This item is from the Yet Another Enhanced IDE/Fast-ATA/ATA-2 FAQ, by John Wehman and Peter den Haan with numerous contributions by others. (v1.92).
The Partial Response Maximum Likelihood or PRML read channel is quickly replacing the ordinary peak detection channel as a mass market technology. Briefly, where a peak detection channel uses a comparatively simple analogue technique to extract the digital data from the signal picked up by the read head, a PRML channel digitizes the signal and employs digital processing techniques to reconstruct the data. Thanks to these DSP techniques a PRML channel can still work reliably on very closely packed data where the distinction between individual bits tends to blur. The end result is a faster, higher capacity drive.
For a more thorough discussion of this topic, take a look at Quantum's excellent white papers <http://www.quantum.com/products/whitepapers/>.