Partial-response decoding of rate 1/2 modulation codes for digital storage

In digital storage systems, receivers for rate 1/2 modulation codes are usually oversampled by a factor of two with respect to the data stream that they attempt to reconstruct. It is shown that oversampling can be avoided by detecting a decimated ternary signal, from which the original data can be recovered by a simple decoder. A method is described to find all such decoders for a given rate 1/2 code. Examples treated are modified frequency modulation, Miller-squared, and two-to-seven. Mean-square performances of the old and new reception schemes are compared for a Lorentzian channel. The relative merits are found to depend quite heavily on the information density. The results obtained seem to contradict the belief that high-density performance benefits from a large minimum run length.<>