Magnetic tape
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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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[edit] Audio recording
Magnetic tape was first invented for recording sound by Fritz Pfleumer in 1926 in Germany, based on the invention of magnetic wire recording by Valdemar Poulsen in 1898. Pfleumer's invention used an iron oxide powder coating on a long strip of paper. This invention was further developed by the German electronics company AEG, which manufactured the recording machines and BASF, which manufactured the tape. An important discovery made in this period was the technique of AC biasing which dramatically improved the fidelity of the recorded audio signal.
Due to the international hostilities preceding World War II, these developments were largely kept secret from the rest of the world. It was only after the war that Americans, particularly Jack Mullin and Major John Herbert Orr, were able to bring this technology out of Germany.
A wide variety of recorders and formats have developed since.
[edit] Video recording
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 very common in homes and television production facilities, though many functions of the VCR are being replaced by optical disc media.
[edit] Data storage
The use of magnetic tape for data storage has been one of the constants of the computer industry.
Magnetic tape was first used to record computer data in 1951 on the Eckert-Mauchly UNIVAC I. The recording medium was a strip of ½″ (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, six were data, one was a parity track, 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.
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.
Most late 1970's and early 1980's home computers used compact audio cassettes encoded with the Kansas City standard.
Most modern magnetic tape systems use reels that are much smaller than the old 10.5 inch open reels and are fixed inside a cartridge to protect the tape and facilitate handling. 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. Modern cartridge formats include DAT/DDC, AIT, DLT and LTO.
Tape has quite a long data latency for random accesses since the deck must wind an average of ⅓ 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.
Tape remains a viable alternative to disk due to its lower cost per bit. Though the areal density is lower than for disk drives, the available surface on a tape is far greater. The highest capacity tape media are generally on the same order as the largest available disk drives (about 1 TB in 2007.) Tape has historically offered enough advantage in cost over disk storage to make it a viable product, particularly for backup, where media removability is also important. The rapid improvement in disk storage density and price, coupled with arguably less-vigorous innovation in tape storage, has reduced the market share of tape storage products.
[edit] See also
- History of sound recording#Magnetic recording - Magnetic tape in the context of the history of sound recording.
- Sound recording and reproduction#Magnetic tape - Magnetic tape in the context of the history of sound recording.
- Audio format - General list of audio recording formats, including magnetic tape.
- Audio tape length and thickness - Details of different audio tape formats.
- Tape recorder - Details regarding the workings of the recording machines.
- Reel-to-reel audio tape recording - Details of using old style recorders.
- Multitrack recording - Advanced usage of sophisticated tape recorders.
[edit] References
This article was originally based on material from the Free On-line Dictionary of Computing, which is licensed under the GFDL.
