1. A decoder for determining the log likelihood logarithmically expressing the probability of passing a given state on the basis of the received value regarded as soft-input and decoding the input by using the log likelihood, said decoder comprising: a linear approximation means for calculating a correction term to be added to the log likelihood, the correction term being expressed in a one-dimensional function relative to a variable; and said linear approximation means being adapted to compute said correction term using a coefficient representing the gradient of said function for multiplying said variable, the coefficient being expressed as a power exponent of 2.