Magnetic tape
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Image:Kaseta magnetofonowa ubt.jpeg 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.
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Magnetic tape audio storage
Template:Main Image:Magtape1.jpg 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:
- Audio tape length and thickness
- Tape recorder
- Reel-to-reel audio tape recording
- Compact audio cassette
- 8-track cartridge
- digital audio tape (DAT)
- digital linear tape (DLT)
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 Mauchly-Eckert UNIVAC I. The recording medium was a 1/2 inch (13 mm) wide thin band of nickel-plated bronze. 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. Making allowance for the empty space between tape blocks, the actual transfer rate was around 7,200 characters per second.
Image:Largetape.jpg IBM computers from the 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 (12.7 mm) wide and wound on removable reels up to 10.5 inches (267 mm) in diameter. Different lengths were available with 2400 feet and 4800 feet (732 and 1463 m) being common.
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. 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.
Image:Tapesticker.jpg 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." Both types of tape required 3/4 inch gaps to be placed between data records to allow the tape to stop, and had reflective marks near each end to signal beginning of tape (BOT) and end of tape (EOT) to the hardware. Recording density improved over time, starting at 200 characters per inch, then 556, 800, 1600 and 3200 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.
Image:Quarter-Inch Cartridges.jpg 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.
References
- This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
Related links
- The Basics of Audio Recording
- Recording Formats Keith Gemmell's Music Technology Site - Excellent resources
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