Abstract:
An apparatus includes a first microphone disposed near a first seat in a car cabin, the first seat configured to seat a first occupant, a second microphone disposed near a second seat in the car cabin, the second seat configured to seat a second occupant, a processor, and a memory storing a computer program. The computer program, when executed, causes the processor to execute operations including: estimating a first mixed speech using a second signal acquired by the second microphone, the first mixed speech including a first speech that has been uttered by the second occupant and has entered the first microphone, and removing the first mixed speech from a first signal acquired by the first microphone.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The present disclosure relates to a speech collector and speech collection method in a car cabin. 
         [0003]    2. Description of the Related Art 
         [0004]    Japanese Unexamined Patent Application Publication No. 2003-114699 discloses detecting the directions of speakers while continuously changing directional characteristics of a microphone array, dividing speech data acquired by the microphone array into pieces of speech data corresponding to the directions of the speakers by signal processing, and recognizing speeches from the pieces of speech data (paragraphs [0027], [0028]). Thus, when speakers 1010, 1020, and 1030 are seated on a driver&#39;s seat, a passenger seat, and the like as shown in FIG. 10, Japanese Unexamined Patent Application Publication No. 2003-114699 can acquire speech data while easily changing directional characteristics ranges 1110, 1120, and 1130 of a microphone array 1000 and can easily recognize the speeches of the speakers 1010, 1020, and 1030 on the basis of the speech data (paragraph [0044], FIG. 8). 
       SUMMARY 
       [0005]    However, the related art disclosed in Japanese Unexamined Patent Application Publication No. 2003-114699 has room for further improvement. 
         [0006]    One non-limiting and exemplary embodiment provides a car-mounted speech collector and the like that when multiple speakers utter speeches simultaneously in a car cabin, prevent mixing into the speech of a particular speaker which is desired to be collected, of the speech of another speaker. 
         [0007]    In one general aspect, the techniques disclosed here feature an apparatus including a first microphone disposed near a first seat on which a first occupant is seated in a car cabin, a second microphone disposed near a second seat on which a second occupant is seated in the car cabin, a mixed speech estimation unit that estimates a mixed speech on the basis of a second signal acquired by the second microphone, the mixed speech being a speech that has been uttered by the second occupant and has entered the first microphone, and a noise remover that removes the mixed speech from a first signal acquired by the first microphone. 
         [0008]    It should be noted that general or specific embodiments may be implemented as a system, a method, an integrated circuit, a computer program, a storage medium, or any selective combination thereof. 
         [0009]    According to the present disclosure, when multiple occupants utter speeches simultaneously in a car cabin, it is possible to extract only the speech of a particular speaker which is desired to be collected and to improve the rate at which the speech of the particular speaker is recognized. 
         [0010]    Additional benefits and advantages of the disclosed embodiments will become apparent from the specification and drawings. The benefits and/or advantages may be individually obtained by the various embodiments and features of the specification and drawings, which need not all be provided in order to obtain one or more of such benefits and/or advantages. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a diagram showing an example of the overall configuration of a speech collection system to which a car-mounted speech collector of a first embodiment of the present disclosure is applied; 
           [0012]      FIG. 2  is a flowchart showing an example of a process performed by the car-mounted speech collector of the first embodiment of the present disclosure; 
           [0013]      FIG. 3  is a diagram showing an example of the overall configuration of a speech collection system to which a car-mounted speech collector of a second embodiment of the present disclosure is applied; 
           [0014]      FIG. 4  is a diagram showing a modification of the car-mounted speech collector of the second embodiment of the present disclosure; 
           [0015]      FIG. 5  is a diagram showing an example of the overall configuration of a speech collection system to which a car-mounted system of a third embodiment of the present disclosure is applied; 
           [0016]      FIG. 6  is a flowchart showing an example of a process performed by the car-mounted speech collector of the third embodiment of the present disclosure; 
           [0017]      FIG. 7  is a diagram showing an example of the overall configuration of a speech collection system to which a car-mounted speech collector of a fourth embodiment of the present disclosure is applied; 
           [0018]      FIG. 8  is a diagram showing a process in which a speech determination unit detects a speech section; 
           [0019]      FIG. 9  is a flowchart showing an example of a process performed by the car-mounted speech collector of the fourth embodiment of the present disclosure; and 
           [0020]      FIG. 10  is a diagram showing related art. 
       
    
    
     DETAILED DESCRIPTION 
     Underlying Knowledge Forming Basis of the Present Disclosure 
       [0021]    There has been commercialized a technology that collects a speech uttered by an occupant using a speech input device in a car cabin, recognizes a control command on the basis of the speech collected, and controls a corresponding device. The speech input device is typically a microphone mounted on a console called an overhead console, which is located on the ceiling and between the driver&#39;s seat and passenger seat. Thus, the occupant can input a speech in a hands-free manner. On the other hand, the microphone on the overhead console is several tens of centimeters away from the mouse of the speaker. For this reason, not only the speech of the speaker but also noise in the car cabin is more likely to enter the microphone. This results in problems such as the degradation of the speech collected by the microphone and a reduction in speech recognition rate. 
         [0022]    One conceivable solution to these problems is the use of a directional microphone. 
         [0023]    However, the directional microphone has some directional angle in its collection range and also has sensitivity to a direction other than the directivity direction, which is called a side lobe. For this reason, it has difficult in collecting only the speech of the target speaker. Further, the space in the car cabin is narrow, and many glass windows, which easily reflect sound, are disposed on the front, side, and rear of the car cabin. The front and side windows are near the occupants. For this reason, the speech of a speaker other than a particular speaker is reflected by the glass windows, and speeches from various directions reach the directional microphone. Consequently, when the multiple speakers utter speeches simultaneously, the directional microphone would collect the speech of the speaker other than the particular speaker even if the directivity of the directional microphone is directed toward the particular speaker, whose speech is desired to be collected. Thus, a conventional speech recognition device, which is assumed to recognize the speech of a single speaker, recognizes the speech of the particular speaker at a reduced rate. 
         [0024]    Japanese Unexamined Patent Application Publication No. 2003-114699 described above identifies the direction of a speaker while changing the direction of the directivity of the microphone array. 
         [0025]    However, as described above, the speech of the speaker flies about in various directions in the car cabin, and speeches from various directions enter the microphone array. For this reason, Japanese Unexamined Patent Application Publication No. 2003-114699 may determine that there are also speakers in directions other than the direction of that speaker, which is actually present in the car cabin. Accordingly, it cannot accurately extract the speech of the particular speaker in the car cabin. 
         [0026]    The present disclosure aims to solve these problems and provides a car-mounted speech collector and the like that can accurately extract the speech of a particular speaker in a car cabin. 
         [0027]    A car-mounted speech collector of one aspect of the present disclosure includes a first microphone disposed near a first seat on which a first occupant is seated in a car cabin, a second microphone disposed near a second seat on which a second occupant is seated in the car cabin, a mixed speech estimation unit that estimates a mixed speech on the basis of a second signal acquired by the second microphone, the mixed speech being a speech that has been uttered by the second occupant and has entered the first microphone, and a noise remover that removes the mixed speech from a first signal acquired by the first microphone. 
         [0028]    According to this configuration, the mixed speech, which is a speech that has been uttered by the second occupant and has entered the first microphone, is estimated on the basis of the second signal acquired by the second microphone and then removed from the first signal acquired by the first microphone. 
         [0029]    Thus, when multiple occupants utter speeches simultaneously in a car cabin, it is possible to extract only the speech of a particular speaker which is desired to be collected and to improve the rate at which the speech of the particular speaker is recognized. Also, even when a hands-free speech is uttered, it is possible to eliminate interference caused by a speech uttered by another occupant and to obtain a high recognition rate. 
         [0030]    In the above aspect, the car-mounted speech collector may further include a first switch disposed near the first seat and a second switch disposed near the second seat. (1) When the first switch is pressed, the mixed speech estimation unit may estimate a first mixed speech on the basis of the second signal, the first mixed speech being a speech that has been uttered by the second occupant and has entered the first microphone, and the noise remover may remove the first mixed speech from the first signal. (2) When the second switch is pressed, the mixed speech estimation unit may estimate a second mixed speech on the basis of the first signal, the second mixed speech being a speech that has been uttered by the first occupant and has entered the second microphone, and the noise remover may remove the second mixed speech from the second signal. 
         [0031]    According to this configuration, the car-mounted speech collector includes the first and second switches and therefore can accurately determine which of the first and second occupants is the particular speaker. 
         [0032]    In the above aspect, the car-mounted speech collector may further include a speech determination unit that determines whether any of the first and second signals includes a speech section. (1) If the speech determination unit determines that the first signal includes a speech section, the mixed speech estimation unit may estimate a first mixed speech using the second signal, the first mixed speech being a speech that has been uttered by the second occupant and has entered the first microphone, and the noise remover may remove the first mixed speech from the first signal. (2) If the speech determination unit determines that the second signal includes a speech section, the mixed speech estimation unit may estimate a second mixed speech using the first signal, the second mixed speech being a speech that has been uttered by the first occupant and has entered the second microphone, and the noise remover may remove the second mixed speech from the second signal. 
         [0033]    According to this configuration, if the first signal includes a speech section, the speech of the first occupant is extracted; if the second signal includes a speech section, the speech of the second occupant is extracted. Thus, it can be automatically determined which of the first and second occupants is the particular speaker. 
         [0034]    In the above aspect, the first and second signals may be previously assigned priorities. (3) If the speech determination unit determines that both the first and second signals include speech sections and if the priority of the first signal is higher than the priority of the second signal, the mixed speech estimation unit may estimate the first mixed speech, and the noise remover may remove the first mixed speech from the first signal. (4) If the speech determination unit determines that both the first and second signals include speech sections and if the priority of the second signal is higher than the priority of the first signal, the mixed speech estimation unit may estimate the second mixed speech, and the noise remover may remove the second mixed speech from the second signal. 
         [0035]    According to this configuration, if both the first and second signals include speech sections and if the priority of the first signal is higher than the priority of the second signal, the speech of the first occupant is extracted; if both the first and second signals include speech sections and if the priority of the second signal is higher than the priority of the first signal, the speech of the second occupant is extracted. Thus, when the first and second occupants utter speeches simultaneously, it is possible to prevent a failure to determine which of the first and second occupants is the particular speaker. 
         [0036]    In the above aspect, the first and second microphones may be unidirectional microphones. 
         [0037]    According to this configuration, the speech of the particular speaker can be accurately extracted. 
         [0038]    In the above aspect, the first and second microphones may be array microphones each consisting of multiple microphone units. 
         [0039]    According to this configuration, the speech of the particular speaker can be accurately extracted. 
         [0040]    In the above aspect, the mixed speech estimation unit may estimate the mixed speech using the adaptive filter whose filter coefficient has been corrected so that an output signal from the noise remover becomes uncorrelated with or independent of an input signal. 
         [0041]    According to this configuration, the mixed speech is estimated using the adaptive filter. Thus, the mixed speech can be accurately obtained. Also, the accuracy of the estimation of the mixed speech can be increased as the adaptive filter is used. 
         [0042]    In the above aspect, the speech determination unit may calculate envelopes of the first and second signals, detect bottom levels of the envelopes calculated, set a threshold on the basis of the bottom levels detected, and determine a section in which levels of the first and second signals are higher than the threshold set, as the speech section. 
         [0043]    According to this configuration, the threshold is set on the basis of the bottom levels of the envelopes. Thus, an appropriate threshold corresponding to the scene can be set compared to when the threshold is fixed. 
       First Embodiment 
       [0044]      FIG. 1  is a diagram showing an example of the overall configuration of a speech collection system to which a car-mounted speech collector  100  of a first embodiment of the present disclosure is applied. The speech collection system includes the car-mounted speech collector  100  and a postprocessing device  200 . 
         [0045]    The car-mounted speech collector  100  includes microphones  11  and  12 , a mixed speech estimation unit  2 , and a noise remover  3 . The microphone  11  (an example of a first microphone) is disposed near a seat  51  (an example of a first seat) on which an occupant P 1  (an example of a first occupant) is seated in a car cabin  50  and mainly collects the speech of the occupant P 1 . The microphone  12  (an example of a second microphone) is disposed near a seat  52  (an example of a second seat) on which an occupant P 2  (an example of a second occupant) is seated in the car cabin  50  and mainly collects the speech of the occupant P 2 . 
         [0046]    In the example shown in  FIG. 1 , the microphone  11  is disposed above the occupant P 1  and near the ceiling of the car cabin  50 , and the microphone  12  is disposed above the occupant P 2  and near the ceiling of the car cabin  50 . For example, the microphone  11  is mounted on a sun visor for the occupant P 1  on the ceiling, and the microphone  12  is mounted on a sun visor for the occupant P 2  on the ceiling. The microphone  11  may be mounted on a dashboard in front of the occupant P 1 , and the microphone  12  may be mounted on a dashboard in front of the occupant P 2 . The microphones  11  and  12  may also be mounted on a center console between the seats  51  and  52  or on an overhead console disposed above the center console and on the ceiling. The microphones  11  and  12  may be disposed in any positions in the car cabin  50  as long as they can collect the speeches of the occupants P 1  and P 2 . 
         [0047]    The microphones  11  and  12  are unidirectional microphones or array microphones. If the microphones  11  and  12  are unidirectional microphones, the microphones are disposed, for example, in such a manner that the directivities thereof are directed toward the heads of the occupants P 1  and P 2 , respectively. An array microphone is a microphone in which multiple microphones are arranged close to each other in an array and whose directivity can be directed in any direction by signal processing. If the microphones  11  and  12  are array microphones, the microphones previously detect the directions of the occupants P 1  and P 2 , and the directions of directivities thereof are determined so as to be directed in the directions detected. This process is performed by, for example, a processor or CPU (not shown). 
         [0048]    The mixed speech estimation unit  2  estimates a mixed speech signal S 2 ′ that has been generated by the occupant P 2  and has entered the microphone  11 , on the basis of a signal S 2  (an example of a second signal) acquired by the microphone  12 . 
         [0049]    The mixed speech estimation unit  2  includes an adaptive filter  2   a  and estimates the mixed speech signal S 2 ′ using the adaptive filter  2   a.  Specifically, the mixed speech estimation unit  2  corrects the filter coefficient of the adaptive filter  2   a  using a predetermined adaptive algorithm so that a signal SO (an example of an output signal) outputted from the noise remover  3  becomes uncorrelated with or independent of the input of the adaptive filter  2   a.  As will be described later, the signal SO is a signal obtained by subtracting the mixed speech signal S 2 ′ from a signal S 1  (an example of a first signal) acquired by the microphone  11 . By correcting the filter coefficient of the adaptive filter  2   a  so that the signal SO becomes uncorrelated with or independent of the input of the adaptive filter  2   a,  the mixed speech signal S 2 ′, which has been generated by the occupant P 2  and mixed into a speech uttered by the occupant P 1  and which is contained in the signal S 1 , is outputted from the adaptive filter  2   a.    
         [0050]    The mixed speech estimation unit  2  may perform this filter coefficient correction process periodically or each time the microphone  12  acquires a signal having a predetermined level or higher. The predetermined adaptive algorithm may be the least-mean-square (LMS) algorithm or independent component analysis (ICA) algorithm. The adaptive filter may be an LMS filter. 
         [0051]    The noise remover  3  generates the signal SO by subtracting the mixed speech signal S 2 ′ from the signal S 1 . The mixed speech signal S 2 ′ is a speech which has been uttered by the occupant P 2  and which is contained in the signal S 1 . Accordingly, when the occupants P 1  and P 2  utter speeches simultaneously, the signal SO indicates the speech uttered by the occupant P 1 , which is the particular speaker. 
         [0052]    In  FIG. 1 , the mixed speech estimation unit  2  and noise remover  3  may be a processor such as a DSP, or may be implemented by the execution of a control program stored in ROM by a CPU. This also applies to later embodiments. 
         [0053]    The postprocessing device  200  is, for example, a speech recognition device and recognizes a speech using the signal SO. The speech recognition device recognizes the speech using, for example, a known speech recognition method. Since the signal SO indicates the speech uttered by the occupant P 1  in the example shown in  FIG. 1 , the speech recognition device recognizes the speech uttered by the occupant P 1 . 
         [0054]    Examples of speeches which can be recognized include control commands for controlling devices included in a vehicle  5 . Examples of the devices included in the vehicle  5  include an air conditioner, a navigation system, a cruse controller, a television, and lights. For example, if the speech recognition device recognizes a control command for controlling the temperature of the air conditioner, the temperature of the air conditioner is set to a temperature indicated by the control command recognized. 
         [0055]    Instead of the speech recognition device, the postprocessing device  200  may be, for example, a hands-free device, which is a device used by an occupant to operate a mobile phone by speech. For example, the hands-free device transmits the signal SO to a mobile phone using a communication system such as Bluetooth®. 
         [0056]    The vehicle  5  is, for example, a four-wheel car and includes the car cabin  50 . In the example shown in  FIG. 1 , the vehicle  5  is a five-seater passenger car, but not limited thereto. It may be a two-, seven-, or other number-seater passenger car, or may be a bus or truck rather than a passenger car. 
         [0057]    The car cabin  50  includes seats  51  to  54 . The seat  51  is a driver&#39;s seat which is disposed behind a steering wheel  55  and on which the occupant P 1  as a driver is seated. 
         [0058]    The seat  52  is a passenger seat which is disposed on the left side of the seat  51  in a top view and on which the occupant P 2  is seated. The seats  53  and  54  are rear seats disposed behind the seats  51  and  52 , respectively. 
         [0059]      FIG. 2  is a flowchart showing an example of a process performed by the car-mounted speech collector  100  of the first embodiment of the present disclosure. The process shown in  FIG. 2  may be performed at predetermined time intervals or may be performed when the microphones  11  and  12  collect speeches having a predetermined level or higher. This also applies to processes shown by flowcharts in  FIGS. 6 and 9  (to be discussed later). First, the microphones  11  and  12  acquire signals S 1  and S 2 , respectively (S 301 ). 
         [0060]    Then, the mixed speech estimation unit  2  estimates a mixed speech signal S 2 ′ indicating a speech uttered by the occupant P 2  and contained in the signal S 1 , by filtering the signal S 2  using the adaptive filter  2   a  (S 302 ). 
         [0061]    Then, the noise remover  3  generates a signal SO by subtracting the mixed speech signal S 2 ′ from the signal S 1 . 
         [0062]    As seen above, according to the present embodiment, when multiple occupants utter speeches simultaneously in a car cabin, it is possible to extract only the speech of a particular speaker which is desired to be collected and to improve the rate at which the speech of the particular speaker is recognized. 
         [0063]    While the occupant P 1  is the particular speaker in the above description, the occupant P 2  may be the particular speaker. In this case, it is only necessary to connect the mixed speech estimation unit  2  between the microphone  11  and noise remover  3  and to connect the microphone  12  to the noise remover  3 . 
       Second Embodiment 
       [0064]    A second embodiment is characterized in that microphones are disposed near front seats, as well as near rear seats. In the present embodiment, the same elements as those in the first embodiment are given the same reference signs and will not be described.  FIG. 3  is a diagram showing an example of the overall configuration of a speech collection system to which a car-mounted speech collector  100  of a second embodiment of the present disclosure is applied. 
         [0065]      FIG. 3  differs to  FIG. 1  in that microphones  13  and  14  are disposed near seats  53  and  54  as rear seats and that three mixed speech estimation units,  21 ,  22 , and  23 , are disposed so as to correspond to microphones  12 ,  13 , and  14 . 
         [0066]    While an occupant P 1  is a particular speaker in the example shown in  FIG. 3 , one of occupants P 2 , P 3 , and P 4  may be the particular speaker. 
         [0067]    The microphone  13  is disposed, for example, above the seat  53  and near the ceiling and mainly collects the speech of the occupant P 3 . The microphone  14  is disposed, for example, above the seat  54  and near the ceiling and mainly collects the speech of the occupant P 4 . As in the first embodiment, the microphones  13  and  14  may be unidirectional microphones or array microphones. The directivities of the microphones  13  and  14  are directed toward the occupants P 3  and P 4 , respectively. 
         [0068]    Hereafter, a signal acquired by the microphone  13  will be referred to as a signal S 3 , and a signal acquired by the microphone  14  as a signal S 4 . 
         [0069]    The mixed speech estimation units  21 ,  22 , and  23  include adaptive filters  21   a,    21   b,  and  21   c,  respectively. Using a signal S 2 , the mixed speech estimation unit  21  estimates a mixed speech signal S 2 ′ indicating a speech which has been uttered by the occupant P 2  and has entered the microphone  11 . Specifically, as in the first embodiment, the mixed speech estimation unit  21  corrects the filter coefficient of the adaptive filter  21   a  using a predetermined adaptive algorithm so that a signal SO becomes uncorrelated with or independent of the input of the adaptive filter  2   a  and estimates the mixed speech signal S 2 ′ by filtering the signal S 2  using the adaptive filter  21   a  thus corrected. Since the mixed speech signal S 2 ′ is a signal estimated using the signal S 2 , it mainly indicates the speech of the occupant P 2  mixed into the signal S 1 . 
         [0070]    As with the mixed speech estimation unit  21 , the mixed speech estimation units  22  and  23  estimate mixed speech signals S 3 ′ and S 4 ′ using the signals S 3  and S 4 , respectively. Since the mixed speech signals S 3 ′ and S 4 ′ are signals estimated using the signals S 3  and S 4 , respectively, they mainly indicate the speeches of the occupants P 3  and P 4 , respectively, mixed into the signal S 1 . 
         [0071]    The noise remover  3  generates the signal SO by subtracting the mixed speech signals S 2 ′, S 3 ′, and S 4 ′ from the signal S 1 . In this way, the signals other than the speech uttered by the occupant P 1 , which is the particular speaker, are removed as noise from the signal S 1 , and the resulting signal is obtained as the signal SO. 
         [0072]    As seen above, according to the third embodiment, even when the microphones  13  and  14  are disposed near the rear seats  53  and  54 , respectively, only the speech of the particular speaker can be extracted. 
         [0073]    If an occupant other than the occupant P 1 , that is, one of the occupants P 2 , P 3 , and P 4  is the particular speaker in  FIG. 3 , it is only necessary to connect the mixed speech estimation units  21 ,  22 , and  23  to the three microphones corresponding to the occupants other than the one occupant. 
         [0074]      FIG. 4  is a diagram showing a modification of the car-mounted speech collector  100  of the second embodiment of the present disclosure. In  FIG. 3 , the microphones  11  to  14  are disposed in the horizontal centers of the corresponding seats in a top view. In  FIG. 4 , on the other hand, microphones  11  to  14  are disposed so as to sandwich a center line passing through the horizontal center of a car cabin  50  and be horizontally close to each other in a top view. If the aspect shown in  FIG. 4  is used, the microphones  11  and  12  may be disposed on an overhead console for front seats on the ceiling of the car cabin  50 , and the microphones  13  and  14  may be disposed on an overhead console for rear seats on the ceiling of the car cabin  50 . Whichever of the aspects in  FIGS. 3 and 4  is used, the directivities of the microphone  11  to  14  are set toward the heads of the corresponding occupants, P 1  to P 4 . 
       Third Embodiment 
       [0075]    A car-mounted speech collector  100  of a third embodiment is characterized in that it includes switches for specifying a particular speaker.  FIG. 5  is a diagram showing an example of the overall configurlation of a car-mounted system to which the car-mounted speech collector  100  of the third embodiment of the present disclosure is applied. In the present embodiment, the same elements as those in the first embodiment are given the same reference signs and will not be described. 
         [0076]      FIG. 5  differs from  FIG. 1  in that the car-mounted speech collector  100  includes switches SW 1  and SW 2  and selectors  31  and  32 . The switches SW 1  and SW 2  are, for example, push-button switches and are disposed near seats  51  and  52 , respectively. In the example shown in  FIG. 5 , the switches SW 1  and SW 2  are disposed on an overhead console on the ceiling in a car cabin  50  so as to sandwich a center line passing through the horizontal center of the car cabin  50  and be horizontally close to each other in a top view. However, the switches SW 1  and SW 2  need not be disposed in such positions and may be disposed in any positions as long as occupants P 1  and P 2  can operate the switches SW 1  and SW 2 , respectively. For example, the switches SW 1  and SW 2  may be disposed on a dashboard or center console, or on the back sides of the seats  51  and  52 . 
         [0077]    The switches SW 1  and SW 2  are configured to be recessible into the ceiling and, for example, are each turned on when recessed into the ceiling and turned off when becoming flush with the ceiling. The switches SW 1  and SW 2  are also configured to be exclusively turned on so that they are not turned on simultaneously. When the switch SW 1  is turned on, the occupant P 1  is set as a particular speaker; when the switch SW 2  is turned on, the occupant P 2  is set as a particular speaker. Hereafter, it is assumed that the switches SW 1  and SW 2  are not turned on simultaneously. 
         [0078]    When the switch SW 1  is on and the switch SW 2  is off, the selector  31  selects a signal S 1  and outputs it to a noise remover  3 , and the selector  32  selects a signal S 2  and outputs it to a mixed speech estimation unit  2 . When the switch SW 1  is off and the switch SW 2  is on, the selector  31  selects the signal S 2  and outputs it to the noise remover  3 , and the selector  32  selects the signal S 1  and outputs it to the mixed speech estimation unit  2 . 
         [0079]    The selectors  31  and  32  may be a DSP, or may be implemented by the execution of control program stored in ROM by a CPU. This also applies to a fourth embodiment. 
         [0080]    When the mixed speech estimation unit  2  receives the signal S 2 , it estimates a mixed speech signal S 2 ′ (an example of a first mixed speech signal) using an adaptive filter  2 α. The mixed speech signal S 2 ′ indicates a speech uttered by the occupant P 2  and contained in the signal S 1 . At this time, as in the first embodiment, the mixed speech estimation unit  2  corrects the correction coefficient of the adaptive filter  2 α using a predetermined adaptive algorithm so that a signal SO becomes uncorrelated with or independent of the input of the adaptive filter  2 α. 
         [0081]    On the other hand, when the mixed speech estimation unit  2  receives the signal S 1 , it estimates a mixed speech signal S 1 ′ (an example of a second mixed speech signal) using the adaptive filter  2 α. The mixed speech signal S 1 ′ indicates a speech uttered by the occupant P 1  and contained in the signal S 2 . At this time, the mixed speech estimation unit  2  corrects the correction coefficient of the adaptive filter  2 α using a predetermined adaptive algorithm so that a signal SO becomes uncorrelated with or independent of the input of the adaptive filter  2 α. 
         [0082]      FIG. 6  is a flowchart showing an example of a process performed by the car-mounted speech collector  100  of the third embodiment of the present disclosure. First, the microphones  11  and  12  acquire signals S 1  and S 2 , respectively (S 601 ). 
         [0083]    If the switch SW 1  is on (YES in S 602 ), the selector  31  selects the signal S 1  and outputs it to the noise remover  3 , and the selector  32  selects the signal S 2  and outputs it to the mixed speech estimation unit  2  (S 603 ). 
         [0084]    The mixed speech estimation unit  2  filters the signal S 2  using the adaptive filter  2 α, estimates a mixed speech signal S 2 ′, and outputs it to the noise remover  3  (S 604 ). 
         [0085]    The noise remover  3  generates a signal SO by subtracting the mixed speech signal S 2 ′ from the signal S 1  (S 605 ). 
         [0086]    If the switch SW 1  is off and the switch SW 2  is on (NO in S 602  and YES in S 606 ), the selector  31  selects the signal S 2  and outputs it to the noise remover  3 , and the selector  32  selects the signal S 1  and outputs it to the mixed speech estimation unit  2  (S 607 ). 
         [0087]    The mixed speech estimation unit  2  filters the signal S 1  using the adaptive filter  2 α, estimates a mixed speech signal S 1 ′, and outputs it to the noise remover  3  (S 608 ). 
         [0088]    The noise remover  3  generates a signal SO by subtracting the mixed speech signal S 1 ′ from the signal S 2  (S 609 ). 
         [0089]    If the switch SW 1  is off and the switch SW 2  is off (NO in S 602  and NO in S 606 ), the process ends. 
         [0090]    As seen above, in the present embodiment, the car-mounted speech collector  100  includes the switches SW 1  and SW 2  and thus can accurately determine which of the occupants P 1  and P 2  is the particular speaker. 
         [0091]    While, in the flowchart shown in  FIG. 6 , the process ends if the switch SW 1  is off and the switch SW 2  is off (NO in S 602  and NO in S 606 ), the present disclosure may be configured otherwise. For example, if the switch is off and the switch SW 2  is off, the selectors  31  and  32  may determine that the occupant P 1  is the particular speaker and select the signals S 1  and S 2 , respectively, or may determine that the occupant P 2  is the particular speaker and select the signals S 2  and S 1 , respectively. If the signals S 1  and S 2  are assigned priorities as described in a fourth embodiment (to be discussed later), the selectors  31  and  32  may determine that an occupant corresponding to the signal S 1  or S 2  having a higher priority is the particular speaker. The selectors  31  and  32  may also determine that an occupant corresponding to the signal S 1  or S 2  having a higher level is the particular speaker. Note that the priorities are previously assigned and stored in, for example, a storage device. The priorities may be changeable on the basis of an operation of any occupant. 
         [0092]    While the two microphones,  11  and  12 , are provided in the example shown in  FIG. 5 , the present disclosure may be configured otherwise. As shown in  FIG. 3 , microphones  13  and  14  may be provided. 
       Fourth Embodiment 
       [0093]    A car-mounted speech collector  100  of a fourth embodiment is characterized in that it determines an occupant uttering a speech as a particular speaker.  FIG. 7  is a diagram showing an example of the overall configuration of a speech collection system to which the car-mounted speech collector  100  of the fourth embodiment of the present disclosure is applied. In the present embodiment, the same elements as those in the first embodiment are given the same reference signs and will not be described. 
         [0094]      FIG. 7  differs from  FIG. 1  in that the car-mounted speech collector  100  includes a speech determination unit  6  and selectors  31  and  32 . The speech determination unit  6  determines whether any of signals S 1  and S 2  includes a speech section.  FIG. 8  is a diagram showing a process in which the speech determination unit  6  detects a speech section. The vertical axis of  FIG. 8  represents the level of a signal (hereafter referred to as a “speech signal Sig 1 ”) acquired by microphones  11  and  12 , and the horizontal axis thereof represents the time. 
         [0095]    The speech signal Sig 1  has a waveform in which the amplitude varies at high frequencies. The speech determination unit  6  sets a processing section ΔT in the speech signal Sig 1 . The processing section ΔT is set so as to be shifted at predetermined time intervals along the time axis. The speech determination unit  6  then obtains an envelope Sig 2  of the speech signal Sig 1  in the processing section T. While the envelope Sig 2  is set over the speech signal Sig 1  in  FIG. 8 , it may be set thereunder. 
         [0096]    The speech determination unit  6  then detects the bottom level of the envelope Sig 2 . The speech determination unit  6  then sets, as a threshold Th, a value obtained by multiplying the bottom level A 1  by a predetermined factor. At this time, the bottom level A 1  is multiplied by a number greater than 1 (e.g., 2, 3). 
         [0097]    The speech determination unit  6  then determines a section in which the level of the speech signal Sig 1  is equal to or greater than the threshold Th, as a speech section ΔT 1 . 
         [0098]    The speech determination unit  6  may be a DSP, or may be implemented by the execution of a control program stored in ROM by a CPU. 
         [0099]    Referring back to  FIG. 7 , if the speech determination unit  6  determines that a signal S 1  includes a speech section, the selector  31  selects the signal S 1  and outputs it to a noise remover  3 , and the selector  32  selects a signal S 2  and outputs it to a mixed speech estimation unit  2 . 
         [0100]    If the speech determination unit  6  determines that the signal S 2  includes a speech section, the selector  31  selects the signal S 2  and outputs it to the noise remover  3 , and the selector  32  selects the signal S 1  and outputs it to the mixed speech estimation unit  2 . 
         [0101]    If the speech determination unit  6  determines that both the signals S 1  and S 2  include speech sections and if the signal S 1  has a higher priority than the signal S 2 , the selector  31  selects the signal S 1  and outputs it to the noise remover  3 , and the selector  32  selects the signal S 2  and outputs it to the mixed speech estimation unit  2 . 
         [0102]    If the speech determination unit  6  determines that both the signals S 1  and S 2  include speech sections and if the signal S 2  has a higher priority than the signal S 1 , the selector  31  selects the signal S 2  and outputs it to the noise remover  3 , and the selector  32  selects the signal S 1  and outputs it to the mixed speech estimation unit  2 . 
         [0103]    As in the third embodiment, the mixed speech estimation unit  2  includes an adaptive filter  2 α. When the mixed speech estimation unit  2  receives the signal S 2  selected by the selector  32 , it estimates a mixed speech signal S 2 ′ (an example of a first mixed speech signal) using the adaptive filter  2 α. When the mixed speech estimation unit  2  receives the signal S 1  selected by the selector  32 , it estimates a mixed speech signal S 1 ′ (an example of a second mixed speech signal) using the adaptive filter  2 α. 
         [0104]      FIG. 9  is a flowchart showing an example of a process performed by the car-mounted speech collector  100  of the fourth embodiment of the present disclosure. First, the microphones  11  and  12  acquire signals S 1  and S 2 , respectively (S 901 ). Then, the speech determination unit  6  detects speech sections in the signals S 1  and S 2  (S 902 ). 
         [0105]    If only the signal S 1  includes a speech section (YES in S 903  and NO in S 904 ), the selector  31  selects the signal S 1  and outputs it to the noise remover  3 , and the selector  32  selects the signal S 2  and outputs it to the mixed speech estimation unit  2  (S 906 ). 
         [0106]    Then, the mixed speech estimation unit  2  filters the signal S 2  using the adaptive filter  2 α, estimates a mixed speech signal S 2 ′, and outputs it to the noise remover  3  (S 907 ). 
         [0107]    Then, the noise remover  3  generates a signal SO by subtracting the mixed speech signal S 2 ′ from the signal S 1  (S 908 ). 
         [0108]    If only the signal S 2  includes a speech section (NO in S 903  and YES in S 910 ), the selector  31  selects the signal S 2  and outputs it to the noise remover  3 , and the selector  32  selects the signal S 1  and outputs it to the mixed speech estimation unit  2  (S 911 ). 
         [0109]    Then, the mixed speech estimation unit  2  filters the signal S 1  using the adaptive filter  2 α, estimates a mixed speech signal S 1 ′, and outputs it to the noise remover  3  (S 912 ). 
         [0110]    Then, the noise remover  3  generates a signal SO by subtracting the mixed speech signal S 1 ′ from the signal S 2  (S 913 ). 
         [0111]    If both the signals S 1  and S 2  include speech sections (YES in S 903  and YES in S 904 ) and if the signal S 1  has a higher priority than the signal S 2  (YES in S 905 ), S 906 , S 907 , and S 908  described above are performed. 
         [0112]    On the other hand, if the signal S 2  has a higher priority than the signal S 1  (NO in S 905 ), S 911 , S 912 , and S 913  described above are performed. 
         [0113]    If none of the signals S 1  and S 2  includes a speech section (NO in S 903  and NO in S 910 ), the process ends. 
         [0114]    As seen above, according to the present embodiment, if the signal S 1  includes a speech section, the speech of the occupant P 1  is extracted; if the signal S 2  includes a speech section, the speech of the occupant P 2  is extracted. Thus, it can be automatically determined which of the occupants P 1  and P 2  is the particular speaker. 
         [0115]    If both the signals S 1  and S 2  include speech sections, the speech of one of the occupants P 1  and P 2  is extracted in accordance with the priorities of the signals S 1  and S 2 . Thus, when the occupants P 1  and P 2  utter speeches simultaneously, it is possible to prevent a failure to determine which of the occupants P 1  and P 2  is the particular speaker. 
         [0116]    While, in the present embodiment, if both the signals S 1  and S 2  include speech sections, the signal to be noise-removed is determined in accordance with the priorities of the signals S 1  and S 2 , the present disclosure may be configured otherwise. For example, a signal having a higher level, of the signals S 1  and S 2  may be determined as a signal to be noise-removed. In this case, the speech determination unit  6  may calculate the average levels of the signals in the speech sections and determine a signal having a higher average level as a signal to be noise-removed. If the speech determination unit  6  determines that the signal S 1  has a higher level than the signal S 2 , the selectors  31  and  32 , mixed speech estimation unit  2 , and noise remover  3  may perform the above process performed when the signal S 1  has a higher priority than the signal S 2 . If the speech determination unit  6  determines that the signal S 2  has a higher level than the signal S 1 , the selectors  31  and  32 , mixed speech estimation unit  2 , and noise remover  3  may perform the above process performed when the signal S 2  has a higher priority than the signal S 1 . 
         [0117]    While the car-mounted speech collector  100  includes the two microphones,  11  and  12 , in the example shown in  FIG. 7 , the present disclosure may be configured otherwise. As shown in  FIG. 3 , it may further include microphones  13  and  14 . 
         [0118]    While, in the first to fourth embodiments, the microphones and switches are disposed near the seats, they may be disposed on the seats. For example, the microphones and switches may be disposed on the headrests or the like of the seats. Also, the microphones and switches of the rear seats may be disposed on the surfaces adjacent to the rear seats, of the front seats. That is, in the present disclosure, the expression “near the seat” refers to a concept including “on the seat.” 
         [0119]    The car-mounted speech collectors of the present disclosure can, when multiple occupants utter speeches simultaneously in a car cabin, extract only the speech of a particular speaker which is desired to be collected. For this reason, the car-mounted speech collectors are useful as a technology that recognizes a speech in a car cabin.