A soft output Viterbi algorithm (SOVA) decoder arranged to decode symbols received over a transmission channel, the symbols indicating a state transition between two states of a plurality of states that determines a decoded data value, the SOVA decoder comprising a reliability memory unit including at least four stages of logic units, each logic unit including a single buffer and at least four stages including a plurality of full stages comprising a separate logic unit corresponding to each of the plurality of states; and a plurality of compact stages including half or less than half the number of logic units than the number of the plurality of states, each logic unit corresponding to two of the plurality of states.