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

An optical information recording medium processor that sets a target level of a signal to be employed in maximum likelihood decoding in PRML processing of a reproduction signal of an optical information recording medium, comprising:

• a means for producing a reproduction signal from reflected light of laser light irradiated to an optical information recording medium;
• a means for performing waveform equalization processing, which depends on a predetermined partial response characteristic, on the reproduction signal to produce a waveform-equalized reproduction equalization signal;
• a means for detecting values of peak levels of a mark and a space of a shortest symbol and marks and spaces of symbols of 5T or more, respectively, from the reproduction equalization signal irrespective of a condition for a symbol adjoining the symbol;
• and
• a setting means for:
• selecting a symbol of 5T or more that has a mark peak level attaining a maximum value among said symbols of 5T or more when a low-to-high recording method has been used in the optical information recording medium, or selecting a symbol of 5T or more that has a mark peak level attaining a minimum value among said symbols of 5T or more when a high-to-low recording method has been used in the optical information recording medium,
• normalizing the respective peak levels of the mark and the space of the shortest symbol in accordance with the peak levels of the mark and the space of the selected symbol of 5T or more, and
• determining and setting a value of a center level of a plurality of target levels used in Viterbi decoding in accordance with an intermediate level between the normalized peak level of the mark and the normalized peak level of the space of the shortest symbol.