Chapter 12 - PRML channels

Abstract

When recording densities are low, each transition written on a magnetic medium results in a relatively isolated voltage peak, and a peak detection channel works well to recover written information. At high channel densities, however, the peak detection channel cannot provide reliable data detection. Superposition of pulses, referred to as linear intersymbol interference (ISI), shifts the peaks of readback signal and increases the probability of errors in the zero-crossing detector. At the same time, the signal amplitude is lowered at high densities and the errors in the threshold detector also increase. The partial-response maximum likelihood (PRML) channels were proposed to overcome the problem of linear ISI and are now the dominant detection schemes in commercial magnetic disk drives. The PRML detection method is not based on voltage peaks, but rather takes into account the fact that the signals from adjacent transitions interfere. PRML channels consist of two relatively independent parts―partial response equalization and maximum likelihood detector.