Note that, in this specification and the appended drawings, structural elements that have substantially the same function and structure are denoted with the same reference numerals, and repeated explanation of these structural elements is omitted. 
     A “DETAILED DESCRIPTION OF EMBODIMENT” will be described in the order shown below: 
     [1] Purpose of the embodiment 
     [2] Functional configuration of the sound processing apparatus 
     [3] Operation of the sound processing apparatus 
     [1] Purpose of the Embodiment 
     First, the purpose of the embodiment of the present invention will be described. A call voice, sound of a shooting target and the like are generally recorded by a device equipped with a sound recording apparatus capable of recording sound such as a mobile phone and camcorder. Sound recorded in a sound recording apparatus has sounds originating from various sound sources including a voice uttered by a person and ambient noise mixed therein. If sounds originating from various sound sources are mixed and a sound originating from a desired sound source is recorded relatively lower than sounds originating from other sound sources, there is an issue that it is difficult to determine content of the desired sound. 
     Thus, technologies to separate a mixed sound in which sounds originating from various sound sources are mixed and then each separated sound is remixed with a desired sound volume are disclosed. For example, a technology to learn characteristic data representing a likeness of voice or that of music in advance and estimate a mixing ratio of a voice signal to a music signal for the music signal on which a narration signal is superimposed to emphasize the desired voice is known. Also, a technology to separate a broadcast voice to which additional information is added in advance to separate the broadcast voice into a voice signal and background noise into a voice signal and background noise after the broadcast voice is received so that the voice signal can be remixed with a desired sound volume is known. 
     However, in related art, there is an issue that it is difficult to separate a mixed sound or remix sounds in a desired ratio without learning in advance or addition of information in advance. That is, since it is difficult to learn in advance or add information in advance for content personally shot or the like, instead of sound or broadcast sound input in real time, it is difficult to acquire a desired sound. Thus, with the above situation being focused on, a sound processing apparatus  10  according to an embodiment of the present invention has been developed. According to the sound processing apparatus  10  in the present embodiment, a mixed sound originating from various sound sources can be separated and then remixed in a desired ratio without performing preprocessing. 
     [2] Functional Configuration of the Sound Processing Apparatus 
     Next, the functional configuration of the sound processing apparatus  10  will be described with reference to  FIG. 1 . As described above, the sound processing apparatus  10  according to the present embodiment can separate a mixed sound originating from various sound sources and then remix in a desired ratio without performing preprocessing. As the sound processing apparatus  10 , for example, a sound recording/reproducing apparatus mounted in an imaging apparatus can be exemplified. 
     To record a sound signal by a sound processing apparatus mounted in an imaging apparatus, a sound originating from a desired sound source may not be recorded in an appropriate volume balance intended by an operator of the imaging apparatus because the sound originating from the desired sound source is masked by sounds originating from other sound sources. Moreover, if sounds recorded in a plurality of situations are reproduced, the recording level may fluctuate greatly so that it is frequently difficult to listen to sound comfortably at a fixed reproduction volume. However, according to the sound processing apparatus  10  in the present embodiment, it becomes possible to record a sound originating from a desired sound source in an appropriate volume balance intended by an operator or to listen to sound comfortably by recording the sound at a fixed reproduction volume. 
       FIG. 1  is a block diagram showing the functional configuration of the sound processing apparatus  10  according to the present embodiment. As shown in  FIG. 1 , the sound processing apparatus  10  includes a sound recording unit  110 , a sound separation unit  112 , a recording unit  114 , a storage unit  116 , an identity determination unit  118 , a sound type estimation unit  122 , a mixing ratio calculation unit  120 , and a sound mixing unit  124 . 
     The sound recording unit  110  records a sound and discretely quantizes the recorded sound. The sound recording unit  110  includes two or more physically separated recording units (for example, microphones). The sound recording unit  110  may include two recording units, one recording unit to record a left sound and the other recording unit to record a right sound. The sound recording unit  110  provides the discretely quantized sound to the sound separation unit  112  as an input sound. The sound recording unit  110  may provide the input sound to the sound separation unit  112  in units of blocks of a predetermined length. 
     The sound separation unit  112  has a function to separate the input sound into a plurality of sounds originating from a plurality of sound sources. More specifically, the input sound provided by the sound recording unit  110  is separated using statistical independence of sound sources and differences in spatial transfer characteristics. As described above, when the input sound is provided from the sound recording unit  110  in units of blocks of a predetermined length, the sound may be separated in units of the blocks. 
     As a concrete technique to separate sound sources by the sound separation unit  112 , for example, a technique using the independent component analysis (article 1: Y. Mori, H. Saruwatari, T. Takatani, S. Ukai, K. Shikano, T. Hietaka, T. Morita, Real-Time Implementation of Two-Stage Blind Source Separation Combining SIMO-ICA and Binary Masking, Proceedings of IWAENC2005, (2005).) may be used. A technique that uses a paucity of overlapping between time-frequency components of sound (article 2: O. Yilmaz and S. Richard, Blind Separation of Speech Mixtures via Time-Frequency Masking, IEEE TRANSACTIONS ON SIGNAL PROCESSING, VOL. 52, NO. 7, JULY (2004).) may also be used. 
     The identity determination unit  118  has a function, when an input sound is separated into a plurality of sounds in units of blocks by the sound separation unit  112 , to determine whether the separated sounds are identical among a plurality of blocks. The identity determination unit  118  determines whether separated sounds between consecutive blocks originate from the same sound source using, for example, the distribution of amplitude information, volume, direction information and the like at discrete times of separated sounds provided by the sound separation unit  112 . 
     The recording unit  114  has a function to record volume information of sounds separated by the sound separation unit in the storage unit  116  in units of blocks. Volume information recorded in the storage unit  116  includes, for example, sound type information of each separated sound acquired by the identity determination unit  118  and the average value, maximum value, variance and the like of separated sounds acquired by the sound separation unit  112 . In addition to real-time sound, the average value of volume of separated sounds on which sound processing was performed in the past may be recorded. If volume information of input sound is available prior to the input sound, the volume information may be recorded. 
     The sound type estimation unit  122  has a function to estimate the sound type of a plurality of sounds separated by the sound separation unit  112 . The sound type (steady or non-steady, noise or sound) is estimated, for example, from sound information obtained from the volume of separated sound and the distribution, maximum value, average value, variance, zero crossing number and the like of amplitude information, and direction distance information. Here, detailed functions of the sound type estimation unit  122  will be described. A case in which the sound processing apparatus  10  is mounted in an imaging apparatus will be described below. The sound type estimation unit  122  determines whether any sound originating from the neighborhood of the imaging apparatus such as a voice of an operator of the imaging apparatus or noise resulting from an operation of the operator is contained. Accordingly, by which sound source a sound is caused can be estimated. 
       FIG. 2  is a functional block diagram showing the configuration of the sound type estimation unit  122 . The sound type estimation unit  122  includes a volume detection unit  130  including a volume detector  132 , an average volume detector  134 , and a maximum volume detector  136 , a sound quality detection unit  138  including a spectrum detector  140  and a sound quality detector  142 , a distance/direction estimator  144 , and a sound estimator  146 . 
     The volume detector  132  detects a volume value sequence (amplitude) of input sound given in frames of a predetermined length (for example, several tens msec) and outputs the detected volume value sequence of input sound to the average volume detector  134 , the maximum volume detector  136 , the sound quality detector  142 , and the distance/direction estimator  144 . 
     The average volume detector  134  detects the average value of volume of input sound, for example, in frames based on the volume value sequence in frames input from the volume detector  132 . The average volume detector  134  outputs the detected average value of volume to the sound quality detector  142  and the sound estimator  146 . 
     The maximum volume detector  136  detects the maximum value of volume of input sound, for example, in frames based on the volume value sequence in frames input from the volume detector  132 . The maximum volume detector  136  outputs the detected maximum value of volume of input sound to the sound quality detector  142  and the sound estimator  146 . 
     The spectrum detector  140  detects each spectrum in the frequency domain of input sound by performing, for example, FFT (Fast Fourier Transform) on the input sound. The spectrum detector  140  outputs detected spectra to the sound quality detector  142  and the distance/direction estimator  144 . 
     The sound quality detector  142  has an input sound, average value of volume, maximum value of volume, and spectrum input thereinto, detects a likeness of human voice, that of music, steadiness, and impulse property of the input sound, and outputs detection results to the sound estimator  146 . The likeness of human voice may be information indicating whether a portion or all of the input sound matches human voice or to which extent the input sound resembles human voice. Also, the likeness of music may be information indicating whether a portion or all of the input sound matches music or to which extent the input sound resembles music. 
     Steadiness indicates, for example, like an air-conditioning sound, a property whose statistical property of sound does not change significantly over time. The impulse property indicates, for example, like a blow sound or explosive, a property full of noise in which energy is concentrated in a short period of time. 
     The sound quality detector  142  can detect, for example, a likeness of human voice based on the degree of matching of the spectral distribution of input sound and that of human voice. The sound quality detector  142  may also detect a higher impulse property with an increasing maximum value of volume by comparing maximum values of volume of each frame or other frames. 
     The sound quality detector  142  may analyze sound quality of input sound using signal processing technology such as the zero crossing method and LPC (Linear Predictive Coding) analysis. According to the zero crossing method, a fundamental period of input sound is detected and therefore, the sound quality detector  142  may detect a likeness of human voice based on whether the fundamental period is contained in the fundamental period (for example, 100 to 200 Hz) of human voice. 
     The distance/direction estimator  144  has an input sound, volume value sequence of the input sound, spectrum of the input sound and the like input thereinto. The distance/direction estimator  144  has a function, based on the input, as a positional information calculation unit that estimates the sound source of the input sound or positional information such as direction information and distance information of the sound source from which a dominant sound contained in the input sound originates. The distance/direction estimator  144  can collectively estimate the position of the sound source even if a reverberation or the reflection of sound caused by the main body of imaging apparatus has a great influence by combining the phase, volume, and volume value sequence of input sound and estimation methods of positional information of the sound source based on the average volume value and maximum volume value in the past. An example of the estimation method of the direction information and distance information by the distance/direction estimator  144  will be described with reference to  FIGS. 3 to 6 . 
       FIG. 3  is an explanatory view showing a state that the sound source position of an input sound is estimated based on a phase difference of two input sounds. If the sound source is assumed to be a point sound source, the phase of each input sound reaching a microphone M 1  and a microphone M 2  constituting the sound recording unit  110  and a phase difference of the input sounds can be measured. Further, a difference between the distance from the microphone M 1  to the sound source position of input sound and that from the microphone M 2  can be calculated from the phase difference and values of a frequency f and a sound velocity c of the input sound. The sound source is present on a set of points where the difference of distance is constant. It is known that such a set of points where the difference of distance is constant forms a hyperbola. 
     It is assumed, for example, that the microphone M 1  is positioned at (x1, 0) and the microphone M 2  at (x2, 0) (generality is not lost under this assumption). If a point on a set of the sound source position to be determined is at (x, y) and the difference of distance is d, Formula 1 shown below holds:
 
[Equation 1]
 
√{square root over (( x−x   1 ) 2   +y   2 )}−√{square root over (( x−x   2 ) 2   +y   2 )}= d   (Formula 1)
 
     Further, Formula 1 can be expanded into Formula 2, from which Formula 3 representing a hyperbola is derived: 
     
       
         
           
             
               
                 
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     The distance/direction estimator  144  can also determine to which of the microphone M 1  and the microphone M 2  the distance/direction estimator  144  is closer based on a volume difference between input sounds recorded by the microphone M 1  and the microphone M 2 . Accordingly, for example, as shown in  FIG. 3 , the sound source can be determined to be present on a hyperbola  1  closer to the microphone M 2 . 
     Incidentally, it is necessary for the frequency f of input sound used for calculation of a phase difference to satisfy a condition on a distance between the microphone M 1  and the microphone M 2  in Formula 4: 
     
       
         
           
             
               
                 
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       FIG. 4  is an explanatory view showing a state that the sound source position of an input sound is estimated based on phase differences among three input sounds. Arrangement of a microphone M 3 , a microphone M 4 , and a microphone M 5  constituting the sound recording unit  110  as shown in  FIG. 4  is assumed. The phase of input sound arriving at the microphone M 5  may be delayed when compared with that of input sound arriving at the microphone M 3  or the microphone M 4 . In such a case, the distance/direction estimator  144  can determine that the sound source is positioned on the opposite side of the microphone M 5  with respect to a straight line  1  linking the microphone M 3  and the microphone M 4  (front/back determination). 
     Further, the distance/direction estimator  144  calculates a hyperbola  2  on which the sound source could be present based on a phase difference of input sounds arriving at each of the microphone M 3  and the microphone M 4 . Then, the distance/direction estimator  144  can calculate a hyperbola  3  on which the sound source could be present based on a phase difference of input sounds arriving at each of the microphone M 4  and the microphone M 5 . As a result, the distance/direction estimator  144  can estimate that an intersection P 1  of the hyperbola  2  and the hyperbola  3  is the sound source position. 
       FIG. 5  is an explanatory view showing a state that the sound source position of an input sound is estimated based on volumes of two input sounds. If the sound source is assumed to be a point sound source, the volume measured at a point is inversely proportional to the square of distance based on the inverse square law. If a microphone M 6  and a microphone M 7  constituting the sound recording unit  110  as shown in  FIG. 5  is assumed, a set of points where the ratio of volumes arriving at the microphone M 6  and the microphone M 7  is constant forms a circle. The distance/direction estimator  144  can determine the radius and the center position of the circle on which the sound source is present by determining the ratio of volume from values of volume input from the volume detector  132 . 
     It is assumed, as shown in  FIG. 5 , that the microphone M 6  is positioned at (x3, 0) and the microphone M 7  at (x4, 0). In this case (generality is not lost under this assumption), if a point on a set of the sound source position to be determined is at (x, y), distances r 1  and r 2  from each microphone to the sound source can be expressed as Formula 5 below:
 
[Equation 5]
 
 r   1 =√{square root over (( x−x   3 ) 2   +y   2 )} r   2 =√{square root over (( x−x   4 ) 2   +y   2 )}  (Formula 5)
 
     Here, Formula 6 below holds thanks to the inverse square law: 
     
       
         
           
             
               
                 
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     Formula 6 is transformed to Formula 7 using a positive constant d (for example, 4): 
     
       
         
           
             
               
                 
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     Formula 8 below is derived by substitution into r 1  and r 2  in Formula 7: 
     
       
         
           
             
               
                 
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     From Formula 8, the distance/direction estimator  144  can estimate that, as shown in  FIG. 5 , the sound source is present on a circle  1  whose center coordinates are represented by Formula 9 and whose radius is represented by Formula 10. 
     
       
         
           
             
               
                 
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                     10 
                   
                   ) 
                 
               
             
           
         
       
     
       FIG. 6  is an explanatory view showing a state that the sound source position of an input sound is estimated based on volumes of three input sounds. Arrangement of the microphone M 3 , the microphone M 4 , and the microphone M 5  constituting the sound recording unit  110  as shown in  FIG. 6  is assumed. The phase of input sound arriving at the microphone M 5  may be delayed when compared with that of input sound arriving at the microphone M 3  or the microphone M 4 . In such a case, the distance/direction estimator  144  can determine that the sound source is positioned on the opposite side of the microphone M 5  with respect to a straight line  2  linking the microphone M 3  and the microphone M 4  (front/back determination). 
     Further, the distance/direction estimator  144  calculates a circle  2  on which the sound source could be present based on a volume ratio of input sounds arriving at each of the microphone M 3  and the microphone M 4 . Then, the distance/direction estimator  144  can calculate a circle  3  on which the sound source could be present based on a volume ratio of input sounds arriving at each of the microphone M 4  and the microphone M 5 . As a result, the distance/direction estimator  144  can estimate that an intersection P 2  of the circle  2  and the circle  3  is the sound source position. If four or more microphones are used, the distance/direction estimator  144  can estimate more precisely including spatial arrangement of the sound source. 
     The distance/direction estimator  144  estimates, as described above, the position of the sound source of input sound based on a phase difference or volume ratio of input sounds and outputs direction information or distance information of the estimated sound source to the sound estimator  146 . Table 1 below lists the input/output of each component of the volume detection unit  130 , the sound quality detection unit  138 , and the distance/direction estimator  144  described above. 
     
       
         
           
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Block 
                 Input 
                 Output 
               
               
                   
               
             
            
               
                 Volume detector 
                 Input sound 
                 Volume value 
               
               
                   
                   
                 sequence 
               
               
                   
                   
                 (amplitude) in frame 
               
               
                 Average volume 
                 Volume value sequence 
                 Average value of 
               
               
                 detector 
                 (amplitude) in frame 
                 volume 
               
               
                 Maximum volume 
                 Volume value sequence 
                 Maximum value of 
               
               
                 detector 
                 (amplitude) in frame 
                 volume 
               
               
                 Spectrum detector 
                 Input sound 
                 Spectrum 
               
               
                 Sound quality 
                 Input sound 
                 Likeness of human 
               
               
                 detector 
                 Average value of volume 
                 voice 
               
               
                   
                 Maximum value of volume 
                 Likeness of music 
               
               
                   
                 Spectrum 
                 Steady or non-steady 
               
               
                   
                   
                 Impulse property 
               
               
                 Distance/direction 
                 Input sound 
                 Direction information 
               
               
                 estimator 
                 Volume value sequence 
                 Distance information 
               
               
                   
                 (amplitude) in frame 
               
               
                   
                 Spectrum 
               
               
                   
               
            
           
         
       
     
     If sounds originating from a plurality of sound sources are superimposed on an input sound, it is difficult for the distance/direction estimator  144  to precisely estimate the sound source position of a sound predominantly contained in the input sound. However, the distance/direction estimator  144  can estimate a position close to the sound source position of the sound predominantly contained in the input sound. The estimated sound source position may be used as an initial value for sound separation by the sound separation unit  112  and thus, the sound processing apparatus  10  can perform a desired operation even if there is an error in the sound source position estimated by the distance/direction estimator  144 . 
     The description of the configuration of the sound type estimation unit  122  will be resumed with reference to  FIG. 2 . The sound estimator  146  collectively determines whether any neighborhood sound originating from a specific sound source in the neighborhood of the sound processing apparatus  10  such as a voice of the operator or noise resulting from an operation of the operator is contained in the input sound based on at least one of the volume, sound quality, and positional information of input sound. If the sound estimator  146  determines that a neighborhood sound is contained in the input sound, the sound estimator  146  has a function as a sound determination unit that outputs a message that a neighborhood sound is contained in the input sound (operator voice present information) and positional information estimated by the distance/direction estimator  144  to the sound separation unit  112 . 
     More specifically, if the distance/direction estimator  144  estimates that the position of the sound source of input sound is behind an imaging unit (not shown) imaging video in the imaging direction and the input sound has sound quality that matches or resembles that of human voice, the sound estimator  146  may determine that a neighborhood sound is contained in the input sound. 
     If the position of the sound source of input sound is behind an imaging unit in the imaging direction and the input sound has sound quality that matches or resembles that of human voice, the sound estimator  146  may determine that the voice of the operator is predominantly contained as a neighborhood sound in the input sound. As a result, a mixed sound in which the sound ratio of the voice of the operator is reduced can be obtained from the sound mixing unit  124  described later. 
     The sound estimator  146  has the position of the sound source of input sound within the range of a setting distance (neighborhood of the sound processing apparatus  10 , for example, within 1 m of the sound processing apparatus  10 ) from the recording position. If the input sound contains an impulse sound and the input sound is higher than an average volume in the past, the sound estimator  146  may determine that the input sound contains a neighborhood sound caused by a specific sound source. Here, an impulse sound such as “click” and “bang” is frequently caused when the operator of an imaging apparatus operates a button of the imaging apparatus or shifts the imaging apparatus from one hand to the other. Moreover, the impulse sound is caused by an imaging apparatus equipped with the sound processing apparatus  10  and thus, it is highly likely that the impulse sound is recorded at a relatively large volume. 
     Therefore, the sound estimator  146  has the position of the sound source of input sound within the range of a setting distance from the recording position. If input sound contains an impulse sound and the input sound is higher than an average volume in the past, the input sound can be determined to predominantly contain noise resulting from an operation of the operator as a neighborhood sound. As a result, a mixed sound in which the sound ratio of noise resulting from an operation of the operator is reduced can be obtained from the sound mixing unit  124  described later. 
     In addition, Table 2 summarizes examples of information input into the sound estimator  146  and determination results of the sound estimator  146  based on the input information. By combining with a proximity sensor, temperature sensor or the like, precision of determination by the sound estimator  146  can be improved. 
     
       
         
           
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Sound estimator input 
                   
               
            
           
           
               
               
               
            
               
                   
                 Sound quality 
                   
               
            
           
           
               
               
               
               
               
            
               
                 Volume 
                 Likeness 
                   
                 Direction 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                   
                 Average 
                 Maximum 
                 of human 
                 Likeness 
                 Steady or 
                 Impulse 
                 and distance 
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                 Volume 
                 volume 
                 volume 
                 voice 
                 of music 
                 non-steady 
                 property 
                 Direction 
                 Distance 
                 Determination results 
               
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 High 
                 Higher than 
                 High 
                 High 
                 Low 
                 Non-steady 
                 Normal 
                 Behind 
                 Close 
                 Non-steady 
                 Operator voice 
               
               
                   
                 average 
                   
                   
                   
                   
                   
                 main body 
                   
                 sound 
               
               
                   
                 volume in the 
               
               
                   
                 past 
               
               
                 Medium 
                 Comparatively 
                 Medium 
                 Normal 
                 Normal 
                 Non-steady 
                 Normal 
                 In front of 
                 Close to far 
                   
                 Object sound 
               
               
                   
                 higher than 
                 to high 
                   
                   
                   
                   
                 main body 
               
               
                   
                 average 
               
               
                   
                 volume in the 
               
               
                   
                 past 
               
               
                 High 
                 Higher than 
                 High 
                 Low 
                 Low 
                 Non-steady 
                 High 
                 All 
                 Close 
                 Non-steady 
                 Operation noise 
               
               
                   
                 average 
                   
                   
                   
                   
                   
                 directions 
                   
                 noise 
               
               
                   
                 volume in the 
               
               
                   
                 past 
               
               
                 Low 
                 Comparatively 
                 Medium 
                 Low 
                 Low 
                 Non-steady 
                 High 
                 All 
                 Far 
                   
                 Impulsive 
               
               
                   
                 lower than 
                   
                   
                   
                   
                   
                 directions 
                   
                   
                 environmental sound 
               
               
                   
                 average 
               
               
                   
                 volume in the 
               
               
                   
                 past 
               
               
                 Low 
                 Lower than 
                 Low 
                 Normal 
                 Normal 
                 Steady 
                 Low 
                 Direction 
                 Far 
                 Steady 
                 Environmental sound 
               
               
                   
                 average 
                   
                   
                   
                   
                   
                 unknown 
                   
                 noise 
               
               
                   
                 volume in the 
               
               
                   
                 past 
               
               
                   
               
            
           
         
       
     
     Returning to  FIG. 1 , the mixing ratio calculation unit  120  has a function to calculate the mixing ration of each sound in accordance with the sound type estimated by the sound type estimation unit  122 . For example, a mixing ratio that lowers the volume of a dominant sound is calculated using separated sounds separated by the sound separation unit  112 , sound type information by the sound type estimation unit  122 , and volume information recorded in the recording unit  114 . 
     When the sound type is more steady, a mixing ratio so that volume information does not change significantly between consecutive blocks is also calculated with reference to output information of the sound type estimation unit  122 . When the sound type is not steady (non-steady) and noise is more likely, the mixing ratio calculation unit  120  lowers the volume of the sound concerned. On the other hand, if the sound type is non-steady a voice uttered by a person is more likely, the volume of the sound concerned is not much lowered when compared with noise sound. 
     Here, a method of fine-tuning the reduction rate will be described with reference to  FIG. 7 . Frequency characteristics (loudness characteristics) of human audition or a masking effect can be used as a method of fine-tuning the reduction rate. More specifically, a method below can be considered. In human audition characteristics, frequency components of 2 to 4 kHz are sensitive. If separated sounds whose volume is dominant mainly contain this band, the mixing ratio is set with an inclination so that the band concerned is relatively more suppressed when compared with other bands. 
     As shown in  FIG. 7 , a relatively smaller mixing ratio is set for 2 to 4 kHz (band a), which is a band more easily perceived by humans. Accordingly, other separated sounds can avoid being masked by separated sounds of dominant volume. The mixing ratio is relatively reduced for frequency bands (band b) with less separation precision. 
     Also, a spectrum masking effect (phenomenon in which if there is a loud sound at some frequency at a certain time, sounds in neighboring frequencies are inaudible by being masked) is considered. In this case, the mixing ratio of sound of frequency bands (band b) in which precision of separation by the sound separation unit  112  is not adequately secured is relatively reduced. Accordingly, a mixing ratio with an inclination so as to be masked by sound of neighboring frequencies (whose separation precision is adequately secured) can be set. 
     By using the above technique, a remixing ratio of separated sounds that allows to hear a sound being masked by a dominant sound source due to low amplitude is automatically calculated. At this point, the total volume may be made constant if possible within a range smoothly linkable in the time direction with no significant change in volume of each sound source between the previous block and the current block determined from volume information of separated sounds and the remixing ratio. Alternatively, a mixing ratio to significantly reduce a specific sound source may be calculated in accordance with settings specified by the user. 
     Returning to  FIG. 1 , the sound mixing unit  124  has a function to mix a plurality of sounds separated by the sound separation unit  112  in the mixing ratio provided by the mixing ratio calculation unit  120 . For example, the sound mixing unit  124  may mix a neighborhood sound of the sound processing apparatus  10  and a sound to be recorded so that the volume ratio occupied by the neighborhood sound is made lower than that of the neighborhood sound occupied in the input sound. Accordingly, if the volume of neighborhood sound of the input sound is unnecessarily high, a mixed sound in which the volume ratio occupied by the sound to be recorded is increased from that of the sound to be recorded occupied in the input sound can be obtained. As a result, the sound to be recorded can be prevented from being buried by the neighborhood sound. 
     [3] Operation of the Sound Processing Apparatus 
     In the foregoing, the functional configuration of the sound processing apparatus  10  according to the present embodiment has been described. Next, the sound processing method executed by the sound processing apparatus  10  will be described with reference to  FIG. 8 .  FIG. 8  is a flow chart showing the flow of processing of the sound processing method executed by the sound processing apparatus  10  according to the present embodiment. As shown in  FIG. 8 , first the sound recording unit  110  of the sound processing apparatus  10  records sound (S 102 ). 
     Next, the sound recording unit  110  determines whether sound has been input (S 104 ). If there has been no input sound at step S 104 , the sound recording unit  110  terminates processing. If there has been input sound at step S 104 , the sound separation unit  112  separates the input sound into a plurality of sounds (S 106 ). At step S 106 , the sound separation unit  112  may separate the input sound in units of blocks of a predetermined length. 
     Then, the identity determination unit  118  determines whether the input sound separated in units of blocks of a predetermined length at step S 106  is identical among a plurality of blocks (S 108 ). The identity determination unit  118  may determine the identity by using the distribution of amplitude information, volume, direction information and the like at discrete times of sounds in units of blocks separated at step S 104 . 
     Next, the sound type estimation unit  122  calculates volume information of each block (S 110 ) to estimate the sound type of each block (S 112 ). At step S 112 , the sound type estimation unit  122  separates the sound into a voice uttered by the operator, sound caused by an object, noise resulting from an operation of the operator, impulse sound, steady environmental sound and the like. 
     Next, the mixing ratio calculation unit  120  calculates a mixing ratio of each sound in accordance with the sound type estimated at step S 112  (S 114 ). The mixing ratio calculation unit  120  calculates a mixing ratio that reduces the volume of a dominant sound based on volume information calculated at step S 110  and sound type information calculated at step S 112 . 
     Then, the plurality of sounds separated at step S 106  is mixed using the mixing ratio of each sound calculated at step S 114  (S 116 ). In the foregoing, the sound separation method executed by the sound processing apparatus  10  has been described. 
     According to the above embodiment, as described above, an input sound input into the sound processing apparatus  10  is separated into sounds caused by a plurality of sound sources and a plurality of separated sound types is estimated. Then, a mixing ratio of each sound is calculated in accordance with the estimated sound type and each separated sound is remixed in the mixing ratio. Accordingly, volumes originating from different sound sources can independently be controlled. Moreover, a desired sound can be prevented from being made inaudible by being masked by a sound whose volume is higher than that of the desired sound. Also, the volume originating from each sound source can be adjusted to a desired volume without the need to arrange a microphone or the like for each different sound source. Further, even if the volume of a desired sound is different from block to block of a predetermined length, the volume can automatically be adjusted without any volume operation by the user. 
     It should be understood by those skilled in the art that various modifications, combinations, sub-combinations and alterations may occur depending on design requirements and other factors insofar as they are within the scope of the appended claims or the equivalents thereof. 
     In the above embodiment, for example, the present invention is described by applying to an imaging apparatus equipped with the sound processing apparatus  10 , but the present invention is not limited to such an example. For example, the present invention may be applied to a sound recording apparatus having no imaging function in general or a communication apparatus. 
     The present application contains subject matter related to that disclosed in Japanese Priority Patent Application JP 2008-283067 filed in the Japan Patent Office on Nov. 4, 2008, the entire content of which is hereby incorporated by reference.