A PRML System for Digital Magnetic Recording

Abstract

Application of partial-response (PR) signaling and maximum-likelihood sequence detection (MLSD) to digital magnetic recording has been shown in theory and practice to further increase the storage densities and reliability that systems using run-length limited (RLL) coding and peak detection (PD) still the prevalent signal processing techniques today can currently achieve. In this paper, the realization of a digital recording system using PR class-IV signaling with MLSD (PRML) is described. To perform MLSD at the high data rates encountered in recording systems, a simple implementation of the Viterbi detector is developed based on a difference-metric algorithm. We present decision Directed schemes for gain control and timing recovery, for tracking variations of the gain and timing phase during data readback, and for fast initial adjustment from a known preamble. The dynamic behavior of the control algorithms is studied by computer simulations. Coding is used to facilitate timing recovery and gain control, to limit the path memory length of the Viterbi detector, and to allow fast and reliable startup of the receiver. The design and properties of rate-8/9 constrained codes are examined. Finally, the problem of equalization is addressed, and analog and combined analog/digital filter implementations are developed. A simple adaptive equalizer capable of compensating variations of the recording channel characteristics with track radius and/or head-to-medium distance is described. © 1992 IEEE