As known in the art, the CD (compact disc) system currently employed as a main-current acoustic apparatus is of such a type that pits are formed in a 12 cm-diameter disc in a manner to correspond to digital PCM (pulse code modulation) data and reproduction is performed by, while rotating the disc in a drive rotation in a CLV (constant linear velocity) system, linearly tracking the data from an inner periphery side to an outer periphery side on the disc by a semiconductor laser and an optical pickup built in a photoelectric conversion element.
Audio playback musical tone data and sub-code data P, Q, R to W defined for control/display are recorded in the disc. Of the aforementioned data, the sub-code Q is also called as address data and represents, in a program area containing disc's musical tone data, a program number (TNO) of recorded musical tone data, a phrase number (INDEX), a lapse time (TIME) for each program, a total time (ATIME) lapsing from a start position of the program area, etc.
Further, the aforementioned sub-code data Q represents, in a lead-in area located on the inner periphery side of the program area, a start address of each program as TOC (table of contents) data. That is, the sub-code data Q is recorded for implementing a search operation, accurately at high speeds, for selective reproduction of playback data from vast information recorded in the disc to enable a stereo reproduction to be performed for about one hour and grasping the state of a reproduction on the disc.
The CD system has initially been developed so as to record and reproduce musical data. In recent years, with attention paid to such a vast recording capacity, a CD-ROM (read only memory) system is determined to be used as a read-only data recording medium for the disc through the use of a musical data recording area of the disc as a digital information recording area. The CD-ROM system is of such a type as to record and reproduce digital information on the disc without varying a recording/reproduction format for musical data reproduction in the CD system and to do so by adding a new format thereto.
In the existing digital data processing system using a CD-ROM system, however, when there occurs a shift in frequency of a clock serving as a reference of a data processing speed relative to a host processing section side and disc reproduction section side, the buffer memory at the host processing section side is placed in an over- or an underflowed state, failing to make a normal data reproduction and thus presenting a problem.
The present invention is achieved, taking the above situations into consideration and the object of the present invention is to provide a much better digital data processing apparatus which, even when there occurs a shift in frequency of a clock serving as a reference of a data processing speed relative to a host processing section side and disc reproduction section side, can prevent a buffer memory on the host processing section side from being placed in an over- or an underflowed state and make a normal data reproduction.