1. A likelihood detector apparatus to receive 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 to output information that specifies 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, which uses silicon area and is configured to calculate state sequence likelihood information based on a first noise model and received data values, and configured to provide said state sequence likelihood information to a second detector; said second detector, which is configured to calculate 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 configured to determine a most likely sequence of data values using said calculated weighting values; and an output configured to output information that specifies said determined sequence of states, wherein the first detector is configured to receive a data input corresponding to said stream of data values from a first equalizer and the second detector is configured to receive a data input from a second equalizer.