Abstract:
The present invention provides an audio apparatus including: a decoding device configured to decode results of channel-by-channel reproduction of a multichannel sound source made up of at least a left channel, a right channel, and a center channel, and to down-mix the decoded results of channel-by-channel reproduction in accordance with the number of speakers configured in a speaker system without a center speaker corresponding to the center channel; an audio processing device configured to perform predetermined audio processing on the decoded results having undergone the down-mixing by the decoding device, and to get the processed results output from the speaker system; and a control device configured to control the audio processing device.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    The present application claims priority from Japanese Patent Application No. JP JP 2009-053593 filed in the Japanese Patent Office on Mar. 6, 2009, the entire content of which is incorporated herein by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an audio apparatus and an audio processing method. More particularly, the invention relates to an audio apparatus and an audio processing method for illustratively allowing an in-car audio apparatus to reproduce and output multichannel (e.g., 5.1 channel) audio signals. 
         [0004]    2. Description of the Related Art 
         [0005]    Typically, when AV (Audio Visual) contents of movies or the like including a multichannel (e.g., 5.1 channel) audio contents are reproduced by a 5.1 channel speaker system, the sound from a particular channel may not be heard properly due to the conditions of the audio equipment in use, surrounding environment status, or recording level variations. 
         [0006]    In that case, the speaker volume of the channel in question need only be turned up because the number of channels found in the audio contents corresponds to the number of speakers configured in the speaker system in use on a one-to-one basis. 
         [0007]    However, not all users possess the 5.1 channel audio reproduction environment; there still exist numerous users not in possession of surround speakers or center speakers. In such cases, if the speaker system of the audio apparatus in question does not have a center speaker, the signal of the channel for the center speaker is assigned to the signals for other speakers through down-mixing, whereby all signals of the audio contents of interest are reproduced and output. 
         [0008]    Specifically, as shown in  FIG. 1 , an ordinary car audio apparatus  1  reproduces from a disk-type recording medium  2  a sound source constituted by 5.1 channel audio contents D 1  (made up of a left speaker channel signal component L, a right speaker channel signal component R, a center speaker channel signal component C, a left surround speaker channel signal component Ls, a right surround speaker channel signal content Rs, and a subwoofer channel signal component LFE). The audio contents D 1  thus reproduced are forwarded to an audio decoder  4  of a DSP (Digital Signal Processor)  3 . 
         [0009]    Until it is attached to a car, the car audio apparatus  1  does not recognize a speaker system SS 1  of that car (e.g., as a 5.1, 4.1, 4 or 2 channel configuration). The car audio apparatus  1  recognizes the configuration of the speaker system SS 1  in response to operation buttons  13  being pressed by the user. 
         [0010]    That is, the speaker system SS 1  of the car may illustratively have a 4.1 channel configuration (with a left speaker  7 , a right speaker  8 , a left surround speaker  9 , a right surround speaker  10 , and a subwoofer  11 ). This configuration is recognized by a microcomputer  12  in response to the operation buttons  13  being pressed by the user. 
         [0011]    The microcomputer  12  of the car audio apparatus  1  then notifies, by a control signal S 1 , the audio decoder  4  in the DSP (Digital Signal Processor)  3  that the onboard speaker system SS 1  has the 4.1 channel configuration. 
         [0012]    In turn, the audio decoder  4  down-mixes the 5.1 channel audio contents D 1  into 4.1 channel contents to comply with the number of speakers of the speaker system SS 1  in accordance with a predetermined method. 
         [0013]    Specifically, the audio decoder  4  mixes the left speaker channel signal component L with the center speaker channel signal component C, and the right speaker channel signal component R with the center speaker channel signal component C. In this manner, the 5.1 channel audio contents D 1  are down-mixed into the 4.1 channel audio contents. 
         [0014]    That is, the audio decoder  4  sends to the audio postprocessor  5  audio data D 2  constituted by a mixed channel signal component L+C (made up of the left speaker channel signal component L and center speaker channel signal component C), a mixed channel signal component R+C (composed of the right speaker channel signal component R and center speaker channel signal component C), the left surround speaker channel signal component Ls, right surround speaker channel signal component Rs, and subwoofer channel signal component LFE. 
         [0015]    Given from the audio decoder  4  the audio data D 2  (L+C, R+C, Ls, Rs, LFE) down-mixed to 4.1 channel data, the audio postprocessor  5  performs such processes as equalizer control and time alignment control on the received data D 2 . The audio data D 4  thus processed is fed to a power amplifier  6 . 
         [0016]    The power amplifier  6  amplifies the audio data D 2  (L+C, R+C, Ls, Rs, LFE) to a predetermined level. The amplified audio data D 2  is eventually reproduced as a 4.1 channel sound output through the left speaker  7 , right speaker  8 , left surround speaker  9 , right surround speaker  10 , and subwoofer  11 . 
         [0017]    There have been proposed information apparatuses for down-mixing multitrack data into 2 channel or 5.1 channel audio data to be reproduced and output. One such information apparatus is disclosed illustratively in Japanese Patent Laid-Open No. 2005-70545. 
       SUMMARY OF THE INVENTION 
       [0018]    In the above-described car audio apparatus  1 , the center speaker channel signal component C is assigned to the left speaker channel signal component L and to the right speaker channel signal component R through the down-mixing process. It may be thus impossible to amplify solely the center speaker channel signal component C. 
         [0019]    As a result, the car audio apparatus  1  can only amplify the mixed channel signal component L+C and mixed channel signal component R+C subsequent to the down-mixing process. The problem remains that the volume of the center speaker channel signal component C may not be controlled singly. 
         [0020]    The present invention has been made in view of the above circumstances and provides an audio apparatus and an audio processing method whereby, even if the number of channels found in a given sound source is larger than the number of speakers configured in the speaker system for reproducing the source, the reproduced volume of the channel for which the corresponding speaker does not exist can be controlled singly. 
         [0021]    In carrying out the present invention and according to one embodiment thereof, there is provided an audio apparatus including: a decoding device configured to decode results of channel-by-channel reproduction of a multichannel sound source made up of at least a left channel, a right channel, and a center channel, and to down-mix the decoded results of channel-by-channel reproduction in accordance with the number of speakers configured in a speaker system without a center speaker corresponding to the center channel; an audio processing device configured to perform predetermined audio processing on the decoded results having undergone the down-mixing by the decoding device, and to get the processed results output from the speaker system; and a control device configured to control the audio processing device; wherein the control device causes the decoding device to decode the results of channel-by-channel reproduction of the sound source, without performing the down-mixing in accordance with the number of speakers configured in the speaker system; and the control device causes the audio processing device to gain-control within the sound source solely a center channel signal component corresponding to the center speaker not found in the speaker system, and to mix the gain-controlled center channel signal component with a left channel signal component and a right channel signal component corresponding to the left channel and the right channel respectively, thereby carrying out the down-mixing process for output through the speaker system. 
         [0022]    According to another embodiment of the present invention, there is provided an audio processing method for use with a decoding device configured to decode results of channel-by-channel reproduction of a multichannel sound source made up of at least a left channel, a right channel, and a center channel, and to down-mix the decoded results of channel-by-channel reproduction in accordance with the number of speakers configured in a speaker system without a center speaker corresponding to the center channel; an audio processing device; and a control device; the audio processing method including the steps of: causing the control device to control the decoding device so as to decode the results of channel-by-channel reproduction of the sound source without performing the down-mixing in accordance with the number of speakers configured in a speaker system; and causing the control device to control the audio processing device so as to gain-control within the sound source solely a center channel signal component corresponding to the center speaker not found in the speaker system, and to mix the gain-controlled center channel signal component with a left channel signal component and a right channel signal component corresponding to the left channel and the right channel respectively, thereby carrying out the down-mixing process for output through the speaker system. 
         [0023]    According to the above-outlined embodiments of the present invention, without getting the decoding device to down-mix the center channel signal component, the control device causes the audio processing device to amplify solely the center channel signal component and mix the amplified center channel signal component with the left channel signal component and right channel signal component in a down-mixing process. Thus even if the number of speakers configured in the speaker system is smaller than the number of channels found in the multichannel sound source, the center channel signal component corresponding to the center speaker not found in the speaker system can be gain-controlled before being output. 
         [0024]    According to a further embodiment of the present invention, there is provided an audio apparatus including: a decoding device configured to decode results of channel-by-channel reproduction of a multichannel sound source and to down-mix the decoded results of channel-by-channel reproduction in accordance with the number of speakers configured in a speaker system; an audio processing device configured to perform predetermined audio processing on the decoded results having undergone the down-mixing by the decoding device, and to get the processed results output from the speaker system; and a control device configured to control the audio processing device; wherein the control device causes the decoding device to decode the results of channel-by-channel reproduction of the sound source, without performing the down-mixing in accordance with the number of speakers configured in the speaker system; and wherein the control device causes the audio processing device to gain-control within the sound source solely a particular channel signal component corresponding to a particular speaker not found in the speaker system, and to mix the gain-controlled particular channel signal component with the other channel signal components of the sound source, thereby carrying out the down-mixing process for output through the speaker system. 
         [0025]    According to the embodiment of the present invention outlined immediately above, without getting the decoding device to down-mix the particular channel signal component corresponding to the speaker not found in the speaker system, the control device causes the audio processing device to amplify the particular channel signal component and then to mix the amplified particular channel signal component with the other channel signal components. Thus even if the number of speakers configured in the speaker system is smaller than the number of channels found in the multichannel sound source, the particular channel signal component corresponding to the particular speaker not found in the speaker system can be gain-controlled before being output. 
         [0026]    As outlined above, the present invention provides an audio apparatus and an audio processing method whereby, without getting the decoding device to down-mix the center channel signal component, the audio processing device is caused to amplify solely the center channel signal component and mix the amplified center channel signal component with the left channel signal component and right channel signal component in a down-mixing process. Thus even if the number of speakers configured in the speaker system is smaller than the number of channels found in the multichannel sound source, the center channel signal component corresponding to the center speaker not found in the speaker system can be gain-controlled before being output. In other words, where the number of channels found in the sound source is larger than the number of speakers configured in the speaker system, the reproduced volume of the channel corresponding to a nonexistent speaker can be controlled. 
         [0027]    The present invention also provides an audio apparatus whereby, without getting the decoding device to down-mix the particular channel signal component corresponding to the speaker not found in the speaker system, the audio processing device is caused to amplify the particular channel signal component and then to mix the amplified particular channel signal component with the other channel signal components. Thus even if the number of speakers configured in the speaker system is smaller than the number of channels found in the multichannel sound source, the particular channel signal component corresponding to the particular speaker not found in the speaker system can be gain-controlled before being output. In other words, where the number of channels found in the sound source is larger than the number of speakers configured in the speaker system, the reproduced volume of the channel corresponding to a nonexistent speaker can be controlled. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0028]      FIG. 1  is a schematic block diagram showing a circuit structure of an ordinary car audio apparatus; 
           [0029]      FIG. 2  is a schematic perspective view showing an external structure of a car audio apparatus according to a first, a second and a third embodiment of the present invention; 
           [0030]      FIG. 3  is a schematic block diagram showing a circuit structure of the car audio apparatus according to the first embodiment of the present invention; 
           [0031]      FIG. 4  is a flowchart of steps constituting a center speaker component control procedure performed by the first embodiment; 
           [0032]      FIGS. 5A ,  5 B,  5 C and  5 D are schematic views explanatory of sound function transitions; 
           [0033]      FIGS. 6A ,  6 B,  6 C and  6 D are schematic views explanatory of the volume control of a center speaker component; 
           [0034]      FIG. 7  is a schematic view explanatory of leaving center speaker component control mode and going back to sound function selection mode; 
           [0035]      FIG. 8  is a schematic view explanatory of fine control of the center speaker component; 
           [0036]      FIG. 9  is a schematic block diagram showing a circuit structure of the car audio apparatus according to the second embodiment; 
           [0037]      FIG. 10  is a flowchart of steps constituting the center speaker component control procedure performed by the second embodiment; 
           [0038]      FIG. 11  is a schematic block diagram showing a circuit structure of the car audio apparatus according to the third embodiment; and 
           [0039]      FIG. 12  is a schematic block diagram of the car audio apparatus according to other variations of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0040]    The preferred embodiments of the present invention will now be described below under the following headings: 
         [0041]    1. First embodiment 
         [0042]    2. Second embodiment 
         [0043]    3. Third embodiment 
         [0044]    4. Other variations 
       1. First Embodiment 
     1-1. External Structure of the Car Audio Apparatus 
       [0045]    In  FIG. 2 , reference numeral  20  denotes a car audio apparatus for use as the first embodiment of the present invention. A rectangular enclosure  21  is embedded in the dashboard of the car. The car audio apparatus  20  has a front panel  22  of the enclosure  21  furnished with a power button  23 , a rotary encoder  24  that can be rotated and pressed, a liquid crystal display (LCD)  25  for various display, and a group of operation buttons  26 . 
       1-2. Circuit Structure of the Car Audio Apparatus 
       [0046]    In  FIGS. 1 and 3 , like reference numerals designate like or corresponding parts. As shown in  FIG. 3 , the car audio apparatus  20  as a whole is controlled by a microcomputer (CPU)  27  reading basic programs from a ROM (Read Only Memory), not shown, into a RAM (Random Access Memory), not shown, for program execution. 
         [0047]    Diverse functions of the car audio apparatus  20  are implemented by the microcomputer  27  reading various application programs from the ROM into the RAM and executing these application programs to carry out various processes. 
         [0048]    The car audio apparatus  20  accommodates a disk-type recording medium  2  such as a DVD (Digital Versatile Disc) from which to reproduce 5.1 channel audio contents as a target sound source. Meanwhile, it is assumed that the car on which to mount the car audio apparatus  20  for sound reproduction and output is equipped with a 4.1 channel speaker system SS 1 . 
         [0049]    That is, whereas the audio contents of the sound source have the 5.1 channel format, the speaker system SS 1  by which the audio contents are to be reproduced and output has a 4.1 channel configuration made up of a left speaker  7 , a right speaker  8 , a left surround speaker  9 , a right surround speaker  10 , and a subwoofer  11 , with no center speaker. 
         [0050]    In practice, the microcomputer  27  of the car audio apparatus  20  causes a disk playing device, not shown, to reproduce data from the disk-type recording medium  2 . In response to the user&#39;s rotating or pressing operations on a rotary encoder  24 , the microcomputer  27  enters center speaker component control mode. In this mode, the microcomputer  27  carries out the following processes. 
         [0051]    Whereas the microcomputer  27  of the car audio apparatus  20  should ordinarily notify an audio decoder  4  in a DSP  3  of a control signal S 1  ( FIG. 1 ) indicating correctly that the speaker system SS 1  has the 4.1 channel configuration, the microcomputer  27  instead notifies the audio decoder  4  of a false control signal S 3  indicating incorrectly that the speaker system SS 1  has the 5.1 channel configuration. 
         [0052]    In turn, the audio decoder  4  decodes the audio contents D 1  of the 5.1 channel format reproduced from the disk-type recording medium  2  and, without performing a down-mixing process, sends to an audio postprocessor  5  the decoded contents as decoded audio data D 3  in the 5.1 channel format (made up of a left speaker channel signal component L, a right speaker channel signal component R, a center speaker channel signal component C, a left surround speaker channel signal component Ls, a right surround speaker channel signal component Rs, and a subwoofer channel signal component LFE). 
         [0053]    Since the onboard speaker system SS 1  does not have a center speaker, the audio postprocessor  5  causes an amplifier circuit G 1  to amplify the center speaker channel signal component C by a predetermined gain α (e.g., up 3 dB). 
         [0054]    Meanwhile, the microcomputer  27  receives a gain setting command from the rotary encoder  24  being operated by the user, and outputs to the audio postprocessor  5  a control signal S 2  representative of the gain setting command. This establishes the gain α for the amplifier circuit G 1  beforehand. 
         [0055]    The audio postprocessor  5  mixes the center speaker channel signal component αC amplified by the amplifier circuit G 1  with the right speaker channel signal component L and right speaker channel signal component R. The mixing process generates a left speaker channel signal component L+αC and a right speaker channel signal component R+αC. 
         [0056]    The audio postprocessor  5  then outputs to a power amplifier  6  audio data D 4  constituted by the left speaker channel signal component L-αC, right speaker channel signal component R+αC, left surround speaker channel signal component Ls, right surround speaker channel signal component Rs, and subwoofer channel signal component LFE. 
         [0057]    It should be noted that the audio postprocessor  5  sends the left surround speaker channel signal component Ls, right surround speaker channel signal component Rs, and subwoofer channel signal component LFE in an unmodified manner to the power amplifier  6 . 
         [0058]    The power amplifier  6  amplifies the left speaker channel signal component L-αC, right speaker channel signal component R+αC, left surround speaker channel signal component Ls, right surround speaker channel signal component Rs, and subwoofer channel signal component LFE to a predetermined level. The signal components thus amplified are output through the speaker system SS 1  as a reproduced sound. 
       1-3. Center Speaker Component Control Routine 
       [0059]    Described below in more detail with reference to  FIG. 4  is a center speaker component control routine carried out by the car audio apparatus  20  in the above-mentioned center speaker component control mode. 
         [0060]    Upon starting the routine RT 1 , the microcomputer  27  of the car audio apparatus  20  goes to step SP 1 . In step SP 1 , the microcomputer  27  waits for center speaker component control mode to be selected from among FM radio mode, equalizer control mode, speaker position control mode, and center speaker component control mode. When center speaker component control mode is selected, the microcomputer  27  goes to step SP 2 . 
         [0061]    Specifically, when the rotary encoder  24  is pressed successively by the user, transitions are effected from FM radio mode (displayed as “FM 1 ” on the LCD  25  shown in  FIG. 5A ) to equalizer control mode (displayed as “EQ” (equalizer) on the LCD  25  in  FIG. 5B ) to speaker position control mode (displayed as “POS” (position) on the LCD  25  in  FIG. 5C ) to center speaker component control mode (displayed as “CSO” (center speaker organizer) on the LCD  25  in  FIG. 5D ). 
         [0062]    In step SP 2 , the microcomputer  27  of the car audio apparatus  20  checks to determine whether a center speaker component control function for amplifying the center speaker channel signal component C alone is turned on in the center speaker component control mode selected by the rotary encoder  24  being pressed. 
         [0063]    In that step, if the rotary encoder  24  is found rotated to the extreme counterclockwise position, the microcomputer  27  of the car audio apparatus  20  turns off the center speaker component control function, indicated as “CSO OFF” in  FIG. 6A . 
         [0064]    If the rotary encoder  24  is rotated away from the extreme counterclockwise position, the microcomputer  27  of the car audio apparatus  20  turns on the center speaker component control function indicated as “CSO” and raises progressively the gain α of the center speaker channel signal component C in proportion to how much the rotary encoder  24  is rotated, as shown  FIGS. 6B through 6D . 
         [0065]    Illustratively, in accordance with how much the rotary encoder  24  is rotated, the microcomputer  27  sets the gain α for the amplifier circuit G 1  of the audio postprocessor  5  to “CSO 1 ” (up 3 dB), “CSO 2 ” (up 6 dB), or “CSO 3 ” (up 12 dB). 
         [0066]    Thereafter, if a back button BB among the group of operation buttons  26  is pressed, the microcomputer  27  of the car audio apparatus  20  goes back to the function selection state ( FIGS. 5A through 5D ) in which to select FM radio mode, equalizer control mode, speaker position control mode, or center speaker component control mode, as shown in  FIG. 7 . 
         [0067]    If the result of the check in step SP 2  is negative, that means the rotary encoder  24  stays rotated to the extreme counterclockwise position and that the center speaker component control function remains turned off as shown in  FIG. 6A  (“CSO OFF”). In this case, the microcomputer  27  goes to step SP 3 . 
         [0068]    In step SP 3 , with the center speaker component control function turned off, the microcomputer  27  of the car audio apparatus  20  notifies the audio decoder  4  in the DSP  3  of the control signal S 1  ( FIG. 1 ) saying that the onboard speaker system SS 1  has the 4.1 channel configuration. The microcomputer  27  then goes to step SP 4 . 
         [0069]    In step SP 4 , the microcomputer  27  of the car audio apparatus  20  down-mixes the audio contents D 1  in the 5.1 channel format of the sound source to the 4.1 channel format commensurate with the number of speakers configured in the speaker system SS 1 . The down-mixed audio contents are fed to the audio postprocessor  5  and the power amplifier  6  before being reproduced and output by the speaker system SS 1 . From step SP 4 , the microcomputer  27  goes to step SP 9  and terminates this routine. 
         [0070]    In this case, the microcomputer  27  of the car audio apparatus  20  mixes the center speaker channel signal component C with the left speaker channel signal component L and right speaker channel signal component R, and outputs the down-mixed left speaker channel signal component L+C and right speaker channel signal component R+C. 
         [0071]    On the other hand, if the result of the check in step SP 2  is affirmative, that means the center speaker component control function is turned on and that the amplifier circuit G 1  of the audio postprocessor  5  is set to the gain α of “CSO 1 ,” “CSO 2 ” or “CSO 3 ” in keeping with how much the rotary encoder  24  is rotated. From step SP 2 , the microcomputer  27  goes to step SP 5 . 
         [0072]    In step SP 5 , the microcomputer  27  of the car audio apparatus  20  notifies the audio decoder  4  of a false control signal S 3  feigning that the speaker system SS 1  has the 5.1 channel configuration despite the fact that the system SS 1  actually has the 4.1 channel configuration. From step SP 5 , the microcomputer  27  goes to step SP 6 . 
         [0073]    In step SP 6 , the microcomputer  27  of the car audio apparatus  20  sends the decoded contents from the audio decoder  4  to the audio postprocessor  5  as decoded audio data D 3  in the 5.1 channel format without performing a down-mixing process. 
         [0074]    The microcomputer  27  of the car audio apparatus  20  causes the amplifier circuit G 1  of the audio postprocessor  5  to amplify the center speaker channel signal component C as per the gain α of “CSO 1 ” set beforehand in keeping with how much the rotary encoder  24  is rotated, before mixing the amplified center speaker channel signal component C with the left speaker channel signal component L and right speaker channel signal component R. 
         [0075]    The microcomputer  27  of the car audio apparatus  20  thus generates a left speaker channel signal component L+αC and a right speaker channel signal component R+αC in the down-mixing process. The microcomputer  27  eventually causes the reproduced contents to be output in the 4.1 channel format, before going to step SP 7 . 
         [0076]    In step SP 7 , the microcomputer  27  of the car audio apparatus  20  checks to determine whether the rotary encoder  24  is further rotated in the reproduction state following the down-mixing process. 
         [0077]    If the result of the check in step SP 7  is negative, that means the rotary encoder  24  is no longer rotated in the reproduction state subsequent to the down-mixing process. In this case, the microcomputer  27  of the car audio apparatus  20  goes to step SP 9  and terminates this routine. 
         [0078]    If the result of the check in step SP 7  is affirmative, that means a command to fine-control the volume of the center speaker channel signal component C in keeping with the rotating operation of the rotary encoder  24  is given. In this case, the microcomputer  27  goes to step SP 8 . 
         [0079]    In step SP 8 , the microcomputer  27  of the car audio apparatus  20  reestablishes a raised or lowered gain α of “CSO 1 ” for the amplifier circuit G 1  of the audio postprocessor in accordance with the small amount of rotation of the rotary encoder  24 . The microcomputer  27  then goes to step SP 9  to terminate this routine. 
         [0080]    In practice, upon receipt of a gain fine-control command reflecting the amount of the rotating operation of the rotary encoder  24 , the microcomputer  27  ( FIG. 3 ) of the car audio apparatus  20  outputs to the audio postprocessor  5  a control signal S 4  representative of that gain fine-control command. This reestablishes a slightly raised or lowered gain α for the amplifier circuit G 1 . 
         [0081]    In the case above, as shown in  FIG. 8 , the microcomputer  27  of the car audio apparatus  20  informs the user of how much the volume of the center speaker channel signal component C has changed in accordance with the small amount of operation of the rotary encoder  24 , in the form of an eight-step gradation display on the LCD  25 . 
       1-4. Operation and Effects 
       [0082]    With the above-described structure in use, if the audio contents D 1  of the sound source are in the 5.1 channel format while the speaker system SS 1  for reproducing and outputting the audio contents D 1  has the 4.1 channel configuration with no center speaker, the microcomputer  27  of the car audio apparatus  20  performs the following processes. 
         [0083]    Whereas the speaker system SS 1  of interest has in fact the 4.1 channel configuration, the microcomputer  27  of the car audio apparatus  20  notifies the audio decoder  4  in the DSP  3  of the false control signal S 3  indicating incorrectly that the speaker system SS 1  has the 5.1 channel configuration. 
         [0084]    As a result, the audio decoder  4  in the DSP  3  decodes the audio contents D 1  of the 5.1 channel format, without performing a down-mixing process, into decoded audio data D 3  in the 5.1 channel format (made up of the left speaker channel signal component L, right speaker channel signal component R, center speaker channel signal component C, left surround speaker channel signal component Ls, right surround speaker channel signal content Rs, and subwoofer channel signal component LFE). The decoded audio data D 3  thus acquired is forwarded to the audio postprocessor  5 . 
         [0085]    In turn, the audio postprocessor  5  causes the amplifier circuit G 1  to amplify solely the center speaker channel signal component C by the predetermined gain α. The audio postprocessor  5  proceeds to mix the amplified center speaker channel signal component αC with the left speaker channel signal component L and right speaker channel signal component R in a down-mixing process. 
         [0086]    In this manner, the microcomputer  27  of the car audio apparatus  20  causes the audio decoder  4  to decode the sound source data while leaving intact the number of channels found in the source regardless of the number of speakers configured in the speaker system SS 1 . Then, the microcomputer causes the audio postprocessor  5  to amplify the center speaker channel signal component C desired to be emphasized, before proceeding with the down-mixing process. 
         [0087]    As a result, the car audio apparatus  20  can amplify solely the center speaker channel signal component C without amplifying the left speaker channel signal component L or right speaker channel signal component R containing relatively dominant sounds. This makes it possible to reproduce and output, say, speeches more clearly than before. 
         [0088]    Also, in keeping with how much the rotary encoder  24  is rotated, the microcomputer  27  of the car audio apparatus  20  can set beforehand the gain α for the amplifier circuit G 1  of the audio postprocessor  5  illustratively to “CSO 1 ” (up 3 dB), “CSO 2 ” (up 6 dB), or “CSO 3 ” (up 12 dB), as shown in  FIGS. 6B through 6D . 
         [0089]    Furthermore, in accordance with the predetermined gain α of, say, “CSO 1 ,” the microcomputer  27  amplifies the center speaker channel signal component C and performs the down-mixing process. While the decoded audio data in the 4.1 channel format is being eventually reproduced and output, the microcomputer  27  can fine-control and reestablish the gain α for the amplifier circuit G 1  of the audio postprocessor  5  as per the small amount of rotation of the rotary encoder  24 , as shown in  FIG. 8 . 
         [0090]    As a result, the microcomputer  27  of the car audio apparatus  20  can fine-control the volume of the center speaker channel signal component C to the level desired by the user. 
         [0091]    Where the above-described structure is in place, the car audio apparatus  20  can control the reproduced volume of solely the center speaker channel signal component C to the output level desired by the user, even if the audio contents D 1  of the sound source are in the 5.1 channel format while the 4.1 channel speaker system SS 1  for reproducing and outputting the audio contents D 1  of interest does not have a center speaker. 
       2. Second Embodiment 
     2-1. External Structure of the Car Audio Apparatus 
       [0092]    In  FIG. 2 , reference numeral  30  denotes a car audio apparatus for use as the second embodiment of the present invention. The car audio apparatus  30  is the same as the car audio apparatus  20  for use as the first embodiment and thus the external structure of the apparatus  30  will not be discussed further. 
       2-2. Circuit Structure of the Car Audio Apparatus 
       [0093]    In  FIGS. 3 and 9 , like reference numerals designate like or corresponding parts. As shown in  FIG. 9 , the car audio apparatus  30  as a whole is controlled by a microcomputer (CPU)  37  reading basic programs from a ROM, not shown, into a RAM, not shown, for program execution. 
         [0094]    Diverse functions of the car audio apparatus  30  are implemented by the microcomputer  37  reading various application programs from the ROM into the RAM and executing these application programs to carry out various processes. 
         [0095]    The car audio apparatus  30  accommodates the disk-type recording medium  2  such as a DVD from which to reproduce 5.1 channel audio contents as a target sound source. Meanwhile, it is assumed that the car on which to mount the car audio apparatus  30  for sound reproduction and output is equipped with a 2 channel speaker system SS 2 . 
         [0096]    That is, whereas the audio contents of the sound source are in the 5.1 channel format, the speaker system SS 2  for reproducing and outputting the audio contents has the 2 channel configuration made up of a left speaker  7  and a right speaker  8 . Not only the center speaker but also the left surround speaker, right surround speaker and subwoofer are absent. 
         [0097]    In practice, the microcomputer  37  of the car audio apparatus  30  enters center speaker component control mode in response to the user&#39;s rotating and pressing operations on the rotary encoder  24 . In center speaker component control mode, the microcomputer  37  initially performs the following processes. 
         [0098]    Whereas the microcomputer  37  of the car audio apparatus  30  should ordinarily notify the audio decoder  4  in the DSP  3  of a control signal indicating correctly that the speaker system SS 2  has the 2 channel configuration, the microcomputer  37  instead notifies the audio decoder  4  of the false control signal S 3  indicating incorrectly that the speaker system SS 2  has the 5.1 channel configuration. 
         [0099]    In turn, the audio decoder  4  decodes the audio contents D 1  of the 5.1 channel format reproduced from the disk-type recording medium  2  and, without performing a down-mixing process, sends to the audio postprocessor  5  the decoded contents as the decoded audio data D 3  in the 5.1 channel format (made up of the left speaker channel signal component L, right speaker channel signal component R, center speaker channel signal component C, left surround speaker channel signal component Ls, right surround speaker channel signal component Rs, and subwoofer channel signal component LFE). 
         [0100]    Since the onboard speaker system SS 2  does not have a center speaker, the audio postprocessor  5  causes the amplifier circuit G 1  to amplify the center speaker channel signal component C by a predetermined gain α (e.g., up 3 dB). 
         [0101]    Meanwhile, the microcomputer  37  receives a gain setting command from the rotary encoder  24  being operated by the user, and outputs to the audio postprocessor  5  the control signal S 2  representative of the gain setting command. This establishes the gain α for the amplifier circuit G 1  beforehand. 
         [0102]    The audio postprocessor  5  mixes the center speaker channel signal component αC amplified by the amplifier circuit G 1  with the left speaker channel signal component L and right speaker channel signal component R. The mixing process generates the left speaker channel signal component L+αC and the right speaker channel signal component R+αC. 
         [0103]    Since the onboard speaker system SS 2  does not have a center speaker, a left surround speaker, a right surround speaker or a subwoofer, the audio postprocessor  5  mixes the left speaker channel signal component L+αC with the left surround speaker channel signal component Ls, and also mixes the right speaker channel signal component R+αC with the right surround speaker channel signal component Rs. 
         [0104]    The audio postprocessor  5  thus generates a left speaker channel signal component L+αC+Ls and a right speaker channel signal component R+αC+Rs. 
         [0105]    The audio postprocessor  5  then outputs to the power amplifier  6  audio data D 5  constituted by the left speaker channel signal component L+αC+Ls and the right speaker channel signal component R+αC+Rs. 
         [0106]    It should be noted that the audio postprocessor  5  does not output the subwoofer channel signal component LFE since the speaker system SS 2  does not have a subwoofer. 
         [0107]    The power amplifier  6  amplifies the left speaker channel signal component L+αC+Ls and the right speaker channel signal component R+αC+Rs to a predetermined level. The signal components thus amplified are eventually output as a 2 channel reproduced sound through the left speaker  7  and right speaker  8  of the speaker system SS 2 . 
       2-3. Center Speaker Component Control Routine 
       [0108]    Described below in more detail with reference to  FIG. 10  is a center speaker component control routine carried out by the car audio apparatus  30  in the above-mentioned center speaker component control mode. 
         [0109]    Upon starting the routine RT 2 , the microcomputer  37  of the car audio apparatus  30  goes to step SP 11 . In step SP 11 , the microcomputer  37  waits for center speaker component control mode to be selected from among FM radio mode, equalizer control mode, speaker position control mode, and center speaker component control mode shown in  FIGS. 5A through 5D . When center speaker component control mode is selected, the microcomputer  37  goes to step SP 12 . 
         [0110]    In step SP 12 , the microcomputer  37  of the car audio apparatus  30  checks to determine whether the center speaker component control function for amplifying the center speaker channel signal component C alone is turned on in the center speaker component control mode selected by the rotary encoder  24  being pressed. 
         [0111]    If the result of the check in step SP 12  is negative, that means the rotary encoder  24  stays rotated to the extreme counterclockwise position and that the center speaker component control function remains turned off as shown in  FIG. 6A  (“CSO OFF”). In this case, the microcomputer  37  goes to step SP 13 . 
         [0112]    In step SP 13 , with the center speaker component control function turned off, the microcomputer  37  of the car audio apparatus  30  notifies the audio decoder  4  in the DSP  3  of the control signal saying that the onboard speaker system SS 2  has the 2 channel configuration. The microcomputer  37  then goes to step SP 14 . 
         [0113]    In step SP 14 , the microcomputer  37  of the car audio apparatus  30  down-mixes the audio contents D 1  in the 5.1 channel format of the sound source into the 2 channel format commensurate with the number of speakers configured in the speaker system SS 2 . The down-mixed audio contents are fed to the power amplifier  6  before being reproduced and output by the speaker system SS 2 . From step SP 14 , the microcomputer  37  goes to step SP 19  and terminates this routine. 
         [0114]    In this case, the audio decoder  4  mixes the center speaker channel signal component C with the left speaker channel signal component L and left surround speaker channel signal component Ls, and also mixes the center speaker channel signal component C with the right speaker channel signal component R and right surround speaker channel signal component Rs. 
         [0115]    The audio decoder  4  then outputs the down-mixed left speaker channel signal component L+C+Ls and right speaker channel signal component R+C+Rs which in turn are output from the left speaker  7  and right speaker  8  of the speaker system SS 2 . 
         [0116]    On the other hand, if the result of the check in step SP 12  is affirmative, that means the center speaker component control function is turned on and that the amplifier circuit G 1  of the audio postprocessor  5  is set to the gain α of “CSO 1 ,” “CSO 2 ” or “CSO 3 ” in keeping with how much the rotary encoder  24  is rotated. From step SP 12 , the microcomputer  37  goes to step SP 15 . 
         [0117]    In step SP 15 , the microcomputer  37  of the car audio apparatus  30  notifies the audio decoder  4  of the false control signal S 3  feigning that the speaker system SS 2  has the 5.1 channel configuration despite the fact that the system SS 2  actually has the 2 channel configuration. From step SP 15 , the microcomputer  37  goes to step SP 16 . 
         [0118]    In step SP 16 , the microcomputer  37  of the car audio apparatus  30  sends the decoded contents from the audio decoder  4  to the audio postprocessor  5  as the decoded audio data D 3  in the 5.1 channel format without performing a down-mixing process. 
         [0119]    The microcomputer  37  of the car audio apparatus  30  causes the amplifier circuit G 1  of the audio postprocessor  5  to amplify the center speaker channel signal component C as per the gain α of “CSO 1 ” set beforehand in keeping with how much the rotary encoder  24  is rotated. 
         [0120]    Thereafter, the microcomputer  37  of the car audio apparatus  30  mixes the center speaker channel signal component αC amplified by the amplifier circuit G 1  with the left speaker channel signal component L and left surround speaker channel signal component Ls, and also mixes the center speaker channel signal component αC with the right speaker channel signal component R and right surround speaker channel signal component Rs. 
         [0121]    The microcomputer  37  of the car audio apparatus  30  thus generates a left speaker channel signal component L+αC+Ls and a right speaker channel signal component R+αC+Rs in the down-mixing process. The microcomputer  37  eventually causes the reproduced contents to be output in the 2 channel format, before going to step SP 17 . 
         [0122]    In step SP 17 , the microcomputer  37  of the car audio apparatus  30  checks to determine whether the rotary encoder  24  is further rotated in the reproduction state following the down-mixing process. 
         [0123]    If the result of the check in step SP 17  is negative, that means the rotary encoder  24  is no longer rotated in the reproduction state subsequent to the down-mixing process. In this case, the microcomputer  37  of the car audio apparatus  30  goes to step SP 19  and terminates this routine. 
         [0124]    If the result of the check in step SP 17  is affirmative, that means a command to fine-control the volume of the center speaker channel signal component C in keeping with the rotating operation of the rotary encoder  24  is given. In this case, the microcomputer  37  goes to step SP 18 . 
         [0125]    In step SP 18 , the microcomputer  37  of the car audio apparatus  30  reestablishes a raised or lowered gain α of “CSO 1 ” for the amplifier circuit G 1  of the audio postprocessor in accordance with the small amount of rotation of the rotary encoder  24 . The microcomputer  37  then goes to step SP 19  to terminate this routine. 
         [0126]    In practice, upon receipt of a gain fine-control command reflecting the amount of the rotating operation of the rotary encoder  24 , the microcomputer  37  ( FIG. 3 ) of the car audio apparatus  30  outputs to the audio postprocessor  5  the control signal S 4  representative of that gain fine-control command. This reestablishes a slightly raised or lowered gain α for the amplifier circuit G 1 . 
       2-4. Operation and Effects 
       [0127]    With the above-described structure in use, if the audio contents D 1  of the sound source are in the 5.1 channel format while the speaker system SS 2  for reproducing and outputting the audio contents D 1  has the 2 channel configuration that does not include a center speaker, a left surround speaker or a right surround speaker, the microcomputer  37  of the car audio apparatus  30  performs the following processes. 
         [0128]    Whereas the speaker system SS 2  of interest has in fact the 2 channel configuration, the microcomputer  37  of the car audio apparatus  30  notifies the audio decoder  4  in the DSP  3  of the false control signal S 3  indicating incorrectly that the speaker system SS 2  has the 5.1 channel configuration. 
         [0129]    As a result, the audio decoder  4  in the DSP  3  decodes the audio contents D 1  of the 5.1 channel format, without performing a down-mixing process, into decoded audio data D 3  in the 5.1 channel format. The decoded audio data D 3  thus acquired is forwarded to the audio postprocessor  5 . 
         [0130]    In turn, the audio postprocessor  5  causes the amplifier circuit G 1  to amplify solely the center speaker channel signal component C by the predetermined gain α. The audio postprocessor  5  proceeds to mix the amplified center speaker channel signal component αC with the left speaker channel signal component L and left surround speaker channel signal component Ls, and also mix the amplified center speaker channel signal component αC with the right speaker channel signal component R and right surround speaker channel signal component Rs in a down-mixing process. 
         [0131]    In this manner, the microcomputer  37  of the car audio apparatus  30  causes the audio decoder  4  to decode the sound source data while leaving intact the number of channels found in the source regardless of the number of speakers configured in the speaker system SS 2 . Then the microcomputer causes the audio postprocessor  5  to amplify the center speaker channel signal component C desired to be emphasized, before proceeding with the down-mixing process. 
         [0132]    As a result, the car audio apparatus  30  can amplify solely the center speaker channel signal component C without amplifying the left speaker channel signal component L or right speaker channel signal component R containing relatively dominant sounds. This makes it possible to reproduce and output, say, speeches more clearly than before. 
         [0133]    In this case, the car audio apparatus  30  down-mixes the left surround speaker channel signal component Ls and right surround speaker channel signal component Rs. This enables the 2 channel speaker system SS 2  to output eventually the kind of reproduced sound appreciably close to the original sound source. 
         [0134]    Also, in keeping with how much the rotary encoder  24  is rotated, the microcomputer  37  of the car audio apparatus  30  can set beforehand the gain α for the amplifier circuit G 1  of the audio postprocessor  5  illustratively to “CSO 1 ” (up 3 dB), “CSO 2 ” (up 6 dB), or “CSO 3 ” (up 12 dB), as shown in  FIGS. 6B through 6D . 
         [0135]    Furthermore, in accordance with the predetermined gain α of, say, “CSO 1 ,” the microcomputer  37  amplifies the center speaker channel signal component C and performs the down-mixing process. While the decoded audio data in the 2 channel format is being eventually reproduced and output, the microcomputer  37  can fine-control and reestablish the gain α for the amplifier circuit G 1  of the audio postprocessor in keeping with the small amount of rotation of the rotary encoder  24 , as shown in  FIG. 8 . 
         [0136]    As a result, the microcomputer  37  of the car audio apparatus  30  can fine-control the volume of the center speaker channel signal component C to the level desired by the user. 
         [0137]    Where the above-described structure is in place, the car audio apparatus  30  can control the reproduced volume of solely the center speaker channel signal component C to the output level desired by the user, even if the audio contents D 1  of the sound source are in the 5.1 channel format while the channel speaker system SS 2  for reproducing and outputting the audio contents D 1  of interest does not have a center speaker, a left surround speaker, a right surround speaker or a subwoofer. 
       3. Third Embodiment 
     3-1. External Structure of the Car Audio Apparatus 
       [0138]    In  FIG. 2 , reference numeral  40  denotes a car audio apparatus for use as the third embodiment of the present invention. The car audio apparatus  40  is the same as the car audio apparatus  20  for use as the first embodiment and thus the external structure of the apparatus  40  will not be discussed further. 
       3-2. Circuit Structure of the Car Audio Apparatus 
       [0139]    In  FIGS. 3 and 11 , like reference numerals designate like or corresponding parts. As shown in  FIG. 11 , the car audio apparatus  40  as a whole is controlled by a microcomputer (CPU)  47  reading basic programs from a ROM, not shown, into a RAM, not shown, for program execution. 
         [0140]    Diverse functions of the car audio apparatus  40  are implemented by the microcomputer  47  reading various application programs from the ROM into the RAM and executing these application programs to carry out various processes. 
         [0141]    The car audio apparatus  40  accommodates the disk-type recording medium  2  such as a DVD from which to reproduce 5.1 channel audio contents as a target sound source. Meanwhile, as with the car audio apparatus  30  of the second embodiment, it is assumed that the car on which to mount the car audio apparatus  40  for sound reproduction and output is equipped with the 2 channel speaker system SS 2 . 
         [0142]    That is, whereas the audio contents of the sound source are in the 5.1 channel format, the speaker system SS 2  for reproducing and outputting the audio contents has the 2 channel configuration made up of the left speaker  7  and right speaker  8 . 
         [0143]    In practice, the microcomputer  47  of the car audio apparatus  40  enters center speaker component control mode in response to the user&#39;s rotating and pressing operations on the rotary encoder  24 . In center speaker component control mode, the microcomputer  47  initially performs the following processes. 
         [0144]    Whereas the microcomputer  47  of the car audio apparatus  40  should ordinarily notify the audio decoder  4  in the DSP  3  of a control signal indicating correctly that the speaker system SS 2  has the 2 channel configuration, the microcomputer  47  instead notifies the audio decoder  4  of the false control signal S 3  indicating incorrectly that the speaker system SS 2  has the 5.1 channel configuration. 
         [0145]    In turn, the audio decoder  4  decodes the audio contents D 1  of the 5.1 channel format reproduced from the disk-type recording medium  2  and, without performing a down-mixing process, sends to the audio postprocessor  5  the decoded contents as the decoded audio data D 3  in the 5.1 channel format. 
         [0146]    Since the onboard speaker system SS 2  does not have a center speaker, the audio postprocessor  5  causes the amplifier circuit G 1  to amplify the center speaker channel signal component C by a predetermined gain α (e.g., up 3 dB). 
         [0147]    Furthermore, whereas the onboard speaker system SS 2  does not have a left surround speaker and a right surround speaker, the audio postprocessor  5  causes amplifier circuits G 2  and G 3  to amplify the left surround speaker channel signal component Ls and right surround speaker channel signal component Rs, respectively, by a predetermined gain β (e.g., up 3 dB). 
         [0148]    Meanwhile, the microcomputer  47  receives a gain setting command from the rotary encoder  24  being operated by the user, and outputs to the audio postprocessor  5  the control signal S 2  representative of the gain setting command. This establishes the gain α for the amplifier circuit G 1  and the gain β for the amplifier circuits G 2  and G 3  beforehand. 
         [0149]    The audio postprocessor  5  mixes the center speaker channel signal component αC amplified by the amplifier circuit G 1  with the left speaker channel signal component L and right speaker channel signal component R. The mixing process generates the left speaker channel signal component L+αC and the right speaker channel signal component R+αC. 
         [0150]    Also, the audio postprocessor  5  mixes the left surround speaker channel signal component βLs amplified by the amplifier circuit G 2  with the left speaker channel signal component L+αC to generate a left speaker channel signal component L+αC+βLs. 
         [0151]    Likewise, the audio postprocessor  5  mixes the right surround speaker channel signal component βRs amplified by the amplifier circuit G 3  with the right speaker channel signal component R+αC to generate a right speaker channel signal component R+αC+βRs. 
         [0152]    The audio postprocessor  5  then outputs to the power amplifier  6  audio data D 6  constituted by the left speaker channel signal component L+αC+βLs and right speaker channel signal component R+αC+βRs. 
         [0153]    It should be noted that the audio postprocessor  5  does not output the subwoofer channel signal component LFE since the speaker system SS 2  does not have a subwoofer. 
         [0154]    The power amplifier  6  amplifies the left speaker channel signal component L+αC+βLs and right speaker channel signal component R+αC+βRs to a predetermined level. The signal components thus amplified are eventually output as a 2 channel reproduced sound through the left speaker  7  and right speaker  8  of the speaker system SS 2 . 
       3-3. Center Speaker Component Control Routine 
       [0155]    The center speaker component control routine performed by the car audio apparatus  40  in the above-mentioned center speaker component control mode is basically the same as the center speaker component control routine shown as the routine RT 2  in  FIG. 10  and carried out by the car audio apparatus  30 , and thus the routine will not be discussed further. 
       3-4. Operation and Effects 
       [0156]    With the above-described structure in use, whereas the speaker system SS 2  of interest has in fact the 2 channel configuration, the microcomputer  47  of the car audio apparatus notifies the audio decoder  4  in the DSP  3  of the false control signal S 3  indicating incorrectly that the speaker system SS 2  has the 5.1 channel configuration. 
         [0157]    As a result, the audio decoder  4  in the DSP  3  decodes the audio contents D 1  of the 5.1 channel format, without performing a down-mixing process, into decoded audio data D 3  in the 5.1 channel format. The decoded audio data D 3  thus acquired is forwarded to the audio postprocessor  5 . 
         [0158]    In turn, the audio postprocessor  5  causes the amplifier circuit G 1  to amplify the center speaker channel signal component C by the predetermined gain α. Also, the audio postprocessor  5  causes the amplifier circuits G 2  and G 3  to amplify the left surround speaker channel signal component Ls and right surround speaker channel signal component Rs by the predetermined gain β. 
         [0159]    The audio postprocessor  5  proceeds to mix the center speaker channel signal component αC amplified by the amplifier circuit G 1  with the left surround speaker channel signal component βLs and right surround speaker channel signal component βRs amplified by the amplifier circuits G 2  and G 3 , and also with left speaker channel signal component L and left surround speaker channel signal component Ls in a down-mixing process. 
         [0160]    In this manner, the microcomputer  47  of the car audio apparatus  40  causes the audio decoder  4  to decode the sound source data while leaving intact the number of channels found in the source regardless of the number of speakers configured in the speaker system SS 2 . Then, the microcomputer  47  causes the audio postprocessor  5  to amplify the center speaker channel signal component C, left surround speaker channel signal component Ls, and right surround speaker channel signal component Rs desired to be emphasized, before proceeding with the down-mixing process. 
         [0161]    As a result, the car audio apparatus  40  can amplify the center speaker channel signal component C, left surround speaker channel signal component Ls, and right surround speaker channel signal component Rs without amplifying the left speaker channel signal component L or right speaker channel signal component R containing relatively dominant sounds. This makes it possible to increase the volume of, say, speeches for a clearer comprehension of their contents and to provide better ambience enhancement by raising the volume of surround sounds. 
         [0162]    Also, in keeping with how much the rotary encoder  24  is rotated, the microcomputer  47  of the car audio apparatus  40  can set beforehand the gain α for the amplifier circuit G 1  of the audio postprocessor  5  and the gain β for the amplifier circuits G 2  and G 3  illustratively to “CSO 1 ” (up 3 dB), “CSO 2 ” (up 6 dB), or “CSO 3 ” (up 12 dB), as shown in  FIGS. 6B through 6D . 
         [0163]    Furthermore, in accordance with the predetermined gains α and β of, say, “CSO 1 ,” the microcomputer  47  amplifies the center speaker channel signal component C, left surround speaker channel signal component Ls, and right surround speaker channel signal component Rs and performs the down-mixing process. While the decoded audio data in the 2 channel format is being eventually reproduced and output, the microcomputer  47  can fine-control and reestablish the gains α and β for the amplifier circuits G 1 , G 2  and G 3  of the audio postprocessor  5  in keeping with the small amount of rotation of the rotary encoder  24 , as shown in  FIG. 8 . 
         [0164]    As a result, the microcomputer  47  of the car audio apparatus  40  can fine-control the volume of the center speaker channel signal component C, left surround speaker channel signal component Ls, and right surround speaker channel signal component Rs to the level desired by the user. 
         [0165]    Where the above-described structure is in place, the car audio apparatus  40  can control the reproduced volume of the center speaker channel signal component C, left surround speaker channel signal component Ls, and right surround speaker channel signal component Rs to the output level desired by the user, even if the audio contents D 1  of the sound source are in the 5.1 channel format while the 2 channel speaker system SS 2  for reproducing and outputting the audio contents D 1  of interest does not have a center speaker, a left surround speaker, a right surround speaker or a subwoofer. 
       4. Other Variations 
       [0166]    According to the first through the third embodiments above of the present invention, the gain α is set illustratively to “CSO 1 ” (up 3 dB), “CSO 2 ” (up 6 dB), or “CSO 3 ” (up 12 dB) in keeping with how much the rotary encoder  24  is rotated. 
         [0167]    However, this is not limitative of the present invention. Instead of being amplified, the gain may be attenuated by 3 dB, 6 dB or 12 dB in accordance with how much the rotary encoder  24  is rotated. It should be noted that these values are only examples and are not limitative of the invention. 
         [0168]    According to the first through the third embodiments of the invention discussed above, the audio decoder  4  in the DSP  3  and the amplifier circuit G 1  in the audio postprocessor  5  are used digitally to carry out the center speaker component control routine described above. 
         [0169]    Alternatively, as shown in  FIG. 12  (where like reference numerals designate like or corresponding parts in  FIG. 3 ), the above-described setup with the audio postprocessor  5  in the DSP  3  may be replaced with a car audio apparatus  50  constituted by a DSP  51  containing an audio decoder  4  alone and by an analog circuit device  52  having an amplifier circuit G 1  made of analog circuits, the apparatus  50  carrying out the center speaker component control routine. In this case, the gain α of the amplifier circuit G 1  is fixed so that the volume level of the center speaker channel signal component C may not be controlled. 
         [0170]    Also according to the first through the third embodiments of the present invention described above, there is no center speaker in the speaker system so that the volume level of the center speaker channel signal component C is controlled. 
         [0171]    Alternatively, where neither the left surround speaker nor the right surround speaker is provided, the volume levels of the left surround speaker channel signal component Ls and right surround speaker channel signal component Rs may be arranged to be controlled. 
         [0172]    The point is that solely the speaker channel signal component corresponding to a nonexistent speaker is amplified by the audio postprocessor  5  before being down-mixed with the channel signal components of the other speakers. Any channel may be subject to gain control. 
         [0173]    Furthermore, according to the above-described first through third embodiments of the present invention, the inventive car audio apparatus  20 ,  30  or  40  is constituted by the decoding device composed of the audio decoder  4 , by the audio processing device formed by the audio postprocessor  5 , and by the control device made up of the microcomputer  27 ,  37  or  47 . Alternatively, the audio apparatus of the present invention may be constituted by a decoding device, an audio processing device and a control device structured in ways other than those discussed above. 
         [0174]    The audio apparatus and audio processing method of the present invention can be applied not only to car audio apparatuses but also to home audio equipment, personal computers, car navigation systems, and other appliances. 
         [0175]    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 factor in so far as they are within the scope of the appended claims or the equivalents thereof.