1. Trellis processing apparatus for calculating branch metrics in a reduced state trellis comprising a processor adapted to pre-calculate for a trellis state a sum of products of channel coefficient values and signal symbol values corresponding to at least the oldest part of the section of the channel that is not hypothesized by the state such that the sum can be re-used in calculating branch metrics for different trellis branches leading out of different states and further comprising a memory for storing the products.