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 minimum d and maximum k number of zeroes between consecutive ones. This is used in both telecommunication and storage systems which 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 decoding 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 maximum 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.

Run-length encoding

Run-length encoding (RLE) is a very simple form of lossless data compression in which runs of data (that is, sequences in which the same data value occurs in many consecutive data elements) are stored as a single data value and count, rather than as the original run. This is most useful on data that contains many such runs. Consider, for example, simple graphic images such as icons, line drawings, and animations. It is not useful with files that don't have many runs as it could greatly increase the file size.

RLE may also be used to refer to an early graphics file format supported by CompuServe for compressing black and white images, but was widely supplanted by their later Graphics Interchange Format. RLE also refers to a little-used image format in Windows 3.x, with the extension rle, which is a Run Length Encoded Bitmap, used to compress the Windows 3.x startup screen.

Typical applications of this encoding are when the source information comprises long substrings of the same character or binary digit.