A method and apparatus are provided for word synchronization with large coding
distance and fault tolerance for a partial-response maximum-likelihood (PRML) data
channel in a direct access storage device (DASD). A Viterbi detector receives equalized
PR4 samples including a predefined word synchronization pattern. The Viterbi detector
is a dedicated detector optimized for detecting the predefined word synchronization
pattern. The Viterbi detector includes a two-state Viterbi trellis and a word synchronization
detector for the two-state Viterbi trellis. The predefined word synchronization
pattern includes only even length magnets. The predefined word synchronization
pattern is a repetition code including pairs of ones and pairs of zeros and includes
multiple pattern match sequences. The Viterbi detector is optimized with branches
removed from the Viterbi trellis, thus increasing coding distance. The two-state
Viterbi trellis and word synchronization detector of the Viterbi detector operate
on a 2T basis, where 1/T is the sample rate.