The development of video disc systems has created the possibility of a long play digital (PCM) audio disc system. The band width required for two channels of digital audio signals is less than that of video signals, and reduction of the revolution makes the longer playing time possible. To additionally improve packing density, a kind of run length limited code is adopted. The code is called 3PM (three-position modulation), and the packing density can be 150% of MFM coding for the same minimum wave length to be recorded. This is achieved at the expense of decreasing jitter margin, something relatively easy to solve in optical disc systems. The playing time of two and a half hours is achieved on one side of the optical disc with a diameter of 30cm. The sampling rate is 44.056kHz and each of two channels is coded by 16 bit linear quantization. The revolution is 450 rpm. Code errors are analyzed for each revolution of 1800, 900 and 450 rpm on the plane of bit error rate and bit error correlation coefficient. An effective error correcting scheme named -Cross Interleave- is described which makes it possible to decode by various levels from the simple erasure type to the more complex -Cross Word- type, while maintaining full compatibility.
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