What is RLL ?
| Run Length Limited |
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An improvement on the MFM encoding technique used in earlier hard disks and used on all floppies is run length limited or RLL. This is a more sophisticated coding technique, or more correctly stated, "family" of techniques. I say that RLL is a family of techniques because there are two parameters that define how RLL works, and therefore, there are several different variations. (Of course, you don't need to know which one your disk is using, since this is all internal to the drive anyway). RLL works by looking at groups of bits instead of encoding one bit at a time. The idea is to mix clock and data flux reversals to allow for even denser packing of encoded data, to improve efficiency. The two parameters that define RLL are the run length and the run limit (and hence the name). The run length is the minimum spacing between flux reversals, and the run limit is the maximum spacing between them. As mentioned before, the amount of time between reversals cannot be too large or the read head can become out of sync and lose track of which bit is where. The type of RLL used is expressed as "X,Y RLL" where X is the run length and Y is the run limit. The most commonly used type of RLL is 1,7 RLL. Describing how RLL encodes the data would be too involved to include here because it uses specific patterns of reversals to encode patterns of bits, and I've probably gotten more detailed here than I wanted to anyway. One or another variant of RLL is now used on most hard disk drives. |
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