Experimental error rates in an EPRML channel versus asymmetry

The effects of readback distortions on the ontrack error rate performance (OTER) of a disk drive system with an extended partial response maximum-likelihood (EPRML) channel were determined experimentally. The amount of distortion in the giant magnetoresistive (GMR) sensor was varied by changing the magnitude of the sense current and by applying an external magnetic field. The OTER measurements were performed at 280 Mb/s using linear densities from 240 to 440 kbpi, corresponding to a 1.8-3.3 range in channel bit densities. The OTER degradation was less than one decade for pulse amplitude asymmetries between -20% and +10%. The minimum OTERs were produced when the pulsewidths, not the amplitudes, were equal for the two signal polarities and the OTERs were correlated to the width of the wider of the two polarities. Equal pulse amplitudes do not correspond to equal pulsewidths because the resistance versus magnetic field response does not have inversion symmetry through any quiescent biasing point: a cubic polynomial is an insufficient representation, and higher order terms are required for a valid representation.