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Run Length Limited codes, or RLL codes are widely used in hard disk drives (and notably digital optical discs, such as CD, DVD and BluRay disc) to prevent long stretches of no transitions, and therefore decoding uncertainty, from creeping in. Typical hard drives of the mid-1990s. ...
CD may stand for: Compact Disc Canadian Forces Decoration Cash Dispenser (at least used in Japan) CD LPMud Driver Centrum-Demokraterne (Centre Democrats of Denmark) Certificate of Deposit České Dráhy (Czech Railways) Chad (NATO country code) Chalmers Datorförening (computer club of the Chalmers University of Technology) a 1960s...
DVD-R writing/reading side DVD-R with purple dye, 4. ...
Blu-ray discs Blu-ray Disc is a next-generation optical disc format jointly developed by a group of leading consumer electronics and PC companies called the Blu_ray Disc Association (BDA), which succeeds the Blu_ray Disc Founders (BDF). ...
On a hard disk, a bit is encoded by a polarity transition or the lack thereof. Encoding 000000 would then be easy - just keep the magnetic phase unchanged for a few micrometers. However, when decoding, uncertainty creeps in - how many micrometers did we read? Does this correspond to 6 zeroes or 5? To prevent this problem, data is coded such that these long stretches of no transitions do not occur.
History
RLL encoding was invented by IBM engineers, who used it in mainframe disk drives. During the late 1980s, PC hard disks began using RLL. RLL codes have found almost universal application in optical disc recording practice since 1980. In consumer electronics, there is the Eight-to-Fourteen Modulation or EFM code (rate = 8/17, d=2, k=10), which is employed in the Compact Disc (CD), and the EFMPlus code (rate = 8/16, d=2, k=10) used in the DVD. See below. International Business Machines Corporation (IBM, or colloquially, Big Blue) NYSE: IBM (incorporated June 15, 1911, in operation since 1888) is headquartered in Armonk, NY, USA. The company manufactures and sells computer hardware, software, infrastructure services and consulting services. ...
Mainframes (often colloquially referred to as big iron) are large and expensive computers used mainly by government institutions and large companies for legacy applications, typically bulk data processing (such as censuses, industry/consumer statistics, ERP, and bank transaction processing). ...
The 1980s decade refers to the years from 1980 to 1989, inclusive. ...
One of the first PCs from IBM - the IBM PC model 5150. ...
Eight-to-Fourteen Modulation (EFM) is an encoding technique used by CDs and MiniDiscs. ...
Interference colors. ...
DVD-R writing/reading side DVD-R with purple dye, 4. ...
Technical overview The length of time (usually expressed in channel bits) between consecutive transitions 0 to 1 or 1 to 0 is known as the runlength. For instance, the runlengths in the word '0111100111000000' are of length 1, 4, 2, 3, and 6. Run length limited sequences are characterized by two parameters, (d+1) and (k+1), which stipulate the minimum (with the exception of the very first and last runlength) and maximum runlength, respectively, that may occur in the sequence. The parameter d controls the smallest pit/land recorded on an optical disc. The maximum runlength parameter k ensures adequate frequency of clocking information for synchronization of the read clock. The grounds on which d and k values are selected, in turn, depend on various factors such as the channel response, the desired data rate (or information density), and the jitter and noise characteristics.
Coding | 1 | 1 (1000) | | | | | 0 (0100) | | | | 0 | 1 | 1 (001000) | | | | | 0 (100100) | | | | 0 | 1 | 1 (00001000) | | | | | 0 (00100100) | | | | 0 (000100) | | Example:
11 000 011
1000 000100 001000
Other encodings Other schemes include Group Code Recording, FM and MFM (Modified Frequency Modulation). Group Code Recording (GCR) is a floppy disk data encoding format used by the Apple II and Commodore Business Machines in the 5¼ disk drives for their 8-bit computers (the best-known drives being the Disk II for the Apple II family and the Commodore 1541, used with the...
Modified Frequency Modulation, commonly MFM, is a line code used by most floppy disk formats, notably by most CP/M machines, as well as PCs running DOS. MFM is a modification to the original FM (frequency modulation) scheme for encoding data on single-density floppy disks. ...
See also In data storage, Partial Response Maximum Likelihood (PRML) is a method for converting the weak analog signal from the head of a magnetic disk drive into a digital signal. ...
External links Chapter 14: Hard Disk Drives - QUE Upgrading & Repairing PCs, Eighth Edition - This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
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