Digital sum value corrective scrambling in the compact digital disc system

In an apparatus for recording information in the form of a track of optically detectable pit and land areas in an optical record carrier, the information being a stream of successive symbol blocks of binary data bits which prior to being recorded are encoded into corresponding successive symbol blocks of channel bits comprising merging bits for minimizing the d.c. unbalance of successive symbol blocks of channel bits;

• such apparatus comprising an input terminal for receiving the successive symbol blocks of data bits, a first encoder coupled to such input terminal for encoding the symbol blocks of data bits into a stream of corresponding successive symbol blocks of channel bits, and an output terminal to which said stream of channel bits is supplied;
• the improvement characterized in that such apparatus further comprises:
• detector circuit means coupled to said first encoder for producing a control signal when the d.c. unbalance value of any part of the stream of channel bits produced by said first encoder does not comply with predetermined criteria, such control signal continuing until a succeeding part of the stream of channel bits complies with such criteria;
• circuit means coupled to said first encoder for selectively scrambling the data bits of the successive symbol blocks of data bits;
• and switching circuit means selectively coupling said scrambling circuit means and said first encoder to said output terminal, said switching circuit means being further coupled to said detector circuit means and responsive to the control signal produced thereby to selectively supply parts of the stream of scrambled bits produced by said scrambling circuit means to said output terminal in place of those parts of the stream of unscrambled bits produced by said first encoder having a d.c. unbalance value which does not comply with said predetermined criteria;
• whereby the channel bit stream supplied to said output terminal has a d.c. unbalance value which complies with said predetermined criteria.

In an apparatus for reading information which has been recorded in the form of a track of optically detectable pit and land areas in an optical record carrier, the information being a stream of successive symbol blocks of binary data bits which prior to being recorded are encoded into corresponding successive symbol blocks of channel bits comprising merging bits for minimizing the d.c. unbalance of successive symbol blocks of channel bits, parts of the recorded stream of channel bits having been selectively scrambled prior to recording and such parts including information identifying them as having been selectively scrambled;

• such apparatus comprising means for reading the symbol blocks of channel bits from the record carrier, a first decoder coupled to said read means for decoding the stream of symbol blocks of channel bits into a stream of symbol blocks of data bits corresponding thereto, and an output terminal to which the stream of symbol blocks of data bits is supplied;
• the improvement characterized in that such apparatus further comprises:
• a detector circuit connected to said read means and responsive to the recorded information identifying selectively scrambled parts of the stream of channel bits to produce a control signal in response to such information;
• circuit means coupled to said first decoder for selectively descrambling the data bits of the stream of successive symbol blocks of data bits produced by said first decoder, such descrambling being the inverse of the scrambling to which the recorded stream of data bits were subjected during recording;
• and switching circuit means selectively coupling said descrambling circuit means and said detector circuit to said output terminal, said switching circuit means being further coupled to said detector circuit means and responsive to the control signal produced thereby to selectively supply parts of the stream of descrambled data bits produced by said descrambling circuit means to said output terminal in place of those parts of the stream of data bits produced by said first decoder which had been selectively scrambled prior to recording in the record carrier;
• whereby the data bit stream supplied to said output terminal reproduces the data bit stream corresponding to the information recorded in the optical record carrier.