magnetic tape
magnetic tape
[mag′ned·ik ′tāp]magnetic tape
(storage)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.
magnetic tape
A sequential storage medium used for data collection, backup and archiving. The first electronic storage medium, magnetic 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 Tapes were originally open reels but were superseded by cartridges and cassettes of many sizes and shapes. Today, LTO is the only surviving tape technology. See LTO.Tape has always been more economical than disks for archival data; however, disk capacities have increased enormously while the cost per bit has been reduced dramatically. In addition, 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 tapes today are used for archiving rather than real-time updating, some drives allow rewriting in place if the byte count remains the same. Otherwise, updating requires reading the original tape, changing the data or inserting new records and rewriting everything onto another tape.
Track Formats
Data are recorded in blocks of contiguous bytes, separated by a space called an "interrecord gap" or "interblock gap." Drive speed is measured in inches per second (ips). Over the decades, storage density jumped from 200 bits per square inch to millions and billions of bits. See magnetic tape formats.
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| Tracks on Magnetic Tape |
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| Except for helical scan, most tracks on tape run parallel to the length of the tape. See helical scan and serpentine recording. |
Magnetic Tape
a magnetic recording medium in the form of a thin, flexible ribbon composed of a base and a magnetic layer. Its operating characteristics are described by the recording sensitivity and the signal distortion of the recording and playback processes.
The most commonly used tape is of the multilayered type, with a working layer consisting of acicular particles of hardmagnetic powdered gamma iron oxide (γ-Fe2O3), chromiumdioxide (CrCO2), and gamma iron oxide modified with cobalt; the particles are usually oriented in the direction of the magnetization used for recording. In 1973 the Philips company of the Netherlands developed a high-quality powder with very fine acicular iron particles.
The base of a magnetic tape may be a polyethylene telephthalate, polyvinylchloride, diacetate, or triacetate film (the first type is the best). The working layer is deposited on the base in the form of a magnetic lacquer composed of the magnetic powder, a binder, a solvent, a plasticizer, and various additives to improve the quality of the tape. After the deposited magnetic lacquer has hardened, the magnetic tape is wound on reels and then is cut into strips of the desired width. The surface quality of the working layer is improved by calendering or polishing.
Magnetic tape should be stored in a dust-free, airconditioned room at a temperature of 20° ± 5°C and a relative humidity of 60 ± 5 percent. For service under particularly difficult climatic conditions, metallic or bimetallic magnetic tape is used.
The width and thickness of magnetic tape depends on its function. The tapes used in sound recording are 3.81 or 6.25 mm wide and 9, 12, 18, 27, 37, or 55 microns (μ) thick (household cassette and reel-to-reel tape recorders and studio tape decks). Video tape recording is done on magnetic tapes 50.8 and 25.4 mm wide and 37 μ thick (studio videotape recorders) or 6.25 and 12.7 mm wide and 37 μ thick (household videotape recorders). The magnetic tape used in memory units is 12.7 mm wide and 37 ju thick (in first-generation electronic computers, magnetic tapes 19.05 and 35 mm wide and more than 50 μ thick were also used). The magnetic tapes used in measurement apparatus are 6.25 mm wide and 18 μ thick, and also 12.7 and 25.4 mm wide and 37 μ thick. Perforated magnetic tape 35 mm wide and 150 μ thick is used in the motion-picture industry.
In the USSR the type of magnetic tape is designated by combinations of five units. The first unit is a letter that indicates the function (for example, A is for sound recording, T is for video recording), the second unit is a number (from 0 to 9) that indicates the base material, the third unit is a number (from 0 to 9) that indicates the thickness of the tape (for example, 2 is 18 μ, 3 is 27 μ), the fourth unit is a number (from 01 to 99) that indicates the technological processing, and the fifth unit indicates the width of the tape in millimeters. Sometimes a sixth letter code is used (P for perforated magnetic tapes, R for studio tapes, B for household tapes). For example, A-4402-6 designates a magnetic tape for sound recording on a dacron base, 37 μ thick, and 6.25 millimeters wide (technological processing 02).
Metallized magnetic tapes with a thin working layer made of Co-Ni, Co-P, Co-N-P, and Co-W alloys, applied by electrodeposition, chemical reduction, or sputtering in a vacuum, are being developed.
REFERENCES
Mazo, Ia. A. Magnitnaia lenta. Moscow, 1968.Kagan, B. M., V. I. Adas’ko, and R. R. Pure. Zapominaiushchie ustroistva bol’shoi emkosti. Moscow, 1968.
IA. A. MAZO and D. P. BRUNSHTEIN