magnetic tape

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An information storage medium consisting of a magnetic coating on a flexible backing in tape form. Data is recorded by magnetic encoding of tracks on the coating according to a particular tape format.

Magnetic tape is wound on reels (or spools). These may be used on their own, as open-reel tape, or they may be contained in some sort of magnetic tape cartridge for protection and ease of handling. Early computers used open-reel tape, and this is still sometimes used on large computer systems although it has been widely superseded by cartridge tape. On smaller systems, if tape is used at all it is normally cartridge tape.

Magnetic tape is used in a tape transport (also called a tape drive, tape deck, tape unit, or MTU), a device that moves the tape over one or more magnetic heads. An electrical signal is applied to the write head to record data as a magnetic pattern on the tape; as the recorded tape passes over the read head it generates an electrical signal from which the stored data can be reconstructed. The two heads may be combined into a single read/write head. There may also be a separate erase head to erase the magnetic pattern remaining from previous use of the tape.

Most magnetic-tape formats have several separate data tracks running the length of the tape. These may be recorded simultaneously, in which case, for example, a byte of data may be recorded with one bit in each track (parallel recording); alternatively, tracks may be recorded one at a time (serial recording) with the byte written serially along one track. For parallel recording and some serial recording, there is a separate head (or set of read and write heads) for each track, assembled into a single multitrack head unit; other mechanisms have a single track head that is moved across the width of the tape to record separate tracks. A third method is helical-scan recording where the heads are mounted in a rotating drum around which the tape is wrapped on the skew, as in a video recorder, so that tracks run diagonally across the tape.

Where write and read heads are close together, the magnetic signals may be read back and checked for correctness as soon as they are written; this is called a read-while-write check.

Standard open-reel tape is ½ inch wide and carries nine data tracks, recorded in parallel; the most widely used reel is 10.5 inches in diameter holding 2400 feet of tape, and such a volume holds up to 140 megabytes of data depending on the tape format. 1200 or 600 foot tapes, on smaller reels, are sometimes used. Other formats are employed for special purposes. Tape cartridges are much more variable in size and capacity because there are so many different formats; volume capacity varies from a few megabytes to many gigabytes.

Magnetic tape has been used for offline data storage, backup, archiving, data interchange, and software distribution, and in the early days (before disk storage was available) also as online backing store. For many of these purposes it has been superseded by magnetic or optical disk or by online communications. For example, although tape is a nonvolatile medium, it tends to deteriorate in long-term storage and so needs regular attention (typically an annual rewinding and inspection) as well as a controlled environment. It is therefore being superseded for archival purposes by optical disk. Magnetic tape is still extensively used for backup; for this purpose, interchange standards are of minor importance, so proprietary cartridge-tape formats are widely used.


Subjects: Computing.

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