Magnetic tape

Magnetic tape is an information storage medium consisting of a magnetisable coating on a thin plastic strip. Nearly all recording tape is of this type, whether used for video with a video cassette recorder, audio storage (reel-to-reel tape, compact audio cassette, digital audio tape (DAT) and other formats including 8-track cartridges) or general purpose digital data storage using a computer (specialized tape formats, as well as the above-mentioned compact audio cassette, used with home computers of the 1980s, and DAT,used for backup in workstation installations of the 1990s).

Magneto-optical and optical tape storage products have been developed using many of the same concepts as magnetic storage, but have achieved little commercial success.

Magnetic Tape Audio Storage

See: Sound Recording: Magnetic Recording

Magnetic Tape Data Storage

half-inch reel tape

Magnetic tape was first used to record data in 1951 on the Mauchly-Eckert Univac. The recording medium was a thin band of solid steel. Recording density was 128 characters per inch at a linear speed of 100 ips, yielding a data rate of 12800 characters per second.

IBM computers of the late 1950s used oxide-coated tape similar to that used in audio recording, and IBM's technology soon became the de facto 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.

IBM's drives were mechanically sophisticated floor-standing drives that used vacuum columns to buffer long u-shaped loops of tape. Between active control of powerful reel motors and vacuum control of these u-shaped tape loops, extremely rapid start and stop of the tape at the tape-to-head interface could be achieved. When active, the two tape reels thus spun in rapid, uneven, unsynchronized bursts resulting in visually-striking action. Stock shots of such vacuum-column tape drives in motion were widely used to represent "the computer" in movies and television.

LINCtape (and its derivative, DECtape) were variations on this "round tape." They were essentially a personal storage medium. They featured a fixed formatting track which, unlike standard tape, made it feasible to read and rewrite blocks repeatedly in place. LINCtapes and DECtapes had similar capacity and data transfer rate to the diskettes that displaced them, but their "seek times" were on the order of thirty seconds to a minute.

Most modern magnetic tape systems use reels that are reels are much smaller and are fixed inside a cartridge to protect the tape and facilitate handling. Cartridge formats include QIC, DAT, and Exabyte.

File:Cartridge tape drive.jpg
cartridge tapes in drives

A tape drive (or "transport or "deck") uses precisely-controlled motors to wind the tape from one reel to the other, passing a read/write head as it does. 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. Since then, a multitude of tape formats have been used, but common features emerge.

In a typical format, data is written to tape in blocks with inter-block gaps between them, and each block is written in a single operation with the tape running continuously during the write.

However, since the rate at which data is written or read to the tape drive is not deterministic, a tape drive usually has to cope with a difference between the rate at which data goes on and off the tape and the rate at which data is supplied or demanded by its host.

Various methods have been used alone and in combination to cope with this difference. A lage memory buffer can be used to queue the data. The tape drive can be stopped, backed up, and restarted. The host can assist this process by choosing appropriate block sizes to send to the tape drive.

There is a complex tradeoff between block size, the size of the data buffer in the record/playback deck, the percentage of tape lost on inter-block gaps, and read/write throughput.

Tape has quite a long data latency for random accesses since the deck must wind an average of 1/3 the tape length to move from one arbitrary data block to another. Most tape systems attempt to alleviate the intrinsic long latency using either indexing, whereby a separate lookup table is maintained which gives the physical tape location for a given data block number, or marking, whereby a tape mark that can be detected while winding the tape at high speed is written to the 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). The actual compression algorithms used are not the most effective known today, and better results can usually be obtained by turning off the compression built into the device and using a software compression program instead.

Tape remains a viable alternative to disk due to its higher bit density and cost per bit. Tape has historically offered enough advantage in these two areas above disk stroage to make it a viable product. The recent vigorous innovation in disk storage density and price, coupled with less-vigorous innovation in tape storage, has reduced the viability of tape storage products.

see also: cut a tape, flap, Group code recording, spool, macrotape, microtape, Non Return to Zero Inverted, Phase encoded, Tape Drive, Error Correction, Helical scan, print-through

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This article (or an earlier version of it) contains material from FOLDOC, used with permission.