1. A signal processing device comprising: a first waveform equalizer configured to equalize a waveform of an input signal according to a first partial response characteristic and to output a first partial response signal; a second waveform equalizer configured to equalize the waveform of the input signal according to a second partial response characteristic and to output a second partial response signal; a first Viterbi decoder configured to decode the first partial response signal output from the first waveform equalizer by means of Viterbi decoding process; and a second Viterbi decoder configured to decode the second partial response signal output from the second waveform equalizer by means of Viterbi decoding process; and a synthesizer configured to perform weighting addition for a log likelihood ratio of a posteriori probability value output from the first Viterbi decoder and a log likelihood ratio of an extrinsic value output from the second Viterbi decoder, and wherein the input signal is reproduced based on an output of the synthesizer.