Disk drive comprising a trellis detector having a read signal whitener in the ACS circuit

A disk drive is disclosed comprising a disk, a head actuated over the disk to generate a read signal, and a trellis detector for detecting an estimated data sequence from the read signal. The trellis detector comprises a sampling device operable to sample the read signal to generate a sequence of signal sample values, and a plurality of add/compare/select (ACS) circuits each corresponding to a state in a trellis. Each ACS circuit comprises a first and second branch metric calculators for computing first and second branch metrics in response to first and second errors adjusted in response to first and second deltas that compensate for a distortion in the read signal.