Patent ID: 6970522

Claim:
A method of retrieving data from a partial response channel having implicit rules, said data having been read from a helical scan magnetic tape storage medium that stores said data in tracks having alternate azimuth magnetizations, the data read from the tracks having alternate azimuth magnetizations having a predominate crosstalk noise component at a relatively low frequency compared to the frequency of the read data, said stored and read data comprising a series of codewords encoded from input data using a DC-free code, each of said codewords having a digital sum variation (DSV) bounded within a first preset range at the codeword boundary, said method comprising: equalizing said data to a partial response target having a substantial DC response; and reducing by a substantial amount the predominate low frequency noise component from the equalized data by applying a set of equalized data to a time-varying trellis decoder that rejects candidate paths that violate (a) the DSV bounds and (b) the partial response rules.