Patent ID: 8599670

Claim:
A Viterbi detector for identifying recorded data in a reproduced signal from an information recording medium, comprising: a branch-metric calculation unit that calculates branch metric values corresponding to the reproduced signal and a plurality of possible paths; a path metric calculator that includes an adder that adds a path metric value currently retained and the branch metric values corresponding to the reproduced signals of a plurality of possible paths, a comparator that compares results of addition by the adder corresponding to the plurality of possible paths with one another, a path selection circuit that refers to a result of comparison by the comparator and selects a single path out of the plurality of possible paths, and an update unit that updates the retained path metric value by the result of addition corresponding to the single path; and a decision circuit that performs a path decision based on the path metric value currently retained by the path metric calculator and the reproduced signal at a specific time instant that is a specified number of channel clocks earlier, wherein: the path selection circuit selects the single path based on the result of comparison by the comparator and a result of decision by the decision circuit, the Viterbi detector further comprising: decision means that decides whether or not a result of selection by the path selection circuit selected a mark-continuing path under the condition that the result of selection by the path selection circuit is not used to select the mark-continuing path, a counter that counts results of decision by the decision means, and a distortion decision circuit that calculates a mark distortion factor based on a result of count by the counter.