1. A likelihood detector apparatus for receiving a stream of data values from a data medium, wherein the received data values correspond to ideal values but may include added noise that is dependent on previous noise and dependent on data on the data medium, said ideal values being determined by possible values of data on the medium, and for outputting information specifying a sequence of states corresponding to the stream of received data values, said sequence of states corresponding to possible data values on the medium, the apparatus comprising: a first detector, for calculating state sequence likelihood information based on a first noise model and received data values, and for providing said state sequence likelihood information to a second detector; a second detector for calculating weighting values indicating likelihoods that a data value received at a particular time corresponds to a particular state transition, based on a second noise model, received data values, and state sequence likelihood information from the first detector; a traceback unit for determining a most likely sequence of data values using said calculated weighting values; and an output for outputting information specifying said determined sequence of states.