1990	IBM 681
	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.

Discussion

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.

PRML was first introduced to tape storage somewhat earlier, the Ampex DCRS, shipped in 1984, is believed to be the first PRML tape recorder.

Additional information

"Last products of Disk-File Development at Hursley and Millbrook", W J P Case, IBM Corporation, October 12, 1990, p.1-28