1. A two-stage detector for a partial response channel having a known channel transfer function; the channel having a channel code encoder for encoding information sequences into code words in accordance with a predetermined channel code enabling detection of any single occurrence within a code word of an error-event of a list of error-events known likely to occur on the partial response channel, the list of error-events being characterized in that adding any error-event from the list to any code word of the channel code will not produce another code word; the channel code including but not being limited to modulation codes concatenated with systematic parity codes, the channel code not including matched-spectral-null codes and linear block codes with Hamming distance greater than a number of bit errors in each of the error-events on the list; the two-stage detector including: a channel sampler for providing samples of the partial response channel equalized in accordance with a time domain response characteristic of the partial response channel; a first-stage detector connected to receive the samples from the channel sampler, the first-stage detector being matched to characteristics of the partial response channel and not to the channel code, for putting out unchecked bit estimates; and, a second-stage post-processor connected to receive the unchecked bit estimates and the samples; the post-processor including a sequence checker for detecting error-events in the unchecked code words, and a sequence corrector for correcting the most-likely combination of error-events based on channel metric information generated from the samples, the unchecked bit estimates, the known channel transfer function, and the channel code, and for putting out checked bit estimates.