Performance degradation of PRML channels due to nonlinear distortions

The effects of nonlinear distortions on the performance of a partial response/maximum likelihood detection (PRML) channel are presented. The nonlinear distortions are introduced during the process of writing (transition shift) and reading (head distortion) data from a magnetic medium. Measurements were taken on the bench using simulated waveforms based on the Lorentzian wave shape, digital equalizer, and experimental PRML channel hardware. Head asymmetry and saturation, typical of experimental magnetoresistive heads, were modeled by adding quadratic and cubic distortions, respectively. Noise was injected to the unequalized signal, presenting a colored spectrum to the input of the detector. The errors were detected by comparing the output of the detector and a reference non-return-to-zero (NRZ) pattern. The system signal-to-noise ratio was computed using a sample distribution method. The effect of various kinds of nonlinearities on the eye diagram was studied, and templates which are useful in identifying the type of nonlinearity present in a real system were generated