Partial-response maximum-likelihood: Difference between revisions

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=== Write precompensation ===

The presence of nonlinear transition-shift (NLTS) distortion on [[Non-return-to-zero|NRZ]]{{dn|date=November 2019}} recording at high density and/or high data-rate was recognized in 1979.<ref>[https://ieeexplore.ieee.org/document/1060300 R. Wood, R. Donaldson, "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>[https://ieeexplore.ieee.org/document/1065310 D. Palmer, P. Ziperovich, R. Wood, T. Howell, "Identification of Nonlinear Write Effects Using Pseudo-Random Sequences", IEEE Trans. Magn., Vol. MAG-23, no. 5, pp. 2377-2379, Sept. 1987]</ref><ref>[https://ieeexplore.ieee.org/document/5680698/ D. Palmer, J. Hong, D. Stanek, R. Wood, "Characterization of the Read/Write Process for Magnetic Recording", IEEE Trans. Magn., Vol. MAG-31, No. 2, pp. 1071-1076, Mar. 1995 (invited)]</ref> <br>

Ampex was the first to recognize the impact of NLTS on PR4.<ref>[https://ieeexplore.ieee.org/document/1064566 P. Newby, R. Wood, "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 />. 'Precomp.'is viewed as a necessity for a PRML system and is important enough to appear in the [[BIOS]] HDD setup<ref>[http://www.kva.kursk.ru/bios1/HTML1/standard.html Kursk: BIOS Settings - Standard CMOS Setup, Feb 12, 2000]</ref> although it is now handled automatically by the HDD.