Abstract:
A method and apparatus for reading multisession discs in which both standard CD-DA format and compressed music formats, such as MP3, may be utilized on the same disc. High speed reading of data into shock proof memory, analysis of available memory and storage of analyzed record structures are used to provide playback of music in both formats and, if desired, in a random playback order.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a method for playing back a multisession disc. In particular, the present invention relates to a method for playing back a multisession disc in which music data of a CD-DA format is recorded in at least one session and compressed music data of another format is recorded in another session.  
           [0003]    2. Description of the Related Art  
           [0004]    One session corresponds to from the start to the end of a writing onto a recordable medium. A disc, such as a CD-R medium, which allows data to be written multiple times, may yield a multisession disc. FIG. 7 illustrates a multisession disc. In the multisession disc, each of added sessions includes a program area P 1 , P 2 , or P 3 , and a lead-in area LI 1 , LI 2 , or LI 3  and a lead-out area LO 1 , LO 2 , or LO 3  sandwiching the program area. A volume descriptor (described later) and a path table of an added session and an already written session are recorded in each of the lead-in areas LI 1  to LI 3 . Therefore, by reading the lead-in area of the last session, the file structure of the entire disc volume can be obtained.  
           [0005]    Music data of various formats can be recorded in each session of the above-described multisession disc. For example, non-compressed music data can be recorded in a session in the CD-DA format (the same format as in an ordinary compact disc), and compressed music data can be recorded in another session in a CD-ROM mode  1  format. When an enhanced CD is used, CD-DA data is recorded in the first session and CD-ROM data (compressed music data) is recorded in the second session and subsequent sessions. Compressed music data may use MP3, WMA or other compression methods.  
           [0006]    [0006]FIG. 8 illustrates a CD-DA signal format. The sample data of an acoustic signal has 6 samples of L and R channels (12 samples in total, one sample is 16 bits). The sample data is divided into 24 symbols, each of which having 8 bits. An 8-symbol correction code (CIRC) is added to the 24-symbol sample data so that 32 symbols form one unit. A frame synchronization signal and a sub-code are added to the 32-symbol unit to form one frame ((a) and (b) of FIG. 8). The 8 bit sub-code and each symbol are converted to 14 bits by EFM modulation. The frame synchronization signal has 24 bits. Further, a 3-bit junction bit is inserted between the individual symbols, whereby one frame has 588 bits.  
           [0007]    98 frames form a large frame ((a) of FIG. 8). As shown in (c) of FIG. 8, the sub-code includes 8 bits of P, Q, R, S, T, U, V, and W. The sub-codes of the first two frames in the large frame are synchronization signals S 0  and S 1 . Each of the sub-codes P, Q, R, S, T, U, V, and W includes 98 bits. Among them, the sub-code Q includes 2-bit synchronization data S 0  and S 1 , 4-bit control data, 4-bit address data, 72-bit data, and 16-bit CRC data, as shown in FIG. 9.  
           [0008]    The control data indicates whether a track is a 2-channel music track without pre-emphasis a 2-channel music track with pre-emphasis, or a normal data track. The track is a music track (CD-DA) if the second bit is 0 and is a data track if the second bit is 1. Thus, by referring to the control data of the sub-code Q in the lead-in area of each session, it can be determined whether the session is a CD-DA format session.  
           [0009]    The 72-bit data packet specifies the song number, the elapsed time from the start of the song, the absolute time indicating the absolute current position with respect to the head of a program area with minute/second/frame, and so on. FIG. 9 shows the 72-bit data format in the lead-in area (TOC). (1) MNR is a song number (track number) and MNR=00 in the lead-in area. (2) POINT indicates the song number (track number) when the start of a song is indicated in TOC. (3) MIN/SEC/FRAME is the elapsed time from the start of a song. (4) ZERO indicates that all is 0. (5) PMIN/PSEC/PFRAME is the absolute position time in which the start of a song from the head of the program area is indicated with minute/second/frame, and corresponds to the index of a disc.  
           [0010]    [0010]FIG. 10 illustrates the TOC in the lead-in area in case of CD-DA. The start of each song is repeatedly (three times) recorded at the MNR=00. For example, the start position of the song  01 : 0 minute, 2 seconds, and frame  32 , is indicated by the sub-code Q of the large frames Nos. n, n+1, and n+2, and the start position of the song  02 : 10 minutes, 15 seconds, and frame  12 , is indicated by the sub-code Q of the large frames Nos. n+3, n+4, and n+5. Further, POINT=A 0  indicates the first song number in the disc, POINT=A 1  indicates the last song number in the disc, and POINT=A 2  indicates the head position of the lead-out area. In FIG. 10, the number of songs is 6.  
           [0011]    [0011]FIG. 11 shows the structure of one sector in the CD-ROM mode  1 . Herein, the signal format is the same as in CD-DA. Also, FIG. 12 is a diagram for comparing one sector of the CD-DA mode and one sector of the CD-ROM mode  1 .  
           [0012]    In the CD-ROM mode  1 , the length of a sector is the same as that of the CD-DA mode, that is, 2352 (=24×98) bytes. One sector includes:  
           [0013]    (1) synchronization signal data SKD of 12 bytes;  
           [0014]    (2) header HDD of 4 bytes;  
           [0015]    (3) user data USD of 2048 bytes; and  
           [0016]    (4) auxiliary data EDCC of 288 bytes for error correction/detection.  
           [0017]    Also, the sector includes 98 frames (one frame=24 bytes).  
           [0018]    Among the 4 bytes of the header HDD, 3 bytes (MIN/SEC/SECTOR) corresponds to address information and 1 byte corresponds to mode information indicating the type of data. The address information is represented by minute/second/sector as in the sub-code of CD-DA and basically includes the same elements as in the Q channel. As in the signal format of the CD-DA, a synchronization signal, a sub-code, and an error-correcting code (CIRC) are added to each frame.  
           [0019]    When data is recorded in a CD-ROM, data is compiled in one sector unit, a sub-code and a CIRC are added thereto as in the CD-DA, EFM modulation is performed so as to write the data, and the data is read from the CD-ROM in each sector by using the sub-code (absolute address of the Q-channel).  
           [0020]    As described above, data in the CD-ROM includes a logical sector of 2048 bytes and includes logical blocks whose number is 2 n  (n=0 in many cases). A volume space includes a system area and a data area. The system area occupies 16 sectors (logical sectors No. 0 to 15) from the head. In case of ISO9660, the data area includes a volume descriptor, a path table, a directory, and data, as shown in FIG. 13A. The volume descriptor includes a basic volume descriptor, a sub-volume descriptor, a volume block descriptor, etc.  
           [0021]    As shown in FIG. 13B, the basic volume descriptor includes a volume identifier A 0 , a logical block length A 1 , the size of path table A 2  for specifying the file structure (see FIG. 14), the position of the path table A 3 , and so on.  
           [0022]    As shown in FIG. 13C, the path table includes a directory identifier (folder name) B 1 , the number of parent directory B 2 , and the record position (extent position) of the directory (file) constituting a directory (folder) B 3 . A plurality of path tables of a directory, whose parent directory is a root directory, are sequentially recorded.  
           [0023]    As shown in FIG. 13D, the directory record includes a file identifier C 1 , a file start position (extent position) C 2 , date of file creation C 3 , the data length of a file C 4 , etc. The directory records of a plurality of files belonging to the directory (folder) are sequentially recorded.  
           [0024]    [0024]FIG. 14 is an example of the file structure. The root directory includes a plurality of directories (folders: R&amp;B, ROCK, JAZZ, POPS, etc.), the folder ROCK includes sub-folders ROCKMAN and ROCKWOMAN, the folder R&amp;B includes a plurality of MP3 files (MP3 music files) RB-1, RB-2, etc. Likewise, each of the other folders includes several MP3 music files. This file structure can be obtained by analyzing the above-described basic volume descriptor, path table, and directory.  
           [0025]    In CD-DA of the CD-DA format, the number and the position of songs can be recognized more easily than TOC information. Thus, sound can be quickly output when playback is performed. However, when a multisession disc, in which compressed music data of MP3 or the like is recorded in at least one session, is played back, it takes a long time for sound to be outputted. This is because the record structure (file structure) of each session must be analyzed to recognize the folders, file structure, and path structure, prior to commencement of playback. Thus, when compressed music data of MP3 or the like is recorded in the ISO9660 format, substantial time is required to analyze the record structure of the music data. The time for analysis becomes longer as the number of sessions increases. In this case, a delay ranging from several tens of seconds to one minute may be required for commencement of playback in a random playback as well as in a normal playback. Further, a silent state disadvantageously continues during that time.  
           [0026]    Also, when a random playback is performed by using a CD changer, the record structure of music data of all sessions of all CDs must be analyzed. Thus, even more time is required for starting playback.  
         SUMMARY OF THE INVENTION  
         [0027]    Accordingly, it is an object of the present invention to quickly output sound by shortening a silent time when a normal playback or a random playback is performed, even if non-compressed music data of the CD-DA format or MP3 compressed music data of the ISO9660 format is recorded in each session of multisessions.  
           [0028]    It is another object of the present invention to quickly output sound by shortening a silent time even when music recorded in a plurality of discs in a CD changer is randomly played back.  
           [0029]    According to a first embodiment of the present invention, a method for playing back a multisession disc in which music data of a CD-DA format is recorded in at least one session and compressed music data of another format is recorded in another session is provided. The method comprises the acts of: (1) searching for a session in which music data of the CD-DA format is recorded, reading the music data in the session from the disc at a high speed so as to write the music data in a shock proof memory, and reading the music data from the shock proof memory at a normal speed so as to play back the music data; (2) suspending reading of the music data from the disc every time the amount of music data in the shock proof memory reaches a first set value until the amount of data reaches a second set value, and meanwhile, analyzing the record structure of compressed music data recorded in other sessions so as to store the analysis result; and (3) playing back music in order based on the stored data when analysis of the record structure of all the sessions has been completed.  
           [0030]    In this way, non-compressed music data of the CD-DA format is read and played back first, and meanwhile, the record structure of music data in other sessions is analyzed. Accordingly, the first song is always a CD-DA song, but playback can be rapidly started.  
           [0031]    According to a second embodiment of the present invention, a method for playing back a multisession disc in which music data of a CD-DA format is recorded in at least one session and compressed music data of another format is recorded in another session is provided. The method comprises the acts of: (1) searching for a session in which music data of the CD-DA format is recorded when random playback is instructed, the playback order being randomly set in the random playback, reading the music data in the session from the disc at a high speed so as to write the music data in a shock proof memory, and reading the music data from the shock proof memory at a normal speed so as to play back the music data; (2) suspending reading of the music data from the disc every time the amount of music data in the shock proof memory reaches a first set value until the amount of data reaches a second set value, and meanwhile, analyzing the record structure of compressed music data recorded in other sessions so as to store the analysis result; and (3) randomly deciding a playback order of music so as to play back the music in the order when analysis of the record structure of all the sessions has been completed.  
           [0032]    In this way, non-compressed music data of the CD-DA format is read and played back, and meanwhile, the record structure of music data in other sessions is analyzed so as to decide the random playback order. Accordingly, the first song is always a song which is randomly selected from among the CD-DA songs, but the second song and the subsequent songs can be played back in the randomly decided order. Further, random playback can be rapidly started.  
           [0033]    According to a third embodiment of the present invention, a method for playing back discs in a disc changer is provided. The method comprises the acts of: (1) checking each disc so as to determine whether the disc contains music data of a CD-DA format and storing the result; (2) loading the disc containing music data of the CD-DA format when random playback is instructed, reading randomly-selected music data from the disc at a high speed so as to write the music data in a shock proof memory, and reading the music data from the shock proof memory at a normal speed so as to play back the music data; (3) suspending reading of the music data from the disc every time the amount of music data in the shock proof memory reaches a first set value until the amount of data reaches a second set value, and meanwhile, analyzing the record structure of music data recorded in each session of other discs so as to store the analysis result; and (4) randomly deciding a playback order of music so as to play back the music in the order when analysis of the record structure of all the sessions of all the discs has been completed.  
           [0034]    In this way, non-compressed music data of the CD-DA format is read and is played back first, and meanwhile, the record structure of music data of all the sessions of all the discs is analyzed so as to decide the random playback order. Accordingly, the first song is always a CD-DA song which is randomly selected, but after the record structure has been analyzed, playback can be performed in the randomly decided order from among all the songs in all the discs. Further, random playback can be rapidly started in the disc changer.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0035]    [0035]FIG. 1 shows the configuration of a disc player according to an embodiment of the present invention;  
         [0036]    [0036]FIG. 2 is a flowchart of a normal playback process of a multisession disc;  
         [0037]    [0037]FIG. 3 is a flowchart of a playback process using a mode in which only CD-DA format music is played back and a mode in which all songs are played back;  
         [0038]    [0038]FIG. 4 is a flowchart of a random playback process;  
         [0039]    [0039]FIG. 5 shows the configuration of a CD changer according to another embodiment of the present invention;  
         [0040]    [0040]FIG. 6 is a flowchart of a random playback process in the CD changer;  
         [0041]    [0041]FIG. 7 illustrates a multisession disc;  
         [0042]    [0042]FIG. 8 illustrates a CD-DA signal format;  
         [0043]    [0043]FIG. 9 illustrates a sub-code Q;  
         [0044]    [0044]FIG. 10 illustrates TOC in a lead-in area in a CD-DA mode;  
         [0045]    [0045]FIG. 11 shows the structure of one sector in a CD-ROM mode  1 ;  
         [0046]    [0046]FIG. 12 is a comparative diagram of one sector of the CD-DA and one sector of the CD-ROM;  
         [0047]    [0047]FIGS. 13A to  13 D show the volume structure of ISO9660; and  
         [0048]    [0048]FIG. 14 shows an example of the structure of a file. 
     
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0049]    (A) First Preferred Embodiment  
         [0050]    [0050]FIG. 1 shows the configuration of a disc player. In FIG. 1, the disc player includes a multisession disc  11  such as a CD-R; a turn table  12  on which the disc is loaded; a spindle motor  13  for rotating the disc at a predetermined constant linear velocity; an optical pickup  14  for detecting a digital recording signal of the disc; and a motor  15  for moving the optical pickup  14  in the radius direction of the disc. Also, the disc player includes an RF amplifier  16  for generating an EFM signal, an error signal, a CLV control signal, and so on based on the detection signal generated by the optical pickup  14 ; and a servo circuit  17  for performing servo-control in response to a command from a system controller  25 , which will be described later, by using an error signal (focus-error signal and tracking-error signal), a CLV control signal, and so on. The servo circuit  17  also controls a feed for moving the optical pickup  14  in the radius direction of the disc. Further, the disc player includes a digital signal processing circuit  18  which performs synchronization, deinterleave, and error correction with respect to an EFM signal output from the RF amplifier  16 , and which demodulates audio sample data so as to input the data to a shock proof memory controller  20 . Also, the digital signal processing circuit  18  demodulates TOC information, sub-code data, a volume descriptor of the ISO9660 format, a path table, and a directory so as to input them to the system controller  25 .  
         [0051]    The disc player also includes a DRAM (shock proof memory)  19  which temporarily stores non-compressed music data or compressed music data, the data being freely read/written from/in the DRAM  19 ; and the shock proof memory controller  20  which writes non-compressed music data or compressed music data in the shock proof memory  19  at a high speed and intermittently and which reads the data from the shock proof memory  19  at a high speed and sequentially while performing the writing process. When the shock proof memory  19  is filled with data (when the data amount reaches or surpasses a first set amount), the shock proof memory controller  20  notifies the system controller  25  of that fact, and suspends writing while continuing reading. Then, when a predetermined free space is generated in the shock proof memory  19  by reading out data, that is, when the data amount is reduced to or under a second set amount, the shock proof memory controller  20  notifies the system controller  25  that a free space is generated, and also restarts writing of non-compressed music data or compressed music data which is input from the digital signal processing circuit  18 .  
         [0052]    Further, the disc player includes an MP3 decoder  21  for decompressing the compressed music data which has been sequentially read from the shock proof memory  19  by the shock proof memory controller  20  so as to generate non-compressed normal music data; a switch  22  which selects and outputs the non-compressed normal music data which has been read from the shock proof memory  19  by the shock proof memory controller  20  or non-compressed normal music data which has been read from the MP3 decoder  21 ; a digital filter  23  for performing oversampling to the music data input by the switch  22 ; and a D/A converter  24  which converts the music data input from the digital filter  23  to analog data and which outputs the analog data to an external audio amplifier through an analog output terminal Aout. Further, the disc player includes the system controller  25  for performing analysis of the record structure of music data in each session and playback control of a multisession disc; a RAM  26  for storing TOC information of the CD-DA format and the analysis result of the record structure (file structure) of music data in each session; a display unit  27 ; and an operation unit  28  including various keys.  
         [0053]    (a) Normal Playback Control  
         [0054]    [0054]FIG. 2 is a flowchart of a normal playback process of a multisession disc.  
         [0055]    A multisession disc (mixed disc), in which music data of the CD-DA format (non-compressed music data) is recorded in at least one session and MP3 compressed music data of another format is recorded in another session, is loaded in the player and a playback command is outputted (act  101 ).  
         [0056]    The system controller  25 , referring to the second bit of control data of sub-code Q recorded in the lead-in area of each session, searches for a session of the CD-DA format (act  102 ), and if the session can be found, reads TOC information from the lead-in area of the session and stores it, and then starts high-speed reading of the head CD-DA song (act  103 ).  
         [0057]    The digital signal processing circuit  18  inputs the non-compressed normal music data of the CD-DA format, which has been read at a high speed from the CD, to the shock proof memory controller  20 . The shock proof memory controller  20  writes the input music data in the shock proof memory  19  at a high speed (act  104 ), reads music data from the shock proof memory  19  at a normal speed and inputs the read music data to the DA converter  24  through the switch  22  and the digital filter  23 , and the DA converter  24  DA-converts the input digital music data and inputs the data to the audio amplifier so that music is outputted through a speaker (act  105 ).  
         [0058]    The shock proof memory controller  20  checks the amount of data in the shock proof memory  19  (act  106 ) and continues acts  103  to  106  until the shock proof memory  19  is filled with data. When the shock proof memory  19  is filled with data, that is, when the amount of data reaches or surpasses the first set amount, high-speed reading of music data from the multisession disc  11  is suspended (act  107 ).  
         [0059]    In this state, the system controller  25  moves the pickup  14  to the lead-in area of the last session, analyzes the record structure of music data of all the sessions recorded in the lead-in area, and stores the identifier of the MP3 compressed music recorded in each session and the record position in the RAM  26  (act  108 ). Also, the system controller  25  checks whether analysis of all the sessions has been completed while analyzing the record structure (act  109 ). If not completed, the system controller  25  checks whether free space generation has been notified from the shock proof memory controller  20 , that is, whether the amount of data in the shock proof memory  19  is at the second set value (30%) or less (act  110 ). If free space generation is not notified, the process of analyzing the record structure in act  108  is continued. On the other hand, if free space generation is notified, the process of analyzing the record structure is stopped, high-speed reading of the CD-DA song is restarted (act  103 ), and subsequent steps are performed.  
         [0060]    If the above-described steps continue so that analysis of the record structure of music data of all the sessions is finally completed in act  109 , the songs in the disc are played back in order from the head after the CD-DA song which is now being played back has been completed (act  111 ). In this case, the CD-DA song which has already been played back may or may not be played back. Also, when time is required for analyzing the record structure of all the sessions so that a CD-DA song has been played back during that time, the next CD-DA song is played back.  
         [0061]    In this way, non-compressed music data of the CD-DA format is read and is played back first, and meanwhile, the record structure of music data in other sessions is analyzed. Accordingly, the first song is always a CD-DA song, but playback can be rapidly started.  
         [0062]    (b) Playback Control According To Playback Mode  
         [0063]    [0063]FIG. 3 is a flowchart of a playback process in which a first playback mode for playing back only non-compressed music of the CD-DA format and a second playback mode for playing back both of non-compressed music and compressed music are provided.  
         [0064]    First, a playback mode is set by using the operation unit  28  (act  201 ). When the playback mode is set, the system controller  25  determines whether the set playback mode is the first playback mode (CD-DA only mode) or the second playback mode (CD-DA/MP3 all mode) in act  202 .  
         [0065]    If the first playback mode (CD-DA only mode) is set, the system controller  25  refers to the second bit of the control data of the sub-code Q which is recorded in the lead-in area of each session so as to check whether a session of the CD-DA format exists (act  203 ). If a session of the CD-DA format can be found, TOC information is read from the lead-in area of the session and stored (act  204 ), and playback is started in order from the head CD-DA song (act  205 ). Then, it is determined whether all songs in the session of the CD-DA format have been played back (act  206 ). Playback of act  205  is continued until all songs have been played back. When all songs have been played back, the process returns to act  203 , where another CD-DA format session is searched for. If another session of the CD-DA format exists, the acts  204  to  206  are performed. On the other hand, if a session of the CD-DA format does not exist any more, playback in the first playback mode is completed.  
         [0066]    On the other hand, if it is determined that the second playback mode (CD-DA/MP3 all mode) is set in act  202 , playback control is performed in accordance with act  102  and the subsequent steps of normal playback shown in FIG. 2 (act  207 ).  
         [0067]    In this way, a user can select playback of only CD-DA songs or playback of all songs. In any case, music can be output quickly.  
         [0068]    (c) Random Playback  
         [0069]    [0069]FIG. 4 is a flowchart of a random playback process.  
         [0070]    A multisession disc (mixed disc), in which music data of the CD-DA format (non-compressed music data) is recorded in at least one session and MP3 compressed music data of another format is recorded in another session, is loaded in the player and a random playback command is output (act  301 ).  
         [0071]    The system controller  25  refers to the second bit of control data of sub-code Q recorded in the lead-in area of each session, searches for a session of the CD-DA format (act  302 ), and if the session can be found, reads TOC information from the lead-in area of the session and stores it, randomly selects a song to be played back from among the CD-DA songs (act  303 ), and then starts high-speed reading of the song to be played back (act  304 ).  
         [0072]    The digital signal processing circuit  18  inputs the non-compressed normal music data of the CD-DA format, which has been read at a high speed from the CD, to the shock proof memory controller  20 . The shock proof memory controller  20  writes the input music data in the shock proof memory  19  at a high speed (act  305 ), reads music data from the shock proof memory  19  at a normal speed and inputs the read music data to the DA converter  24  through the switch  22  and the digital filter  23 , and the DA converter  24  DA-converts the input digital music data and inputs the data to the audio amplifier so that music is output through the speaker (act  306 ).  
         [0073]    The shock proof memory controller  20  checks the amount of data in the shock proof memory  19  (act  307 ) and continues the acts  304  to  307  until the shock proof memory  19  is filled with data. When the shock proof memory  19  is filled with data, i.e., when the amount of data reaches or surpasses the first set amount, high-speed reading of music data from the multisession disc  11  is suspended (act  308 ).  
         [0074]    In this state, the system controller  25  moves the pickup  14  to the lead-in area of the last session, analyzes the record structure of music data of all the sessions recorded in the lead-in area, and stores the identifier of the MP3 compressed music recorded in each session and the record position in the RAM  26  (act  309 ). Also, the system controller  25  checks whether analysis of all the sessions has been completed while analyzing the record structure (act  310 ). If not completed, the system controller  25  checks whether free space generation has been notified from the shock proof memory controller  20 , i.e., whether the amount of data in the shock proof memory  19  is at the second set value (30%) or less (act  311 ). If free space generation is not notified, the process of analyzing the record structure in act  309  is continued. On the other hand, if free space generation is notified, the process of analyzing the record structure is stopped, high-speed reading of the CD-DA song is restarted (act  304 ), and subsequent acts are performed.  
         [0075]    If the above-described acts continue so that analysis of the record structure of music data of all the sessions is finally completed in step  310 , the playback order of all the songs except the CD-DA song which is now being played back is randomly decided (act  312 ), and random playback is performed in accordance with the decided order (act  313 ). In this case, the random playback order may be decided by excluding or including the CD-DA song which has already been played back. Also, when time is required for analyzing the record structure of all the sessions so that the CD-DA song has been played back during that time, the next CD-DA song, which has been randomly selected, is played back.  
         [0076]    In this way, non-compressed music data of the CD-DA format is read and is played back, and meanwhile, the record structure of music data in other sessions is analyzed so as to decide the random playback order. Accordingly, the first song is always a song which is randomly selected from among the CD-DA songs, but the second song and the subsequent songs can be played back in the randomly decided order. Further, random playback can be rapidly started.  
         [0077]    (B) Second Embodiment  
         [0078]    [0078]FIG. 5 is the configuration of a CD changer according to a second preferred embodiment of the present invention. In FIG. 5, elements which are the same as in FIG. 1 are denoted by the same reference numerals. A different point from the first embodiment is that the following elements are provided: a magazine  31  for accommodating a plurality of discs DK 1  to DKn; a disc changer mechanism  32  for changing discs; and a changer control unit  33  for controlling the disc changer mechanism  32  in accordance with the instructions from the system controller  25 .  
         [0079]    [0079]FIG. 6 is a flowchart of a random playback process in the CD changer.  
         [0080]    When the magazine  31  is attached, the system controller  25  automatically checks each disc so as to determine whether the disc is a CD-DA disc in which only music data of the CD-DA format is recorded, an MP3 disc in which only MP3 compressed music is recorded, or a multisession mixed disc in which music data of the CD-DA format is recorded in at least one session and MP3 compressed music is recorded in another session, and stores the result in the RAM  26  of a battery backup (act  401 ).  
         [0081]    In this state, when random playback is instructed (act  402 ), the system controller  25  searches for a CD-DA disc based on the above-mentioned storage information. If a CD-DA disc exists, the system controller  25  loads the CD-DA disc. If a CD-DA disc does not exist, a mixed disc is loaded. Then, the system controller  25  reads TOC information and stores it in the RAM  26  (act  403 ).  
         [0082]    Then, the playback order of the CD-DA songs is randomly decided (act  404 ) and high-speed reading of the songs to be played back is started (act  405 ). The digital signal processing circuit  18  inputs the non-compressed normal music data of the CD-DA format, which has been read from the CD at a high speed, to the shock proof memory controller  20 . The shock proof memory controller  20  writes the input music data to the shock proof memory  19  at a high speed (act  406 ), and at the same time, it reads music data from the shock proof memory  19  at a normal speed and inputs the data to the DA converter  24  through the switch  22  and the digital filter  23 . The DA converter  24  DA-converts the input digital music data so as to input the converted data to the audio amplifier. Accordingly, music is outputted through the speaker (act  407 ).  
         [0083]    The shock proof memory controller  20  checks the amount of data in the shock proof memory  19  (act  408 ) and continues acts  405  to  408  until the shock proof memory  19  is filled with data. When the shock proof memory  19  is filled with data, that is, when the amount of data reaches or surpasses the first set amount, high-speed reading of music data from the disc is suspended (act  409 ).  
         [0084]    In this state, the system controller  25  loads another disc, moves the pickup  14  to the lead-in area of the last session, analyzes the record structure of music data of all the sessions recorded in the lead-in area, and stores the identifier of the songs recorded in each session and the record position in the RAM  26  (act  410 ). Also, the system controller  25  checks whether analysis of the record structure of music data in all the sessions of all the discs has been completed while analyzing the record structure (act  411 ). If not completed, the system controller  25  checks whether free space generation has been notified from the shock proof memory controller  20 , i.e., whether the amount of data in the shock proof memory  19  is at the second set value (30%) or less (act  412 ). If free space generation is not notified, the process of analyzing the record structure in act  410  is continued. On the other hand, if free space generation is notified, the process of analyzing the record structure is stopped, high-speed reading of the CD-DA song is restarted (act  405 ), and subsequent acts are performed.  
         [0085]    If the above-described acts continue so that analysis of the record structure of music data of all the sessions of all the discs is finally completed in act  411 , the playback order of all the songs except the CD-DA song which is now being played back is randomly decided (act  413 ), and random playback is performed in accordance with the decided order (act  414 ). In this case, the random playback order may be decided by including the CD-DA song which has already been played back. Also, when time is required for analyzing the record structure of all the sessions of all the discs so that the first CD-DA song has been played back during that time, the next CD-DA song, which has been randomly selected, is played back.  
         [0086]    According to the preferred embodiments, music data of the CD-DA format is read and is played back first, and meanwhile, the record structure of music data in other sessions is analyzed. Therefore, the first song is always a CD-DA song, but playback can be rapidly started.  
         [0087]    Also, according to the preferred embodiments, music data of the CD-DA format is read and is played back first, and meanwhile, the record structure of music data in other sessions is analyzed so as to decide the random playback order. Accordingly, the first song is always a song which is randomly selected from among the CD-DA songs, but the second song and the subsequent songs can be played back in the randomly decided order. Further, random playback can be rapidly started.  
         [0088]    Further, according to the preferred embodiments, music data of the CD-DA format is read and is played back first, and meanwhile, the record structure of music data of all the sessions of all the discs is analyzed so as to decide the random playback order. Accordingly, the first song is a CD-DA song which is randomly selected, but after the record structure has been analyzed, playback can be performed in the randomly decided order from among all the songs in all the discs. Further, random playback can be rapidly started in the disc changer.