Editing Partial-response maximum-likelihood (section)

{{Short description|Method for interpreting data in digital storage systems}}

In [[computer data storage]], '''partial-response maximum-likelihood''' ('''PRML''') is a method for recovering the [[Digital signal (electronics)|digital data]] from the weak analog read-back signal picked up by the [[Disk read-and-write head|head]] of a magnetic [[Hard disk drive|disk drive]] or [[tape drive]]. PRML was introduced to recover data more reliably or at a greater [[areal density (computer storage)|areal-density]] than earlier simpler schemes such as peak-detection.<ref>G. Fisher, W. Abbott, J. Sonntag, R. Nesin, "[https://ieeexplore.ieee.org/document/542278 PRML detection boosts hard-disk drive capacity]", IEEE Spectrum, Vol. 33, No. 11, pp. 70-76, Nov. 1996</ref> These advances are important because most of the digital data in the world is stored using [[magnetic storage]] on [[hard disk drive|hard disk]] or [[tape drive]]s.

Ampex introduced PRML in a tape drive in 1984. IBM introduced PRML in a disk drive in 1990 and also coined the acronym PRML. Many advances have taken place since the initial introduction. Recent [[read/write channel]]s operate at much higher data-rates, are fully adaptive, and, in particular, include the ability to handle nonlinear signal distortion and non-stationary, colored, data-dependent noise ([[noise-predictive maximum-likelihood detection|PDNP or NPML]]).

''Partial response'' refers to the fact that part of the response to an individual bit may occur at one sample instant while other parts fall in other sample instants. ''Maximum-likelihood'' refers to the detector finding the bit-pattern most likely to have been responsible for the read-back waveform.