On the performance of symbol- and fractionally-spaced equalization in digital magnetic recording

Equalization is a mature technology in digital communications and is used in many applications to improve the spectral efficiency (bits/sec/Hz) of the channel and reduce sensitivity to channel variations. It has the same potential for digital magnetic recording systems and may be used to achieve similar benefits, namely, (1) higher storage density on available head/medium combinations, and (2) reduced performance sensitivity to variation in the readback signal resulting from manufacturing tolerances, etc. The purpose of this paper is to describe analytic and simulated results on the performance of digital equalization with partial response methods. Two partial response signals, namely, Class 4 (PR4) and extended-Class 4 (EPR4), are considered. The digital equalizer is configured as a transversal filter (also called finite impulse response, FIR, filter) that linearly combines a finite sequence of samples of the readback signal to form a desired output. The sampling interval equals either the symbol period or a fraction thereof, but the output is computed at the symbol rate, 1/T. The resulting approaches are referred to as synchronous and fractionally-spaced equalization (FSE) respectively