A maximum likelihood detector receiving a data stream corresponding to ideal values which may include noise, and outputting information specifying a sequence of states of maximum likelihood selected from possible states corresponding to the data stream according to weighting value selections made by the processors, the ideal values being determined by the possible states, including: a pre-processor to obtain first weighting values; processors in a hierarchy, each processor in a select level of the hierarchy is programmed to use, respectively, a plurality of the weighting values to calculate subsequent weighting values indicating respective likelihoods that a section of the data stream values corresponds to each of a plurality of possible state sequences, for each possible initial state and each possible final state, to select further weighting value of highest likelihood corresponding to a state sequence from the initial state to the final state.