The role of SP in data-storage systems

Among the key components in the development of a successful storage system are heads, media, and signal processing. In the past, major breakthroughs in the heads and media technologies have been mainly responsible for the spectacular growth in storage capacity, but signal processing is increasingly recognized as a cost-efficient means of improving density. This article addresses the issues relevant to signal processing. While the focus is on magnetic storage, most of the signal-processing strategies discussed are also applicable to optical storage. This article begins with a description of the read/write (R/W) process in magnetic storage. The characteristics of noise and nonlinearity present in real magnetic storage channels are also discussed, and a discussion on channel modeling is followed by a description of different modulation codes. Different suboptimal sequence detectors and equalizers are then presented that represent the techniques that are either already in use in commercial systems or being investigated for near-future applications. We then discuss sequence detectors, with in-depth treatment given to those based on tree search, particularly finite or fixed delay tree search (FDTS) detectors. Finally, performance evaluation results are presented under various channel conditions, which should be useful in assessing the potential benefits of different coding and sequence-detection strategies.