An evolutionary improvement on the PRML design
has been developed over the last few years. Called extended partial response, maximum
likelihood, extended PRML or just EPRML, this advance was the
result of engineers tweaking the basic PRML design to improve its performance. EPRML
devices work in a similar way to PRML ones: they are still based on analyzing the analog
data stream coming form the read/write head to determine the correct data sequence. They
just use better algorithms and signal-processing circuits to enable them to more
effectively and accurately interpret the information coming from the disk.
The chief benefit of using EPRML is that due to its higher performance, areal density (or more correctly, the linear component
of areal density) can be increased without increasing the error rate. Claims regarding
this increase range from around 20% to as much as 70%, compared to "regular"
PRML. Those numbers represent a fairly significant improvement.
EPRML has now been widely adopted in the hard disk industry and is replacing PRML on
new drives.
Next: Tracks, Cylinders and Sectors
