a maximum likelihood decoder includes metric generators for generating metrics based on a plurality of partial responses and a viterbi decoder for realizing maximum likelihood decoding by using a synthetic metric generated by synthesizing the metrics . a first partial response is an original partial response . a second partial response is a differential response generated by subtraction by shifting the first partial response by 1 clock . alternatively , the differential response may be generated by subtraction by shifting the first partial response by 2 clocks . the second partial response may be a response generated by addition by shifting the first partial response by 2 clocks . alternatively , the second partial response may be an integration response generated by adding all previous samples of the first partial response .