1984: Digital Signal Processing Increases Hardware Areal Density

PRML techniques enhance magnetic media storage capacity

 

 

 

Advances in data storage have been accomplished through innovations in recording media, read/write heads, mechanics, and, not least, in the electronics required to produce reliable binary data from the weak/noisy analog readback signals.

PRML (Partial Response Maximum Likelihood) detection provided significant gains in areal density (data stored in a given surface area). PRML operates at higher linear densities by allowing considerable interference between bits then applying relatively sophisticated digital signal processing to ‘unravel’ the original bit pattern. (“Partial Response” means that parts of the bit response appear in several read waveform samples. "Maximum Likelihood" refers to finding the most likely bit pattern) PRML and its successors have allowed 30-40% increase areal density.

PRML was first applied in Ampex’s 1984 magnetic tape recorder, the Digital Cartridge Recording System (DCRS). Development was led by Charles Coleman. For two decades, DCRS was the de-facto standard for high-data rate digital instrumentation recording for flight-test and military airborne applications. DCRS operated at 28 Mbits/sq.in. and 15 Mbytes/s. The largest cartridges offered capacities of 165 GBytes. The first application of PRML on a hard disk drive (HDD) was in the 1990 IBM 0681 developed at the Hursley UK laboratory. The PRML algorithms were developed by a team led by Francois Dolivo, IBM Zurich and implemented in hardware by IBM Rochester. The 0681 operated at 45 Mbits/sq in. and 3 MBytes/s and offered a maximum capacity of 471 Mbytes. All HDD suppliers adopted PRML by 2000.

Modern channels (2019) are about 10,000 times more complex (gate count). They operate by passing probability information iteratively back and forth between an NPML (Noise Predictive Maximum Likelihood) front-end and an LDPC (Low Density Parity check) back-end. Error-correction is implicit. The entire channel and HDD controller are integrated into a single VLSI chip. Other significant software developments that have improved the density, performance, and reliability of data storage equipment, include innovative compression, signal processing and encoding techniques such as RLL (Run-Length Limiting), signal dependent correlation-sensitive sequence detection, LDPC (Low-Density Parity Checking), and ECC (Error Correction) codes.