Patent ID: 7738202

Claim:
A decoder for decoding servo data recorded on a magnetic disk drive, said magnetic disk drive having a magneto-resistive head that includes a pinned layer of a predetermined polarity, the servo data recorded using wide-bi-phase encoding of user data recorded in servo fields, the decoder comprising: an A/D converter for sampling signals emitted by a magneto-resistive sensor which is configured to sense magnetization in domains within servo fields and to generate a sequence of encoded samples; a decoder assembly having a first detector for decoding the sequence of encoded samples generated by the A/D converter, thereby generating a first sequence of decoded bits, said first detector having a first trellis comprising nodes representing states, connected by paths representing transitions among the nodes, said first trellis being arranged to converge if a pinned layer associated with the magneto-resistive sensor has a polarity in a first direction, and said first detector being configured to compute a first quality value for each decoded bit in the first sequence of decoded bits, said first quality value being determined based upon a given actual sample and a corresponding expected sample, in the first sequence decoded bits produced by the A/D converter, said decoder assembly further including a second detector generating a second sequence of decoded bits, said second detector having a second trellis comprising nodes representing states connected by paths representing transitions among the nodes, said second trellis being arranged to converge if the pinned layer has a polarity in a direction opposite to said first direction, and said second detector being configured to compute a second quality value for each decoded bit in the second sequence of decoded bits, said second quality value being determined based upon an actual sample and a corresponding expected sample in the second sequence decoded bits produced by the A/D converter; comparator means for comparing the quality value outputs of said first and second detectors, said quality value outputs representing a distance between each actual sample and a corresponding expected sample in the sequence output by the A/D converter the comparator generating an output control signal, which indicates the sequence of decoded bits that has the lower quality value; and a selector configured to receive the outputs of the first and second detectors of the decoder assembly and the output control signal of said comparator, the output control signal of said comparator signaling the selector to select the output sequence of decoded bits that has the lower quality value as a selected sequence, and the selector being further configured to determine a pinned layer reversal, if a polarity of the selected sequence of decoded bits is opposite to the predetermined polarity.