Partial-response maximum-likelihood: Difference between revisions

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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>T. Wood, "[http://www.thic.org/pdf/Oct96/ampex.twood.pdf Ampex Digital Cassette Recording System (DCRS)]", THIC meeting, Ellicott City, MD, 16 Oct., 1996 (PDF)</ref> It became Ampex' most successful digital product.<ref>R. Wood, K. Hallamasek, "[https://www.computerhistory.org/collections/catalog/102788145 Overview of the prototype of the first commercial PRML channel]", Computer History Museum, #102788145, Mar. 26, 2009</ref>

The heads and the read/write channel ran at the (then) remarkably high data-rate of 117&nbsp;MbitsMbit/s.<ref>C. Coleman, D. Lindholm, D. Petersen, and R. Wood, "[https://web.archive.org/web/20191007051735/https://ieeexplore.ieee.org/document/5261308 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>Computer History Museum, #102741157, "[https://www.computerhistory.org/collections/catalog/102741157 Ampex PRML Prototype Circuit]", circa 1982</ref> The PRML channel outperformed a competing implementation based on "Null-Zone Detection".<ref>J. Smith, "[https://ieeexplore.ieee.org/document/1089924 Error Control in Duobinary Data Systems by Means of Null Zone Detection]", IEEE Trans. Comm., Vil 16, No. 6, pp. 825-830, Dec., 1968</ref> A prototype PRML channel was implemented earlier at 20&nbsp;Mbit/s on a prototype 8-inch HDD,<ref name=8inch>R. Wood, S. Ahlgrim, K. Hallamasek, R. Stenerson, "[https://ieeexplore.ieee.org/document/1063460 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> but Ampex exited the HDD business in 1985. These implementations and their mode of operation are best described in a paper by Wood and Petersen.<ref>R. Wood and D. Petersen, "[https://ieeexplore.ieee.org/document/1096563 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 it was never leveraged by Ampex.<ref>D. Petersen, "[https://patents.google.com/patent/US4504872A/en Digital maximum likelihood detector for class IV partial response]", US Patent 4504872, filed Feb. 8, 1983</ref>

=== Hard disk drives ===

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The PRML channel for the IBM 0681 was developed in [[IBM Rochester]] lab. in Minnesota<ref>J. Coker, R. Galbraith, G. Kerwin, J. Rae, P. Ziperovich, "[https://ieeexplore.ieee.org/document/278677 Implementation of PRML in a rigid disk drive]", IEEE Trans. Magn., Vol. 27, No. 6, pp. 4538-43, Nov. 1991</ref> with support from the [[IBM Zurich]] Research lab. in [[Switzerland]].<ref>R.Cidecyan, F.Dolvio, R. Hermann, W.Hirt, W. Schott "[https://ieeexplore.ieee.org/document/124468 A PRML System for Digital Magnetic Recording]", IEEE Journal on Selected Areas in Comms, vol.10, No.1, pp.38-56, Jan 1992</ref> A parallel R&D effort at IBM San Jose did not lead directly to a product.<ref>T. Howell, et al. "[https://ieeexplore.ieee.org/document/104703 Error Rate Performance of Experimental Gigabit per Square Inch Recording Components]", IEEE Trans. Magn., Vol. 26, No. 5, pp. 2298-2302, 1990</ref> A competing technology at the time was 17ML<ref>A. Patel, "[https://www.researchgate.net/publication/224663211 Performance Data for a Six-Sample Look-Ahead 17ML Detection Channel]", IEEE Trans. Magn., Vol. 29, No. 6, pp. 4012-4014, Dec. 1993</ref> an example of Finite-Depth Tree-Search (FDTS).<ref>R. Carley, J. Moon, "[https://patents.google.com/patent/US5136593A/en Apparatus and method for fixed delay tree search]", filed Oct. 30th, 1989</ref><ref>R. Wood, "[https://ieeexplore.ieee.org/document/42527 New Detector for 1,k Codes Equalized to Class II Partial Response]", IEEE Trans. Magn., Vol. MAG-25, No. 5, pp. 4075-4077, Sept. 1989</ref>

The IBM 0681 read/write channel ran at a data-rate of 24&nbsp;MbitsMbit/s but was more highly integrated with the entire channel contained in a single 68-pin [[Plastic leaded chip carrier|PLCC]] [[integrated circuit]] operating off a 5 volt supply. As well as the fixed analog equalizer, the channel boasted a simple adaptive digital ''cosine equalizer''<ref>T. Kameyama, S. Takanami, R. Arai, "[https://ieeexplore.ieee.org/document/1059216 Improvement of recording density by means of cosine equalizer]", IEEE Trans. Magn., Vol. 12, No. 6, pp. 746-748, Nov. 1976</ref> after the A/D to compensate for changes in radius and/or changes in the magnetic components.

=== Write precompensation ===

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=== Post-processor architecture ===

Given the rapid increase in complexity with longer targets, a post-processor architecture was proposed, firstly for EPRML.<ref>R. Wood, "[https://ieeexplore.ieee.org/document/281375 Turbo-PRML, A Compromise EPRML Detector]", IEEE Trans. Magn., Vol. MAG-29, No. 6, pp. 4018-4020, Nov. 1993</ref> With this approach a relatively simple detector (e.g. PRML) is followed by a post-processor which examines the residual waveform error and looks for the occurrence of likely bit pattern errors. This approach was found to be valuable when it was extended to systems employing a simple parity check<ref>{{Cite journal|last=Conway|first=T.|date=July 1998|title=A new target response with parity coding for high density magnetic recording channels|url=https://ieeexplore.ieee.org/document/703887|journal=IEEE Transactions on Magnetics|volume=34|issue=4|pages=2382–2386|doi=10.1109/20.703887|bibcode=1998ITM....34.2382C }}</ref><ref>R. Cideciyan, J. Coker; E. Eleftheriou; R. Galbraith, "[https://ieeexplore.ieee.org/document/917606 NPML Detection Combined with Parity-Based Postprocessing]", IEEE Trans. Magn. Vol. 37, No. 2, pp. 714–720, March 2001</ref><ref>M. Despotovic, V. Senk, "Data Detection", Chapter 32 in ''[https://www.researchgate.net/publication/328870436 Coding and Signal Processing for Magnetic Recording Systems]'' edited by B. Vasic, E. Kurtas, CRC Press 2004</ref>

=== PRML with nonlinearities and signal-dependent noise ===