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A magnetic recording channel can be regarded as a “partial-response” channel because of its inherent differentiation in the readback process. The conventional NRZI method of recording is shown to be equivalent to the “precoding” of this particular partial-response channel, the purpose of which is to limit the propagation of error in the channel output. Using this new viewpoint, one can readily adopt an error detection scheme (developed for general partial-response channels) that takes full advantage of the inherent redundancy in the three-level channel output. The detection scheme is optimum in the sense that it detects all detectable errors with minimum delay.
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