Partial-response maximum-likelihood: Difference between revisions

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=== PRML with Nonlinearities and Signal-dependent Noise ===

As data detectors became more sophisticated, it was found important to deal with any residual signal nonlinearities as well as pattern-dependent noise (noise tends to be largest when there is a magnetic transition between bits) including changes in noise-spectrum with data-pattern. To this end, the Viterbi-detector was modified such that it recognized the expected signal-level and expected noise variance associated with each bit-pattern. As a final step, the detectors were modified to include a 'noise predictor filter' thus allowing each pattern to havve a different noise=-spectrum. Such detectors are referred to as PatterPattern-Dependent Noise-Prediction (PDNP) detectors<ref>[https://ieeexplore.ieee.org/abstract/document/920181 J. Moon, J. Park, “Pattern-dependent noise prediction in signal dependent noise,” IEEE J. Sel. Areas Commun., vol. 19, no. 4, pp. 730–743, Apr. 2001]</ref> or [[noise-predictive maximum-likelihood detection|noise-predictive maximum-likelihood detectors]] (NPML)<ref>[https://ieeexplore.ieee.org/document/539233 E. Eleftheriou, W. Hirt, "Improving Performance of PRML/EPRML through Noise Prediction". IEEE Trans. Magn. Vol. 32, No. 5, pp. 3968–3970, Sept. 1996]</ref> <br><br>

== Recent Read/Write Electronics ==

The most advanced detectors today (2019) use all these techniques and furthermore they replace the hard-decision Viterbi algorithm with a detector providing soft-outputs (additional information about the reliability of each bit). Such detectors using a 'soft Viterbi algorithm' or [[BCJR]] algorithm are essential in decoding [[LDPC]] codes used in modern HDDs.

Although the PRML acronym is still occasionally used, the most advanced detectors today (as of 2017) are around a million times more complex (gate-count) than the first PRML channel and operate about 100 times the data-rate (up to 3 Gbit/s). The analog front-end typically includes [[AGC]], correction for the nonlinear read-element response, and a low-pass filter with with control over the high-frequency boost or cut. Equalization is done after the A/D with a digital [[FIR]] equalizer. ([https://en.wikipedia.org/wiki/Draft:Two-Dimensional_Magnetic_Recording TDMR] uses a 2-input, 1-output equalizer.) The detector uses the PDNP/NPML approach but the hard-decision Viterbi algorithm is replaced with a detector providing soft-outputs (additional information about the reliability of each bit). Such detectors using a 'soft Viterbi algorithm' or [[BCJR]] algorithm are essential in iteratively decoding [[LDPC]] codes used in modern HDDs. A single integrated circuit contains the entire R/W channel (including the iterative decoder) as well as all the disk control and interface functions. There are two suppliers: [[Broadcom]] and [[Marvell]]<ref>[https://www.marvell.com/storage/assets/Marvell_88i9422_Soleil_pb_FINAL.pdf Marvell 88i9422 Soleil SATA HDD Controller., Sept 2015]</ref>.

== See also ==