APPARATUS FOR USING DIFFERENTIAL DATA DETECTION IN WRITE PROTECTION OF AN OPTICAL RECORDER

摘要

Apparatus for differential decoding of fixed-block encoded data, comprising: a medium having data recorded thereon in a plurality of symbols each having a predetermined number of symbol positions of a normal media state and a further having at least one subset of predetermined positions having only one position of second media state; means for reading data from said medium and generating read signals corresponding to the media states; a symbol counter (46) for counting the symhol positions within a symbol, including means (122, 124) for intialising the symbol counter after reaching the count of the last symbol position of a symbol; at least two sample and hold cells (114) responsive to the read signals; a timing circuit (44) responsive to the symbol counter (46) for triggering a first of the sample and hold cells (114) at a first predetermined symbol position of a symbol and responsive to said symbol counter (46) for triggering a second of said sample and hold cells (114) a second predetermined symbol position, said second symbol position being within the at least one subset of symbol positions having media of said second state at one position; a comparator (116) for comparing said first and second sample and hold cell signal values with each other and for determining and signalling which of the two is greater; means (118) responsive to the symbol counter (46) and to the comparator (116) for retaining the read signal contained in the sample and hold cell (114) having the greater value and for triggering the other sample and hold cell at a next symbol position within the subset which may have media of said second state and for repeating said retention and said triggering of the sample and hold cell having the lowest value at the occurrence of every subsequent symbol position within the subset which may have media of said second state; and means (120) for determining from the states of the comparator the location of the subset position having the media of said second state.