A decoding process for decoding a received block of N systematic binary data samples or N systematic duobinary data samples using a maximum a posteriori probability (MAP) decoding algorithm. The decoding process calculates a set of four log-likelihood values using the corresponding forward state metric, reverse state metric, and branch metric values for each of N/2 pairs of systematic binary data or each of N/2 pairs of duobinary data in the received block. The decoding process also calculates, for each set of four log-likelihood values a delta value corresponding to the difference between the largest and the second largest of the four log-likelihood values in each set. The decoding process repeats for at least a second iteration. The decoding process is stopped based on a plurality of delta values calculated during two consecutive iterations.