Patent ID: 8107339

Claim:
A reproduced signal evaluation method for evaluating a quality of signals reproduced from an information-recorded medium, comprising: a binarized bitstream acquisition step to decode the reproduced signals by using a PRML method to time-sequentially acquire binarized bitstreams, the PRML method comparing reproduced signals to target signals in a plurality of state changes, and selecting a most likely state change, wherein a restraint length of the PRML is equal to or greater than 5, wherein the reproduced signals are modulated using a run length limited code with a shortest run length of 2T, and wherein the quality of the reproduced signals is evaluated by a difference between parameters Pa and Pb, with parameter Pa being an index representing a probability of a first state change, and Pb being an index representing a probability of a second state change; and a predetermined combination detection step to detect a predetermined combination of a most likely state change (first state change), and a secondary most likely state change (second state change), using a main bitstream group of four main bitstreams having a length L=5+2i, wherein a parameter i represents an integer equal to or greater than 0, the predetermined combination detection step including: comparing the main bitstreams with a part of the binarized bitstream, and selecting a first main bitstream from the main bitstream group, generating a second main bitstream by performing on the first main bitstream a particular calculation including the parameter i, determining state changes, including the first main bitstream as the first state change, and the second main bitstream as the second state change.