magnetic tape: see
computer computer, device capable of performing a series of arithmetic or logical operations. A computer is distinguished from a calculating machine, such as an electronic calculator , by being able to store a computer program (so that it can repeat its operations and make
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tape recorder tape recorder, device for recording information on strips of plastic tape (usually polyester) that are coated with fine particles of a magnetic substance, usually an oxide of iron, cobalt, or chromium. The coating is normally held on the tape with a special binder.
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magnetic tape
A sequential storage medium used for data collection, backup and archiving. Like videotape, computer tape is made of flexible plastic with one side coated with a ferromagnetic material. Tapes were originally open reels, but were superseded by cartridges and cassettes of many sizes and shapes.
Tape has been more economical than disks for archival data, but that is changing as disk capacities have increased enormously. If tapes are stored for the duration, they must be periodically recopied or the tightly coiled magnetic surfaces may contaminate each other.
Sequential Medium
The major drawback of tape is its sequential format. Locating a specific record requires reading every record in front of it or searching for markers that identify predefined partitions. Although most tapes are used for archiving rather than routine updating, some drives allow rewriting in place if the byte count does not change. Otherwise, updating requires copying files from the original tape to a blank tape (scratch tape) and adding the new data in between.
Track Formats
Tracks run parallel to the edge of the tape (linear recording) or diagonally (helical scan). A linear variation is serpentine recording, in which the tracks "snake" back and forth from the end of the tape to the beginning.
Legacy open reel tapes used nine linear tracks (8 bits plus parity), while modern cartridges use 128 or more tracks. Data are recorded in blocks of contiguous bytes, separated by a space called an "interrecord gap" or "interblock gap." Tape drive speed is measured in inches per second (ips). Over the years, storage density has increased from 200 to 38,000 bpi. See helical scan and compact tape.
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| Tracks on Magnetic Tape |
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| Except for helical scan recording, most tracks on magnetic tape run parallel to the length of the tape. |
Magnetic Tape Summary
The following magnetic tape technologies are summarized below. See also
magnetic disk and
optical disc.
magnetic tape a long narrow plastic or metal strip coated or impregnated with a ferromagnetic material such as iron oxide, used to record sound or video signals or to store information in computers
| (storage) | magnetic tape - (Or "magtape", "tape" - paper tape is now
obsolete) A data storage medium consisting of a magnetisable
oxide coating on a thin plastic strip, commonly used for
backup and archiving.
Early industry-standard magnetic tape was half an inch wide
and wound on removable reels 10.5 inches in diameter.
Different lengths were available with 2400 feet and 4800 feet
being common. DECtape was a variation on this "round tape".
In modern magnetic tape systems the reels are much smaller and
are fixed inside a cartridge to protect the tape and for
ease of handling ("square tape" - though it's really
rectangular). Cartridge formats include QIC, DAT, and
Exabyte.
Tape is read and written on a tape drive (or "deck") which
winds the tape from one reel to the other causing it to move
past a read/write head. Early tape had seven parallel tracks
of data along the length of the tape allowing six bit
characters plus parity written across the tape. A typical
recording density was 556 characters per inch. The tape had
reflective marks near its end which signaled beginning of tape
(BOT) and end of tape (EOT) to the hardware.
Data is written to tape in blocks with inter-block gaps
between them. Each block is typically written in a single
operation with the tape running continuously during the write.
The larger the block the larger the data buffer required in
order to supply or receive the data written to or read from
the tape. The smaller the block the more tape is wasted as
inter-block gaps. Several logical records may be combined
into one physical block to reduce wastage ("blocked records"). Finding a certain block on the tape generally
involved reading sequentially from the beginning, in contrast
to magnetic disks. Tape is not suitable for random access. The exception to this is that some systems allow
tape marks to be written which can be detected while winding
the tape forward or rewinding it at high speed. These are
typically used to separate logical files on a tape.
Most tape drives now include some kind of data compression.
There are several algorithms which provide similar results:
LZ (most), IDRC (Exabyte), ALDC (IBM, QIC) and
DLZ1 (DLT).
See also cut a tape, flap, Group Code Recording,
spool, macrotape, microtape, Non Return to Zero Inverted, Phase Encoded. | |
