TARGET LEVEL SETTING METHOD FOR A SIGNAL USED IN MAXIMUM-LIKELIHOOD DECODING IN PRML PROCESSING OF A SIGNAL REPRODUCED FROM AN OPTICAL INFORMATION RECORDING MEDIUM, PLAYBACK DEVICE FOR AN OPTICAL INFORMATION RECORDING MEDIUM, AND TARGET LEVEL SETTING PROGRAM FOR A SIGNAL USED IN MAXIMUM-LIKELIHOOD DECODING IN PRML PROCESSING

摘要

Target levels for the PRML method are set adaptively, under conditions of simplicity and low cost. The target level setting method for playback from an optical information recording medium comprises: a step which reproduces a signal from light reflected from an optical information recording medium illuminated with laser light; a step for subjecting the reproduced signal to waveform equalization in accordance with prescribed partial response characteristics, and generating a waveform-equalized reproduced signal; a step for detecting the peak level of each of a plurality of prescribed codes, i.e. marks and spaces, from the waveform-equalized reproduced signal without regard to the code conditions adjacent to the codes; and a setting step which, based on the relative relationships between the detected peak levels, determines and sets a plurality of target levels which are used in Viterbi decoding and are arranged with unequal spacing.