Introduction

In a digital system, the so-called “user-data” is typically an unconstrained sequence of binary digits 0 and 1. In such devices it is the responsibility of the information storage subsystem to record the data and to reproduce it faithfully and reliably upon request. To achieve high densities and fast data rates, storage systems are usually pushed to the limit at which the strength of the readout signal is of the same order of magnitude as that of the noise. Operating under these circumstances, it should come as no surprise that errors of misinterpretation do indeed occur at the time of retrieval. Also, because individual bits are recorded on microscopic areas, the presence of small media imperfections and defects, dust particles, fingerprints, scratches, and the like, results in imperfect reconstruction of the recorded binary sequences. For these and other reasons to be described below, the stream of user-data typically undergoes an encoding process before it finally arrives on the storage medium. The encoding not only adds some measure of protection against noise and other sources of error, but also introduces certain useful features in the recorded bit-pattern that help in signal processing and future data recovery operations. These features might be designed to allow the generation of a clocking signal from the readout waveform, or to maintain the balance of charge in the electronic circuitry, or to enable more efficient packing of data on the storage medium, or to provide some degree of control over the spectral content of the recorded waveform, and so on.