Partial-response maximum-likelihood: Difference between revisions

Revision as of 04:35, 19 October 2019

Revision as of 04:47, 19 October 2019

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The first implementation of PRML was shipped in 1984 in the Ampex Digital Cassette Recording System (DCRS). The chief engineer on DCRS was [[Charles Coleman (engineer)|Charles Coleman]]. The machine evolved from a 6-head, transverse-scan, digital [[video tape recorder]]. DCRS was a cassette-based, digital, instrumentation recorder capable of extended play times at very high data-rate.<ref>[http://www.thic.org/pdf/Oct96/ampex.twood.pdf T. Wood, "Ampex Digital Cassette Recording System (DCRS)", THIC meeting, Ellicott City, MD, 16 Oct., 1996 (PDF)]</ref> It became Ampex' most successful digital product.<ref>[https://www.computerhistory.org/collections/catalog/102788145 R. Wood, K. Hallamasek, "Overview of the prototype of the first commercial PRML channel", Computer History Museum, #102788145, Mar. 26, 2009]</ref>

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The heads and the read/write channel ran at the (then) remarkably high data-rate of 117 Mbits/s.<ref>[https://ieeexplore.ieee.org/document/5261308 C. Coleman, D. Lindholm, D. Petersen, and R. Wood, "High Data Rate Magnetic Recording in a Single Channel", J. IERE, Vol., 55, No. 6, pp. 229-236, June 1985. (invited) (Charles Babbage Award for Best Paper)]</ref> The PRML electronics were implemented with four 4-bit, [[Plessey]] [[analog-to-digital converter]]s (A/D) and [https://en.wikichip.org/wiki/fairchild/100k 100k ECL logic].<ref>[https://www.computerhistory.org/collections/catalog/102741157 Computer History Museum, #102741157, "Ampex PRML Prototype Circuit", circa 1982]</ref>. ItsA similar channel was implemented at 20 Mbit/s on a prototype 8-inch HDD<ref name=8inch>[https://ieeexplore.ieee.org/document/1063460 R. Wood, S. Ahlgrim, K. Hallamasek, R. Stenerson, "An Experimental Eight-inch Disc Drive with One-hundred Megabytes Per Surface", IEEE Trans. Mag., vol. MAG-20, No. 5, pp. 698-702, Sept. 1984. (invited)]</ref>. These implementations and their mode of operation isare best described in a paper by Wood and Petersen.<ref>[https://ieeexplore.ieee.org/document/1096563 R. Wood and D. Petersen, "Viterbi Detection of Class IV Partial Response on a Magnetic Recording Channel", IEEE Trans. Comm., Vol., COM-34, No. 5, pp. 454-461, May 1986 (invited)]</ref>

Petersen was granted a patent on the PRML channel but Ampexit was never tookleveraged advantageby of it.Ampex<ref>[https://patents.google.com/patent/US4504872A/en D. Petersen, "Digital maximum likelihood detector for class IV partial response", US Patent 4504872, filed Feb. 8, 1983]</ref>.

=== Hard Disk Drives (HDD) ===

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=== Write Precompensation ===

The presence of nonlinear transition-shift (NLTS) distortion on [[NRZ]] 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>[https://ieeexplore.ieee.org/document/1063460 R. Wood, S. Ahlgrim, K. Hallamasek, R. Stenerson, "An Experimental Eight-inch Disc Drive with One-hundred Megabytes Per Surface", IEEE Trans. Mag., vol. MAG-20, No. 5, pp. 698-702, Sept. 1984.name=8inch (invited)]</ref>. '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.

== Further Developments ==