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 wherein the input signal is reproduced based on an output of the first Viterbi decoder and an output of the second Viterbi decoder.