The development of video disk systems has made long-play digital (PCM) audio disk systems possible. The bandwidth required for the two channels of digital audio signals is less than that of video signals and, combined with a reduction of the revolution, makes the longer playing time possible. For a further improvement in packing density, a kind of run-length-limited code is adopted. The code is known as 3PM (three-position modulation), and the packing density can attain 150% of MFM coding for the same minimum wavelength to be recorded. This is achieved at the expense of the decreasing jitter margin, which is relatively easy to solve in optical disk systems. A playing time of 2 1/2 hours is compiled on one side of an optical disk with a diameter of 303 mm. The sampling rate is 44.056 kHz and each of the two channels is coded by 16-bit linear quantization. The revolution is 450 r/min. Code errors are analyzed for each revolution of 1800, 900 and 450 r/min on the plane of bit error rate and bit error correlation coefficient. An effective error-correcting scheme called "cross interleave" has been developed which makes it possible to decode using various types of decoder from a simple erasure type to the complex "crossword" type while maintaining full compatibility.
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