Encyclopedia > Magnetic tape
Magnetic tape is an information storage medium[?] consisting of a magnetisable oxide coating on a thin plastic strip. This includes nearly all recording tape, whether used for video with a video cassette recorder, audio (compact audio cassette, digital audio tape (DAT) and other formats) with a cassette tape player , or data storage with a computer system.
In a computing context, the term is often incorrectly used to denote only one-half-inch open-reel magnetic tapes, the oldest common form. However, with the exception of a very small number of historic niche optical tape systems, all computer data tapes are magnetic.
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 greater the percentage of tape wasted on inter-block gaps. Several logical records may be combined into one physical block to reduce waste ("blocked records").
Tape is not suitable for random access since the seek latency is much higher than disk-based storage, but tape remains a viable alternative product because its overall bits/volume density and cost per bit have historically been enough ahead of disk storage that the longer latency was an acceptable tradeoff. The recent vigorous innovation in disk storage products, coupled with less-vigorous innovation in tape storage, has reduced the viability of tape storage.
Most tape systems attempt to alleviate the intrinsic long latency using either a lookup table that logs exactly where on tape a record is, or some sort of tape mark[?] that can be detected while winding the tape at high speed.
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.
see also: cut a tape[?], flap, Group code recording[?], spool[?], macrotape[?], microtape[?], Non Return to Zero Inverted[?], Phase encoded[?], Tape Drive, Error Corrrection[?].
This article (or an earlier version of it) contains material from FOLDOC, used with permission.
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