Patent ID: 8091009

Claim:
An apparatus, comprising: a processing module; and a memory, coupled to the processing module, that is operable to store operational instructions that enable the processing module to employ a trellis to demodulate a plurality of symbols having colored and signal-dependent noise and for each of the plurality of symbols: recursively calculate a corresponding forward metric with respect to the trellis using a first branch metric term based on at least one other symbol of the plurality of symbols; recursively calculate a corresponding backward metric with respect to the trellis using a second branch metric term that is based on at least one other symbol of the plurality of symbols; and calculate a corresponding LLR (log likelihood ratio) by multiplying at least one forward metric corresponding to a first time, at least one backward metric corresponding to a second time, and at least one probability; and wherein: the plurality of symbols having colored and signal-dependent noise generated by processing a signal, having been perpendicularly recorded, received from a read channel coupled to a storage media of a hard disk drive (HDD); and the at least one probability being a product of a plurality of terms including a first term being a duplicate of the at least one forward metric, a second term being a duplicate of the at least one backward metric, and a third term being “a priori” probability of input information bits within the signal corresponding to transition via the trellis from a first state at the first time to a second state at the second time.