Partial Response, Maximum Likelihood (PRML)
Standard
This method works fine as long as the peaks are large enough to be picked out from the background noise of the signal. As data density increases, the flux reversals are packed more tightly and the signal becomes much more difficult to analyze, because the peaks get very close together and start to interfere with each other. This can potentially cause bits to be misread from the disk. Since this is something that must be avoided, in practical terms what happens instead is that the maximum areal density on the disk is limited to ensure that interference does not occur. To take the next step up in density, the magnetic fields must be made weaker. This reduces interference, but causes peak detection to be much more difficult. At some point it becomes very hard for the circuitry to actually tell where the flux reversals are.
To combat this problem a new method was developed that takes a different approach to solving the data interpretation problem. This technology, called partial response, maximum likelihood or PRML, changes entirely the way that the signal is read and decoded from the surface of the disk. Instead of trying to distinguish individual peaks to find flux reversals, a controller using PRML employs sophisticated digital signal sampling, processing and detection algorithms to manipulate the analog data stream coming from the disk (the “partial response” component) and then determine the most likely sequence of bits this represents (“maximum likelihood”).
Conceptual drawing demonstrating the principles behind PRML. The data stream is sampled and analyzed using digital signal processing techniques.While this may seem like an odd (and unreliable) way to read data from a hard disk, it is in fact reliable enough that PRML, and its successor, EPRML, have become the standard for data decoding on modern hard disks. PRML allows areal densities to be increased by a full 30-40% compared to standard peak detection, resulting in much greater capacities in the same number of platters.
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