Magnetic tape

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Compact audio cassette

Magnetic tape is a non-volatile storage medium consisting of a magnetic coating on a thin plastic strip. Nearly all recording tape is of this type, whether used for video, audio storage or general purpose digital data storage using a computer.

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.

Contents

1 Magnetic tape audio storage
2 Magnetic tape video storage
3 Magnetic tape data storage

3.1 History
3.2 Current usage

4 References
5 Related links

Magnetic tape audio storage

$7 inch reel of ¼ inch-wide recording tape, typical of consumer use in the 1950s-70s.$

7 inch reel of ¼ inch-wide recording tape, typical of consumer use in the 1950s-70s.

Magnetic tape was first invented for recording sound by Fritz Pfleumer in 1928 in Germany, based on the invention of magnetic wire recording by Valdemar Poulsen in 1898. A wide variety of recorders and formats have developed since. See:

See also audio storage for a comprehensive list of formats.

Magnetic tape video storage

Video recording demands much higher bandwidth than audio recording and was made practical by the invention of helical scan. Early video recorders were reel-to-reel but modern systems use cartridge tapes and videocassette recorders are quite popular.

Magnetic tape data storage

The use of magnetic tape for data storage has been one of the constants of the computer industry.

History

Magnetic tape was first used to record computer data in 1951 on the Eckert-Mauchly UNIVAC I. The recording medium was a a strip of 1/2" (12.7 mm) wide thin metal, consisting of nickel-plated bronze. (called Vicalloy) Recording density was 128 characters per inch (198 micrometre/character) on eight tracks at a linear speed of 100 in/s (2.54 m/s), yielding a data rate of 12,800 characters per second. Of the eight tracks, seven were data and one was a clock, or timing track. Making allowance for the empty space between tape blocks, the actual transfer rate was around 7,200 characters per second.

10.5 inch computer mag tape.

IBM computers from the 1950s used ferrous-oxide coated tape similar to that used in audio recording. IBM's technology soon became the de facto industry standard. Magnetic tape dimensions was .5" (12.7 mm) wide and wound on removable reels of up to 10.5 inches (267 mm) in diameter. Different tape lengths were available with 1200', 2400' on mil and one half thickness being somewhat standard. Later during the 80's, longer tape lengths such as 3600' became available, but only with a much thinner Mylar plastic(TM) Most tape drives could support a maximum reel size of 10.5"

Early IBM tape 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. (1.5ms from stopped tape to full speed of up to 112.5 IPS) When active, the two tape reels thus fed tape into or pulled tape out of the vacuum columns, intermittently spinning in rapid, 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. --[[User:Woodym1|Woodym1]] 02:35, 16 July 2006 (UTC)

Beginning-of-tape reflective marker.

Early half-inch tape had 7 parallel tracks of data along the length of the tape allowing six-bit characters plus parity written across the tape. This was known as 7-track tape. With the introduction of the IBM System 360, 9-track tapes became common to support 8-bit characters or "bytes." 7-track tapes used a .75 IRG (inter-record-gap) 9-track 800 NRZI and 1600 PE tapes utilized a .60" IRG placed between data records to allow the tape to stop. 6250 GCR tapes used a very tight .3" IRG. Both 7 and 9 track tapes had reflective stickers placed near each end to signal beginning of tape (BOT) and end of tape (EOT) to the hardware. Efective recording density increased over time, starting at 200 characters per inch, then 556, 800, 1600, 3200 and 6250 cpi. Since then, a multitude of tape formats have been used.

LINCtape, and its derivative, DECtape, were variations on this "round tape." They were essentially a personal storage medium. The tape was 3/4 inch wide and 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.

In the 1980's compact audio cassettes were used with home computers of the 1980s, and digital audio tape was used for backup in workstations.

Current usage

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

Quarter-Inch cartridges.

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 large 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, either using indexing, where a separate lookup table is maintained which gives the physical tape location for a given data block number, or by marking blocks with a 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. Software compression also allows encryption to be performed after compression. (Once data has been encrypted, compression algorithms are no longer effective.) However, software compression can place high loads on processors. Future tape drive will likely incorporate hardware encryption after the compression.

Tape remains a viable alternative to disk due to its higher bit density and lower cost per bit. Tape has historically offered enough advantage in these two areas above disk storage to make it a viable product, particularly for backup. The rapid improvement in disk storage density and price (see Kryder's law), coupled with arguably less-vigorous innovation in tape storage, has reduced the market share of tape storage products.

	Magnetic Tape Data Storage Formats
	Linear	Helical-Scan
Three Quarter Inch (~19 mm)	LINCtape (1962) - DECtape (1963) - DECtapeII (1979)
Half Inch (12.65 mm)	UNISERVO (1951) - IBM 729 (1952) - IBM 3480 (1984) - DLT (1984) - IBM 3590 (1995) - LTO Ultrium (2000)	SAIT (2003)
Eight Millimeter (8 mm)	IBM 3570 MP (1997) - Travan (199x)	Exabyte (1987) - Mammoth (1994) - AIT (1996) - M2 (1999)
Quarter Inch (~6 mm)	QIC (1972) - SLR (xxxx)
Four Millimeter (3.8 mm)		DDS/DAT (1989)

References

This article was originally based on material from the Free On-line Dictionary of Computing, which is [Foldoc licenselicensed] under the GFDL.