Magnetic band

Use

Audio

It first of all was used for the recording of the Its (first uses in France in the years 1940-50, in the studios of rtf - Broadcasting French Television - to replace the 78 turns). They were analogical recordings here on Magnétophone S with bands ½ inch (Ampex, Studer etc).

to facilitate the use, the bands were placed in cassettes. That which had greatest success is the Musicassette of Philips.

During the appearance of digitalization, a new format of cassette was launched: the DIGITAL Audio Types (DAT), where the numerical information is recorded transversely on the tape using a rotary head.

Video

The first video recorders were apparatuses with professional vocation used by the Television channels, they functioned with magnetic bands (format to be specified).

Thereafter, the projections of consumer electronics allowed launching, by several manufacturers with various fortunes, various types of cassettes Vidéo associated with their recording material/reader, bound for the Amateur S of video recording:

the V2000 of Philips, good quality or long life of recording, but too expensive because too complex.
the Betamax of Sony, best quality, a little expensive, but very snuffed professionals.
VHS of a consortium of Japanese manufacturer with as leader the JVC, the least powerful but of quality sufficient for the amateurs, the least expensive, ended up becoming the standard of the video tape recorders of living room.
For the video cameras a mini version of the VHS was developed and had a certain success.

All these systems have as a support a magnetic band, the information which is stored there is analogical, it is recorded in diagonal on the tape using a rotary head.

There exist other formats using a video magnetic band and storing it numerically: the format Video 8, the mini FD (used on cameras FD). See Cinema and video amateurs.

Data processing

History

The magnetic bands were used like mass memories as of the beginnings of data processing. As of the years 1950 the format of the bands quickly becomes standard: a band measured to the maximum 2400 foot S length (732 Mètre S) for ½ inch (13 mm) broad; smaller reels of 200,300 or 600 feet were also produced. In the beginning, the recording was carried out on 7 tracks (6 of data plus for the parity) with densities of 556 or 800 bpi but with the development of the computers with character sets to eight bits (e.g. EBCDIC of IBM), the recording passed to 9 tracks (8 of data plus for the parity) and the density could be of 800 (NRZI: Non-Return-to-Zero Inverted), 1650 (EP: Encoded phase) or 6250 (GCR: Group Coded) bpi.

Operation

The recording is linear, i.e. the tracks are parallel to the band, that is to say helicoid.

The formats and densities evolve/move unceasingly. In 2005, the format Ultrium LTO3 (Slap-Open Linear), for example, can store 400 Go on a cartridge of a few centimetres on side (approximately 10 X 10 X 2 cm).

The data are not stored in a contiguous way of the beginning to the run-out. They are stored in blocks separated by intervals ( gap in English). These intervals make it possible to the tape reader to stop before taking again a reading or a writing: it is not possible to stop the unfolding of the band then taking it again where it had stopped. When the reading must be stopped, even temporarily (for example, when the buffer memory is full), the band is rewound until the preceding gap. Sometimes, one calls the size of the block factor of blocking . It is because of this organization that the uncurlers of bands involve the band according to a jerked movement, with long readings (forwards), then a short stop, a short rewinding, before taking again the reading.

It is possible to read information in the two directions, and this faculty was in particular used in the algorithms of sorting.

As for any magnetic medium, it is possible to put the support in reading mode alone. On a magnetic band, this is accomplished by withdrawing the ring of the groove located at the back of the band.

The first meters of band are called the starter . The starter is finished by a reflective label (called sticker ) stuck on the tape. A photoelectric device of detection on the uncurler of bands detects this label which delimits the beginning of the data field and finishes the stage of “loading” of the band. One second label is positioned at the run-out, and marks the end of the data field. Typically, when this one is detected by the reader, it rewinds the band automatically.

This simplistic system knows several defects: if the label of beginning is lost, the reader goes all embobiner on the second hub, to stop when the label of end is detected, indicating (wrongly) that it is at the beginning of the band. If one orders the reading of the band at this time, the reader will roll up the totality of the band on the second hub before producing an error. The rewinding of the band on its initial hub can prove to be a delicate operation. Sometimes only the label of end is lost, and it is necessary to restick a news and to renew the writing of the data of it, the last part being truncated. It will be also noted that the sticker is stuck along an edge of the band: one should not be mistaken in edge if one must stick new, otherwise it will not be detected!

Applications

The price positive ratio/density of the magnetic bands always makes of it a support of choice for the data-processing safeguards. In spite of the very important progress made as regards technologies of storage on magnetic or optical disks at the end of the 20th century, the magnetic bands remain a privileged support of safeguard and filing of the data because of their very great capacity and their removable nature which make it possible to delocalize them easily. They are thus used in the “farms” of waiters on PC which underlie large the Web sites of today. Those manage the volumes of data incomparably more important than largest of the host computers of antan. Technologies of filing on tape followed this same modular approach: each bay can contain ten or a score of magnetic bands and the bays can gather, for example by 8 or 10, for a full capacity going until several tens of terabytes. The libraries of bands are thus the most accessible means to ensure the safeguard and the filing of all these data.

Mixed

Certain formats of tapes magnetic in cartridge (or cassette) allowed their employment in the three above mentioned fields. It is the case for the DAT (DIGITAL Audio Types) used as well in tape recorders, the video cameras, or systems of data-processing safeguard.

Lifespan

The lifespan of the data on a tape ½ inch is about 10 to 20 years.

References

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