Apparatus and method for implementing viterbi butterflies

The Viterbi algorithm (20) is performed with a reduced number of calculations when the comparing step (C, 80, 50) is anticipated before the selecting (S, 62, 64, 66, 68) and adding steps (A, 74, 76). During comparing (C), selection decisions (e.g., D (i)) are obtained by analyzing pairs of old path metrics (e.g., P (2i, j-1) at 91, P (2i+1, j-1) at 92) by subtracting and multiplying the path metrics with branch metrics (e.g., B (i, j) at 95) and combining intermediate sign (e.g., SP, SB, SDELTA) by e.g., and-logic (50). Selecting decisions (e.g., D (i/2, j+1), D (i/2+N/2, j+1) and new path metrics (e.g., P (i, j), P (i+12, j) are continuously stored and updated.