Run length limited

Run-length limited or RLL coding is a line coding technique that is used to send arbitrary data over a communications channel with bandwidth limits. RLL codes are defined by four main parameters: m, n, d, k. The first two, m/n, refer to the rate of the code, while the remaining two specify the minimal d and maximal k number of zeroes between consecutive ones. This is used in both telecommunication and storage systems that move a medium past a fixed recording head.

Specifically, RLL bounds the length of stretches (runs) of repeated bits during which the signal does not change. If the runs are too long, clock recovery is difficult; if they are too short, the high frequencies might be attenuated by the communications channel. By modulating the data, RLL reduces the timing uncertainty in the stored data, which would lead to the possible erroneous insertion or removal of bits when reading the data back. Run-length-limited codes were widely used in hard disk drives until the mid-1980s and are still used in digital optical discs such as CD, DVD, MD, Hi-MD and Blu-ray. This mechanism ensures that the boundaries between bits can always be accurately found (preventing bit slip), while efficiently using the media to reliably store the maximal amount of data in a given space.

Early disk drives used very simple encoding schemes, such as RLL (0,1) FM code, but higher-density RLL (2,7) and RLL (1,7) codes became the de facto industry standard for hard disks by the early 1990s.

On a hard disk drive, information is represented by changes in the direction of the magnetic field on the disk. In a computer, information is represented by the voltage on a wire. No voltage on the wire in relation to a defined ground level would be a binary zero, and a positive voltage on the wire in relation to ground represents a binary one. Magnetic media, on the other hand, always carries a magnetic flux - either a "north" pole or a "south" pole. In order to convert the magnetic fields to binary data, some encoding method must be used to translate between the two.

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