Ho Wai Wong-Lam

Dual decision feedback equalizer

The detector of Dahlman and Gudmundson (1988) ranks among the simplest known extensions of the decision feedback equalizer (DFE). The article develops a simplification of this detector, and evaluates its performance vis-a-vis other near maximum likelihood detectors. Comparable performances are observed at much lower complexity levels.

A class of data-aided timing-recovery schemes

This paper develops a class of data-aided timing-recovery schemes whose structure and operating principle are reminiscent of zero-forcing equalizer adaptation. The schemes exhibit no pattern-dependent jitter and are applicable to a variety of modulation codes and reception techniques. They come in both oversampled and baud-rate forms, and permit maximum-likelihood tracking performance whenever the oversampling factor satisfies the sampling theorem. Subject to this condition, even simple schemes can be close to optimum. It is shown that various existing; schemes belong to the class under study or are closely related to it. >

Structure and adjustment of a novel write-precompensation scheme

At high data rates and information densities, recorded data patterns in digital magnetic recording systems can exhibit severe nonlinearities. Key causes for these nonlinearities are bandwidth limitations of the write path and demagnetizing fields in the recording medium. The present note reviews the structure of these nonlinearities, develops a simple yet generic write-precompensation scheme that is capable of counteracting them, and derives a technique for adjusting this scheme according to information extracted within the data receiver.

A general solution of write equalization for digital magnetic tape recording

A general solution of digital recursive filters which produce two-level output is given. To give a rough idea of the performance of write equalizers, a high-density recording channel corrupted by additive noise is appraised, and the signal-to-noise ratio with and without a write equalizer is compared. Calculations show that a small gain can be expected in using some of the write equalizers. The linearity of the channel was tested using some of the write equalizers. >

A simple transition response model for magnetoresistive heads on thin-film media

Using an analysis similar to that of Potter (1975), we develop a model to describe the transition response for magnetoresistive (MR) heads. The model developed is called the Lorentzian-gap model and includes the effects of shield gap length and MR element thickness. By bringing in a small frequency-independent phase shift, the left-right asymmetry of the transition response and baseline offset for isolated pulse measurements can be modeled. Applying the Lorentzian-gap model to geometrical parameters of the head-media system (HMS) results in a transition response which is close to the measured one. Further improvement of the modeling can be obtained by using the tanh transition model instead of the commonly used arctan model.

Optimization of azimuth angle and other geometrical parameters in digital tape recording

In guardband-less tape recorders, azimuth recording is commonly used to reduce intertrack interference. Previous work on the optimization of the azimuth angle for digital recording [1] uses the criterion of minimum on-track bit error rate (BER). In this article, we consider the maximum tracking margin to be a more relevant criterion. Furthermore, the calculation model in [1] is extended to cover a wider range of equalization and detection schemes, including the increasingly popular partial response Class IV equalization with Viterbi detector (VD). The gain of using VD over the conventional threshold detection is also discussed. BER measurements on several recorders with different geometrical configurations serve to verify the validity of the calculation results. >