Analysis of a Biphase-Based Servo Format for Hard-Disk Drives

Abstract—Biphase modulation in an embedded-servo format for hard-disk drives is investigated. It is shown that for biphase, at the low linear densities typical of servo information, near-max-imum-likelihood performance can be attained by a simple bit de-tector consisting of a full-response linear equalizer and a binary slicer. Compared to the commonly used method of dibit coding, a signal-to-noise ration gain of some 4 dB is achieved. The same equalizer may be used as the basis for near-maximum-likelihood position error signal amplitude estimation and timing recovery. Simulations of a practical servo demodulator based on a fifth-order analog filter show that at typical linear densities, this ideal perfor-mance is closely approached. The equalizer has a band-pass char-acter and yields excellent suppression of the effects of thermal as-perities and magneto-resistive head asymmetry. Index Terms—Biphase modulation, disk drive, position error signal, servo. I