Time-varying modulo N trellis codes for input restricted partial response channels

摘要

A method and apparatus for generating high rate run length limited trellis codes and increasing minimum distance between output sequences of partial response channels with constrained channel inputs without requiring codes with spectral nulls. A Viterbi detector replicates a conventional trellis structure for the channel N times. The N copies of the channel response trellis are interconnected such that a preselected function associates each state in the trellis with a particular integer value modulo N. The number N is selected according to the channel detection and coding constraints so that diverging erroneous sequences of minimum distance lead to detector states which are distinct from the correct detector state. The detector trellis is time-varying such that only certain values of the preselected function are allowed every m bits. The time-variation assures there are no minimum distance extensions of erroneous sequences beyond a predetermined length. Reliability of storage channels is desirably increased, because more noise is required to overcome the additional distance and cause an error in distinguishing the correct encoded sequence.