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Extended PRML (EPRML)
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.