The realization of a digital recording system using partial-response class-IV signaling with maximum-likelihood sequence detection (MLSD) is described. To perform MLSD at the high data rates encountered in recording systems, a simple implementation of the Viterbi detector based on a difference-metric algorithm is developed. 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 are presented. The dynamic behavior of the control algorithms was studied by computer simulations. Coding was 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. 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. >

A PRML system for digital magnetic recording

A class of partial response systems of the form P n (D) = (1 - D)(1 + D)n, where D is the delay operator and n is a non-negative integer, is described for the spectral shaping of the readback signal to achieve higher recording densities. The properties of the desired response at the detector input are examined. It is shown that the use of higher values of n can result in higher recording densities without excessive noise enhancement, or, equivalently, pulse slimming. The cost for increased density is higher complexity in the required maximum likelihood detector.

A class of partial response systems for increasing storage density in magnetic recording

The application of Viterbi detection to Class IV partial response on a magnetic recording channel is discussed. The theoretical performance is analyzed in the presence of noise correlation and quantizing errors. A simplified algorithm is described which was implemented on a transverse-scan magnetic tape recorder operating at a linear density of 1800 bits/mm and a transfer rate of 120 Mbits/s. Following an initial 5 bit analog-to-digital conversion of the ternary waveform, the Viterbi detector is implemented entirely in high-speed digital emitter-coupled logic. An advantage of approximately 1.5 dB in effective signal-to-noise ratio is gained over a simple level detector, a result which agrees well with the theoretical development.

Viterbi Detection of Class IV Partial Response on a Magnetic Recording Channel

The use of adaptive equalization to increase storage density is discussed. Adaptive equalization is attractive since it permits a significant reduction in manufacturing costs by allowing a greater component yield due to relaxed tolerances and also permits a reduction in servicing costs because of a reduced need for fine-tuning on the customer's premises. The differences between data storage and data transmission channels are examined. The storage channel's important signal-processing characteristics are described, covering read and write processes, detection methods, and various types of distortion that can occur in storage channels. The use of signal-to-noise ratio as a performance measure is considered. Equalization methods for peak detection and for sampling detection are discussed. Gains in both linear and areal (track) density are addressed. Some of the basic performance advantages of using adaptive equalization are illustrated. The future of communication technology in storage systems is assessed. >

Adaptive equalization in magnetic-disk storage channels

A modification of the decision feedback equalizer (DFE), RAM-DFE, is presented and analyzed for use in channels with trailing nonlinear intersymbol interference, especially binary saturation-recording channels. In the RAM-DFE, a look-up table, which can be easily implemented with random access memory, (RAM), replaces the transversal filter feedback section of the DFE. The feedforward section of the equalizer remains linear. A general nonlinear Markov (or finite-state machine) model is used to model the nonlinear intersymbol interference (ISI) channel. With this Markov model, a method is introduced for computing the minimum-mean-squared-error settings of the feedforward filter coefficients and the feedback filter and look-up table contents for the linear DFE and the RAM-DFE, respectively. RAM-DFE with these settings can be significantly better than the linear DFE for channels with trailing nonlinear ISI. Globally convergent gradient-type algorithms for updating the feedforward section coefficients and the contents of the feedback table are introduced and analyzed. Results based on data taken from disk storage units are discussed. >

An adaptive RAM-DFE for storage channels

Communication technology has been applied to a magnetic disk recording channel to achieve up to a fourfold increase in linear bit density as compared to conventional binary recording. Among the techniques incorporated were digital data transmission by Class IV Partial-Response signaling (Interleaved NRZI), recording channel pre-emphasis, equalization and filtering, and periodic amplitude sampling of the data signal. The magnetic recording channel was linearized using very high frequency a.c. bias, which also served simultaneously to erase old data. This enabled multilevel recording and the addition of a pilot tone for timing recovery. System block diagrams are presented together with a discussion of the optimization procedure and attained system performance.

H. Melbye

Papers

An experimental, multilevel, high density disk recording system