PURPOSE:To easily perform a detection processing for a pair of state metrics of most likelihood by performing ACS computation with a division processing so as to be unified at every system. CONSTITUTION:In a viterbi decoder which decodes a convolution code, a pair of branch metrics of most likelihood before the ACS computation are detected 10, and the pair of state metrics of mos likelihood last time are detected 12, and the most likelihood value of the state metric this time is detected from the pair of branch metrics of most likelihood before the ACS computation and the pair of state metrics of most likelihood last time. And metric is normalized by using the detected most likelihood value of the state metric this time. At such a case, the ACS computation is performed with the time division processing so as to unify systems to obtain the pair of state metrics. In such a way, the pair of state metrics of most likelihood easily is detected.
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