Nonlinearities in high density digital recording

In high density digital magnetic recording, successful data recovery depends on proper equalization and compensation to minimize both linear and nonlinear distortion. A combined theoretical and experimental investigation of distortion, with particular emphasis on nonlinearities at very high recording flux reversal density, is presented. To clearly expose the contribution of data dependent nonlinear distortion, the linear portion is removed by a versatile equalizer. Several unique data patterns with characteristics pertinent to the investigation have been identified and employed. Results in the range of 1000-5000 flux-changes-per-centimeter on an IBM 3350 type head/disk interface are given for the purpose of experimental confirmation. Among equally spaced recorded transitions, we observed that linear distortion exhibits mostly symmetrical timing spreading while nonlinear distortion shows a directional trend of timing shift (downstream peak shift) accompanied by amplitude loss. Furthermore, predictions based on the linear superposition principle for the given interface are inaccurate beyond 3000 fccm, where nonlinear distortion becomes apparent and gradually emerges as the dominant component.