1. A decoder for a received digital signal comprising: a branch metric unit to determine a correlation between symbols in the digital signal, in part, by adding a common term to all branch metric values before determining the correlation, the common term being based on a combination of input symbols; a path metric unit coupled to the branch metric unit to determine metrics for paths through a decision tree; and a traceback unit coupled to the path metric unit to select a path and a corresponding symbol sequence in the received digital signal based on the path metric determinations.