Optical disk reproducing apparatus

An optical disk reproducing apparatus comprising:

• a pickup irradiating laser light to an optical disk and converting reflected light thereof to an electric signal;
• an analog front end performing an analog processing to the electric signal outputted from the pickup;
• a digitalization circuit digitalizing an analog signal outputted from the analog front end and outputting a multi-level signal;
• a first PRML(Partial Response Maximum Likelihood) processing circuit of a PRML method with constrained length of m;
• and
• a second PRML processing circuit of the PRML method with the constrained length of n where n≧m,
• wherein the first PRML processing circuit includes:
• a first equalization circuit equalizing the multi-level signal with a target signal of the PRML method with the constrained length of m;
• a first reference-value generation circuit generating a first target signal of the PRML method with the constrained length of m from a first equalization signal outputted from the first equalization circuit and a first binarization signal consisting of a maximum-likelihood decoded output;
• a first adaptive learning circuit determining a parameter of the first equalization circuit from the first equalization signal and the first target signal;
• and
• a first maximum-likelihood decoding circuit obtaining the first binarization signal from the first target signal and the first equalization signal based on the PRML method with the constrained length of m,
• wherein the second PRML processing circuit includes:
• a second equalization circuit equalizing the multi-level signal with a target signal of the PRML method with the constrained length of n;
• a second reference-value generation circuit generating a second target signal of the PRML method with the constrained length of n from a second equalization signal outputted from the second equalization circuit and a second binarization signal comprised of a maximum-likelihood decoded output;
• a second adaptive learning circuit determining a parameter of the second equalization circuit from the second equalization signal and the second target signal;
• and
• a second maximum-likelihood decoding circuit obtaining the second binarization signal from the second target signal and the second equalization signal based on the PRML method with the constrained length of n, and
• wherein the optical disk reproducing apparatus includes:
• a switch selecting and outputting one of an output of the first maximum-likelihood decoding circuit and an output of the second maximum-likelihood decoding circuit;
• a decoder performing a decode-processing to an output of the switch and outputting a reproduction data signal;
• and
• a determination circuit performing switching control of the switch between the output of the first maximum-likelihood decoding circuit and the output of the second maximum-likelihood decoding circuit based on a result of comparison between a first equalization error value consisting of a difference between the first equalization signal and the first target signal calculated by the first adaptive learning circuit and a second equalization error value comprised of a difference between the second equalization signal and the second target signal calculated by the second adaptive learning circuit.