|1st PRML channel|
Why it’s important The Partial Response Maximum Likelihood (PRML) channel marks the beginning of sophisticated channel codes to further increase the number of bits per track.
Traditionally magnetic recording detected channel bits thru detection of of peaks in the playback signal. Peak detection had the advantage of being both robust and simple to implement; however by its very nature works best at low linear bit densities. As linear densities increase their magnetic nature can cause the playback signals to interfere causing the timing ot fhe peaks to shift; Called intersymbol interference such peak shift can cause playback errors and therefore peak detection systems attempt to maximize the minimum the spacing between bits. Unlike peak detection systems,Partial Response Maximum Likelihood (PRML) systems allow the bits to be close together and the playback signal with its inherent intersymbol interference is equalized and then detected using a maximum likelhood detector. 
PRML was introduced to magnetic storage with the Ampex DCRS tape recorder, shipped in 1983 and believed to be the first storage product incorporating PRML.
The 0681 was developed at IBM's Hursley UK laboratory under the code name "Redwing" beginning 1983 and first shipped in April 1990.  The 0681-500 had a capacity of 471 MB formatted (8 disks) while the 0681-1000 had a capacity of 857 MB formatted (12 disks); both with an areal density of 45.20 Mbits/sq in (1677 tpi * 26951 bpi). It is likely that the same head disk assemblies were used in the IBM 9336-10 and -20 first shipped in September 1990 since their technical parameters are the same.
The Redwing is believed to be the first instance of the use of a Partial Response Maximum Likelihood (PRML) channel in a disk drive. The channel was designed in IBM's Rochester laboratory, based on the work done in the Zurich research laboratory.
The acronym PRML was invented by John Eggenberger of the IBM San Jose development group around 1986. Before Eggenberger, the industry used various cumbersome names: Partial Response with Viterbi Detection, Class IV equalization with maximum likelihood detector, etc.
 "Modulation and Coding forInformation Storage," Siegel and Wolf,IEEE Communications Magazine, December 1991 “High data rate magnetic recording in a single channel,” Coleman, et. al, JIERE, Vol. 55, No. 6, pp. 229-236. June 1985
 "Last products of Disk-File Development at Hursley and Millbrook", W J P Case, IBM Corporation, October 12, 1990, p.1-28
 "PARTIAL-RESPONSE CODING, MAXIMUMLIKELIHOODDECODING: ... ," H. Kobayashi,IEEE Communications Magazine • March 2009
 US Patent 4707681 A, “Method and apparatus for implementing optimum PRML codes”, Eggenberger & Patel, filed April 24, 1986