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This paper describes a Viterbi detector and margin circuit for PR4 magnetic recording channels. The implementation uses a modified difference-metric formulation of the Viterbi algorithm (VA). A Viterbi margin (VM) function is included for channel quality checking purposes. Modifications and implementation techniques that optimize power consumption and speed are described. The VA+VM circuit includes 24+24 bits of path and margin memory, uses 11 mW of power (7 mW for VA alone) at 480 MHz and occupies 0.052 mm/sup 2/ in a 0.25 /spl mu/m 1.8 V CMOS process.