Partial-response coding, maximum-likelihood decoding: capitalizing on the analogy between communication and recording [History of Communications]

Signal processing and coding technology for digital magnetic recording is the core technology of the channel electronics module in a hard disk drive (HDD) that processes signals read from magnetic media. In this historical review I focus on what is now widely known as partial-response, maximum-likelihood (PRML) technology, which takes advantage of the inherent redundancy that exists in signals read out of magnetic media; its theoretical foundation goes back to 1970, and it capitalizes on the analogy between high-speed data transmission and high-density digital recording, and that between a convolutional code and a partial-response signal. The first PRML-based product was introduced by IBM in 1990, and PRML technology soon became the industry standard for all digital magnetic recording products, ranging from computers´ HDDs and tape drives to micro hard discs used in PCs, mobile phones, and MP3 players; use of the PRML principle has recently been extended to optical recording products such as CDs and DVDs. Its improved version, called NPML (noise-predictive, maximum-likelihood), and variants have been adopted by the HDD industry since 2000. Today, a large number of communication and information theory researchers are investigating use of advanced techniques such as turbo coding/decoding to further improve the density and reliability of both magnetic and optical recording systems.