Editing Partial-response maximum-likelihood (section)

=== Write precompensation ===
The presence of nonlinear transition-shift (NLTS) distortion on [[Non-return-to-zero|NRZ]] recording at high density and/or high data-rate was recognized in 1979.<ref>R. Wood, R. Donaldson, "[https://ieeexplore.ieee.org/document/1060300 The Helical-Scan Magnetic Tape Recorder as a Digital Communication Channel]", IEEE Trans. Mag. vol. MAG-15, no. 2, pp. 935-943, March 1979</ref> The magnitude and sources of NLTS can be identified using the 'extracted dipulse' technique.<ref>D. Palmer, P. Ziperovich, R. Wood, T. Howell, "[https://ieeexplore.ieee.org/document/1065310 Identification of Nonlinear Write Effects Using Pseudo-Random Sequences]", IEEE Trans. Magn., Vol. MAG-23, no. 5, pp. 2377-2379, Sept. 1987</ref><ref>D. Palmer, J. Hong, D. Stanek, R. Wood, "[https://ieeexplore.ieee.org/document/5680698/ Characterization of the Read/Write Process for Magnetic Recording]", IEEE Trans. Magn., Vol. MAG-31, No. 2, pp. 1071-1076, Mar. 1995 (invited)</ref>

Ampex was the first to recognize the impact of NLTS on PR4.<ref>P. Newby, R. Wood, "[https://ieeexplore.ieee.org/document/1064566 The Effects of Nonlinear Distortion on Class IV Partial Response]", IEEE Trans. Magn., Vol. MAG-22, No. 5, pp. 1203-1205, Sept. 1986</ref> and was first to implement [[Write precompensation]] for PRML NRZ recording. 'Precomp.' largely cancels the effect of NLTS.<ref name=8inch /> Precompensation is viewed as a necessity for a PRML system and is important enough to appear in the [[BIOS]] HDD setup<ref>{{Cite web |url=http://www.kva.kursk.ru/bios1/HTML1/standard.html |title=Kursk: BIOS Settings - Standard CMOS Setup, Feb 12, 2000 |access-date=October 8, 2019 |archive-date=October 4, 2018 |archive-url=https://web.archive.org/web/20181004103524/http://www.kva.kursk.ru/bios1/HTML1/standard.html |url-status=dead }}</ref> although it is now handled automatically by the HDD.