K.D. Fisher

Adaptive equalization in magnetic-disk storage channels

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 customers premises. The differences between data storage and data transmission channels are examined. The storage channels 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.<<ETX>>

An adaptive RAM-DFE for 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. >

The ITU-T's new g.vector standard proliferates 100 mb/s dsl

This article explores the recently issued ITUT G.vector (G.993.5) that allows expanded use of 100 Mb/s DSL. A tutorial description on G.vectors crosstalk noise reduction methods leads to specific projections and measurements of expanded DSL 100 Mb/s reach. A discussion on dynamic maintenance to enhance G.vectors practical application then concludes this article.

A MMSE interpolated timing recovery scheme for the magnetic recording channel

Advances in VLSI technology permit the use of interpolated timing recovery (ITR) as a replacement for the conventional VCO-based phase lock loop. Fully digital ITR has the advantage of lower cost and higher stability. In this paper, we present an interpolated timing recovery scheme that requires almost no oversampling, which should be suitable for high-speed storage channels.

A Survey Of Adaptive Equalization For Magnetic-disk Storage Channels

This paper surveys the use of adaptive equalization to increase the density, as well as reduce the manufacturing costs, of commerical magnetic-disk storage systems. We list some of the most important characteristics that must be considered by the adaptive equalization designer in ap- plying equalization and adaptive filtering methods to the magnetic disk storage system. Specifically, we survey the important problems of radial channel variation, offtrack ef- fects, channel nonlinearity, and datadependent noise, and discuss their effects upon conventional equalization meth- ods. We then describe modifications of these conventional equalization methods that can be used to design an adap- tive equalization system that is appropriate for disk storage systems.

An adaptive DFE for storage channels suffering from nonlinear ISI (magnetic recording)

The authors explore the magnetic recording channel in some detail and describe some of the characteristics of the channel that point to the random access memory decision feedback equalizer (RAM-DFE) as a suitable detection method. The structure and adaptation of the RAM-DFE are described. The RAM-DFE with the signed-LMS (least mean square) algorithm and the constrained step size ( mu =2/sup -n/) is shown to be well suited to the magnetic storage application. Not only does RAM-DFE outperform the conventional DFE under many circumstances, but it is also more conducive to a high-speed digital implementation, making the RAM-DFE appropriate for the higher data rates (linear densities) that its improved performance facilitates. Preliminary investigations indicate that the RAM-DFE can be implemented in 1.6- mu m CMOS on a single VLSI chip (about 50000 transistors) with a sampling (data) rate of 54 MHz.<<ETX>>

Modelling thin-film storage channels

A straightforward method for modeling nonlinearity and data-dependent (transition-dependent) media noise in a thin-film storage channel is presented. The least-squares approach for measuring the channel linear pulse response is described, and this FIR (finite impulse response) channel model is contrasted with a nonlinear moving average modeling technique described in terms of a random-access memory used as a finite state machine (FSM). It is shown that the channel nonlinearity is largely trailing in time, and performance figures of modeling accuracy for the channel FSM are presented. The nonlinear distortion increased dramatically at higher data rates for the thin-film disk examined. In addition, the variation of the channel noise power according to the flux transition density is described. A jitter noise effect is identified. A FSM modeling technique for identifying this noise power variation as a function of the channel input data pattern is given. Finally, it is shown how to combine these two modeling techniques to create arbitrarily long, representative read signals for the thin-film disk. By using these compact channel models, one can easily test new coding or equalization techniques. >

A high-speed adaptive equalizer for magnetic disk drives

The design of a high-speed adaptive decision feedback equalizer (DFE) is described for detection of magnetic disk data. A RAM is used in the feedback path instead of the usual transversal filter, and the equalizer is referred to as the RAM-DFE. The RAM compensates for trailing nonlinear distortion in the disk data. However, use of the RAM makes the equalizer more sensitive to latency in the adaptive update and can slow training considerably when using LMS adaptation. A technique is presented for training the RAM which reduces convergence time to near that of a transversal feedback filter. The RAM-DFE design is targeted for a VLSI implementation using BiCMOS technology. Simulation predict clock speeds in excess of 50 MHz, corresponding to data rates in excess of 50 Mb/s. The design also includes circuits to perform digital timing recovery and initial synchronization.<<ETX>>

On the combination of equalization and coding in saturation recording

The authors establish the importance of considering equalization loss and coding gain together in evaluating the performance of a particular code in a saturation-recording storage channel. They illustrate methods by which it is possible, under this measure of performance, to increase the effective density using well-designed equalizers and codes in combination. It is concluded that, while coding gain of existing codes can be used to increase areal density at fixed or reduced linear density, new codes designed specifically for families of partial-response polynomials at increasing clock rates or density are required for significant further linear density increase.<<ETX>>

Analysis of and detector comparisons using the microtrack model of magnetic recording

We rely on accurate and relatively simple models of the magnetic recording process so that signal processing algorithms that operate with different system parameters can be compared fairly. The microtrack model of thin-film recording, posed by researchers to mimic the random, zig-zag transition boundaries, satisfies our requirements for an accurate and simple model. This paper summarizes analysis of the microtrack model done by Caroselli and Wolf (1995, 96) and presents additional results including simulations of detectors. Parallels are drawn between the microtrack model and other models including a stationary additive noise model, a model of partial erasure, and a model of transition jitter, amplitude reduction, and pulse width jitter. The parameters used by the microtrack model can be derived from physical media; we derive parameters for these other models from those of the microtrack model.