Twyla Tharp

Principle

The Fréquence of a signal Vidéo can go until 6,5 MHz. However the memory support usually used in video, the magnetic band, is limited to the level of the density of information. For example, a audio Cassette of excellent quality has its band-width limited around 15 Khz whereas the tape speed of the band is of 4,75 cm/s.

In this basing on this report, simple a Rule of three makes it possible to evaluate the tape speed necessary for the recording of video on magnetic band:

$V_v = \ frac {F_v \ times V_a} {F_a}$ , with:

$V_v$ : speed of the video, which one seeks;
$F_v$ : frequency of the video (6 500 Khz);
$V_a$ : speed of the audio (0,0475 m/s);
$F_a$ : frequency of the audio (15 Khz).

One thus obtains: $V_v= \ frac {6500 \ times 0,0475} {15} = 20,58 \, \ mathrm {m/s}$ (i.e. about 74 km/h).

What means that only 49 is needed; seconds to absorb one kilometer (!) of magnetic band. As much to say that this method is practically unusable, even if at the beginnings of television, the video tape recorder “Vera” of the BBC functioned according to this principle.

Initially, the system “quadruplex system” developped at the point by the Ampex firm was the only able one to reproduce televised images of professional good quality. Principle developped at the point in the Fifties which initially, was only used to remotely transmit complete programs of the east coast of the the United States towards the west coast. At the time, no possibility of assembly.

The recording was carried out on a magnetic band of 2 inches broad. To obtain the tape speed necessary, a drum of 4 video heads turned at the speed of 15 000 turns/minute, each head recording a package of 16 lines. A very elaborate electronics reconstituted the whole signal of a screen. No roll of the dice supported such a mode. It was compressed air which made “float” the axis of the drum on its stages. In addition to the video one recorded transversely, there were a track its, a track auxiliary, called “cue” in the jargon and a timing track.

Enough quickly, appeared devices making it possible to publish the videotape and of the correctors of signals allowing the recording of the color.

One suspects it, it acted of complicated machines, consuming 3 kw and requiring a connection of compressed air. Weight by machine: 500 kg.

A neologism was used on television French: one spoke d' ampexer the programs for the video recordings.

End of the year 60, the helicoid recording was proposed by Ampex and Sony, then taken again since by all the standards of video recording (as well as the audio recorders DAT). The basic idea is simple: it is not possible to record video with less 20 m/s, on the other hand it is not necessary to occupy all the bandwidth (ranging between 4 mm and 25,4 mm) with the video signal, some tenth of millimetre are enough, moreover the magnetic head used for the writing and the reading must be of small size to allow a band-width until 6,5 MHz.

The video heads are thus laid out on a cylindrical drum and the band is rolled up around him. The drum turns at high speed, in such way that the heads laid out on its periphery go at a speed of 20 m/s compared to the band. The axis of the drum is tilted so that the way of the video heads also follows an axis inclined compared to the band. The band moves simultaneously, with the result that to the turn according to the video heads will pass beside their preceding course without overlapping it.

In accordance with the standards of apparatuses, the drum can be equipped with 1,2 or 4 video heads (the systems with a head are very old standards). In addition for reasons of stability of the image, an oblique track corresponds to exactly an image or a screen.

Standards

Use of half of the bandwidth 1/2".
Brightness/Chrominance
STD: Its linear - FM: Its Hi-fi - PCM: Its numerical - brackets = in option
One needs 6 consecutive tracks to record a image
Bandage stored in reel and not in cassette
Duration with cassette compacte
In normal speed and standard PAL/SECAM
Use a cassette W-VHS
Use the cassette 3:00 of the system VCR-Longplay
Announced, but not appeared

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