Thomas D. Howell

Identification of nonlinear write effects using pseudorandom sequences

Nonlinear distortion causes significant degradation at high linear recording densities. Nonlinear distortion cannot generally be corrected by equalization and is thus particularly important to correctly identify and quantify. A technique is described which is easily applied, clearly separates linear from nonlinear effects, can be related to the underlying physical processes, and provides a good basis for modeling the channel. The technique involves the recording of a binary pseudorandom sequence and the capture and processing of the playback waveform. The technique is used to identify four distinct nonlinear effects and to examine their behavior as a function of flying height and record current.

Error rate performance of experimental gigabit per square inch recording components

The authors measured the ontrack and offtrack error-rate performance of several combinations of experimental heads and disks using a partial-response channel at approximately 1 Gb/in/sup 2/. The system components include a magnetoresistive head, a thin-film disk, and the partial response readback channel. Precompensation and equalization were used to tune the channel for best performance. Results indicate that these components can write and read data at 1 Gb/in/sup 2/ with good error rates and offtrack margins. >

A virtual machine emulator for performance evaluation

The Seventh Symposium on Operating Systems Principles, held in December 1979, continues an excellent tradition. Part of that tradition includes the publication of some of the best papers in a subsequent issue of the Communications of the ACM. The papers from the Seventh SOSP are included here, and give a good indication of the quality of the program from which they were selected. The symposium was excellently organized by the General Chairperson, Michael Schroeder, and his assistants. The quality of the program is evidence of the unusually competent performance of the program committee under the direction of Anita Jones. Her efforts materially reduced the difficulties of preparing this special issue of Communications. The remaining papers appear in the Proceedings of the Seventh Symposium on Operating Systems Principles, and are recommended reading for all those interested in current research in operating systems. R. Stockton Gaines Editor, Operating Systems Department

Analysis of correctable errors in the IBM 3380 disk file

A method of analyzing the correctable errors in disk files is presented. It allows one to infer the most probable error in the encoded-data stream given only the unencoded readback and error-correction information. This method is applied to the errors observed in seven months of operation of four IBM 3380 head-disk assemblies. It is shown that nearly all the observed errors can be explained as single-bit errors at the input to the channel decoder. About 90 percent of the errors were related to imperfections in the disk surfaces. The remaining 10 percent were mostly due to heads which were unusually susceptible to random noise-induced errors.

Global properties of tensor rank

Abstract The dependence of tensor rank on the underlying ring of scalars is considered. It is shown that the integers are, in a certain sense, the worst scalars. A ring of scalars can be improved by adjoining algebraic elements but not by adjoining indeterminates. The real closed fields are the best scalars among ordered rings, and the algebraically closed fields are best among all rings. Let B(Rm×n×p) be the maximum tensor rank of any m×n×p array of elements from the ring R. A generalization of Gaussian elimination shows that B(Rn×n×n)⩽ 3 4 n2 for most useful rings R. For every R, B(Rm×n×p)⩾mnp/(m+n+p), and slightly stronger lower bounds are proven for R a field.

Statistical properties of selected recording codes

Most recording systems encode their data using binary run-length-limited (RLL) codes. Statistics such as the density of Is, the probabilities of specific code strings or run lengths, and the power spectrum are useful in analyzing the performance of RLL codes in these applications. These statistics are easy to compute for ideal run-length-limited codes, those whose only constraints are the run-length limits, but ideal RLL codes are not usable in practice because their code rates are irrational. Implemented RLL codes achieve rational rates by not using all code sequences which satisfy the run-length constraints, and their statistics are different from those of the ideal RLL codes. Little attention has been paid to the computation of statistics for these practical codes. In this paper a method is presented for computing statistics of implemented codes. The key step is to develop an exact description of the code sequences which are used. A consequence of the code having rational rate is that all the code-string and run-length probabilities are rational. The method is illustrated by applying it to three codes of practical importance: MFM, (2, 7), and (197).

Exact bounds for Viterbi detector path metric differences

The authors address the problem of computing exact bounds on the path metric differences in maximum-likelihood sequence detectors based upon the Viterbi algorithm. The calculation of the bounds is formulated as a series of linear programming problems. Numerical results are presented for several examples of uncoded and coded partial-response channels of practical interest in digital data recording applications.<<ETX>>

Determining a signal to noise ratio for an arbitrary data sequence by a time domain analysis

An algorithm is developed for determining the signal to noise (S/N) ratio for an arbitrary data sequence by a time domain correlation analysis. By measuring the correlation coefficient of time domain readback voltages, r(V(t), V(t+nT)), the S/N for an arbitrary data sequence can be determined. The authors applied this method to obtain the S/N ratio for a magnetic recording thin film medium written by a 127-b pseudorandom sequence at different linear recording densities. The new algorithm may also provide a simple way to integrate the S/N measurement into a digital oscilloscope. >

Exact field calculations for asymmetrical finite-pole-tip ring heads

An exact solution is given for the fields of an infinitely permeable asymmetrical finite-pole-tip ring head. The solution is obtained by conformal mapping and is expressed in terms of elliptic integrals and elementary functions. Using the method of undetermined coefficients, it is shown how to decompose the Schwarz-Christoffel integral into elementary functions and elliptic integrals. For each given pole geometry, the constants appearing in the Schwarz-Christoffel formula are calculated once. Then the fields at various points in the poles domain are calculated using Newtons method and the formulas for the decomposition of the Schwarz-Christoffel integral. >

Advanced read channels for magnetic disk drives

Advanced read channels which sample the read signal and process those samples using algorithms borrowed from the communications industry allow us to reach higher storage densities and faster data rates than ever before. In this paper we describe some of the design issues involved in using a PRML (partial-response, maximum-likelihood) channel in a disk drive employing thin-film heads, zoned-recording, and embedded servo sectors. We discuss the effect of high transition densities required for a PRML channel on the design of heads and media. Finally, we survey some additional signal processing methods which might lead to even higher performance in the future. >