The DVD is advanced to the magnetoscope

2000/10/01 Azkune Mendia, Iñaki - Elhuyar Fundazioa Iturria: Elhuyar aldizkaria

• Elektronika

In a few months the DVD system has become the main competitor of the video. Just access any videoclub to see how the DVD movies have been played alongside the video films that have been made so far. For this there are several reasons:

1) Thanks to its optical reader, the DVD has an unbeatable image and sound.

2) Unlike what happens in the videos, the films on this support are repeated (thousands of times) but are not spoiled.

3) For the video industry to remove copies of DVD discs (or "clones") in press is very easy and fast. On the VHS cassette, on the other hand, you have to pass the whole tape and for it, even if it passes fast, you need time.

The DVD had to break a limit in order to completely replace the video clip. From the mere fact of reading the discs it was necessary to record the discs, that is, any television program (or the one broadcast by a camcorder) could be recorded, erased and recorded again on the disc. This is what Pioneer and TDK have jointly achieved.

For the DVD, the "magnetoscope"

recording device on one side and the image and sound support (DVD-RW) on the other. In this DVD-RW it means re-writable, that is, it can be removed and recorded on top. Pioneer has prepared a reader/recorder and supports or discs to record TDK houses.

First in Japan, devices and discs of this new technology have been put on sale. Last December, more than ten thousand Pioneer DVR-1000 devices were sold, despite their high price, before the month of its launch. The price of the device is about 325,000 pesetas (13,000 pounds or 1,950 euros) and its high cost is because the preparation of the reader/recorder has had to overcome complicated technical problems. On the one hand, the difficulties were in recording and reading the disc, and on the other, in the treatment that had to give the images of the video to incorporate them to the DVD disc.

Recording technique

The DVR-1000 can burn it to DVD disc, but it can also erase it, reburn and modify it. And all this on thousands of occasions without losing quality. The information recording system, therefore, is much more complex than on the CD-ROM which can only be recorded once. This is an optical magnetic procedure based on the modification of the refractive index of the "active" layer of the disk containing the information data. In this technology, the concepts of optics and interferometry are used.

The purpose of recording is to save on the disk a succession of symbols "0" and "1" because the information is digitized. Being an optical reading of the DVD and based on reflecting or not the ray of light that protrudes from the reader's head, zones are created that reflect or do not reflect according to the signs "0" or "1" to be preserved. For this it is essential that the disc has a sensitive layer very well calibrated and can vary the refractive index.

The sensitive layer of the disc is a mixture of silver, Indian, antimony, germanium and telurium. Therefore, on the disc you have to create “sites” and its optical behavior must be the same as in the micropanels of the traditional DVD. They are generated by the constructive or destructive interferences that have been reflected in the sensitive layer in the layer in which the reflective or opaque zone acts as a mirror. In reflection, the intensity of the original light beam is reinforced or weakened.

Remember that the light produced by the laser is encoded, that is, all the light waves that propagate are in phase. When the light passes from one medium to another, the speed of propagation changes. However, it must be taken into account that, regardless of the medium crossed, the frequency of light waves (or the number of electromagnetic vibrations that occur per second) is constant, so the speed variation occurs slightly changing the wavelength. In other words, the distance between two equal states of vibration changes.

Sensitive layer

In the DVD that is used to record, the thickness of the sensitive layer and refractive indices can be given two cases. With a certain refractive index, the laser beam leaves the self-sensitive layer in phase, that is, there is constructive interference and reflection occurs. With another refractive index, the incoming laser beam comes out with the opposite phase, that is, there is a destructive interference and the light is turned off. The reflected light is removed and means there is no reflection. This is what experts call coding of laser beam reflection by "phase modulation".

Therefore, to move from one state to another, you have to modify the refractive index. For this purpose, the central core of the sensitive layer is the magneto-optical element, which with high temperature and a high intensity magnetic field changes the refractive index. The DVR-1000 recording head features a laser to heat the sensitive layer and electromagnet to create magnetic field. During recording, the laser light heats the point of 0.74 micrometers in diameter to the sensitive layer in less than a million seconds to 180 °C. That hot dot is the base of the recording DVD. But in addition, to encode the disc, in addition to the heat, a magnetic field is needed. Opposite the laser beam, on the other side of the disc, the electromagnet creates a magnetic field. Depending on the direction of the magnetic field, the sensitive layer has a refractive index or another, that is, it is enough to modify the polarity of the electric current that feeds the electromagnet to encode in the sensitive layer the reflective point or the black point.

Disk capacity

TDK says its empty discs can be recorded and erased a thousand times without loss of quality. In the tests carried out, in addition, ten thousand times recorded, the discs have not lost any initial capacity. However, TDK discs are of a "unique" magnetic layer and each of them can store a maximum of 4.7 gigabytes (you can record a two-hour disc).

the recording standard of the DVD is MPEG2. The first task of this coding system is to compress the set of numerical data generated by digitizing the film, but compression has its limits. Although the first great planes, fixed landscapes, etc. They allow to compress much without losing quality, with the action images does not occur the same thing. In the first case, two megabytes per second are sufficient, but when movements are faster a flow of nine megabits per second is required. The entire disc for safe walking can be recorded at maximum flow, but thus the disc would only have an hour of capacity. Another solution would be to record everything with a flow of 4.5 megabytes per second, but so in the fast moving planes there are pixels. Therefore, the best thing is to change the flow, taking into account the type of images that will be recorded.

Empty discs sold by TDK 5.000 pts. Its price is 200 pounds or 30 euros, but other houses are also working on the prtest of sensitive two-layer discs and their engraving and reading devices. Thus they would have capacity of 9 gigabytes, that is, to be able to record a film of 4 hours.

Automatic compression

If in our home room we have a DVD device and we want to record a TV movie, for example, it is not nice that we are changing the self-compression flow by images. For this reason, the Pioneer house has prepared an automatic compression adaptation system. The encoding flow can vary from 2 megabytes per second to 9 megabytes. This is a relatively complex system, since the analysis of images must be done in real time. However, Pioneer also offers the possibility to compress by hand to extract copies of high-quality rolled. If the program to be recorded has no image of action and the user does not consider a great quality essential, the disc is very compressed and has the capacity to record movies of six hours.

Making copies or "clones"

The houses that are making films fear that the DVD that is capable of recording is another tool to hack their works. When copies are removed from the digitized support there is no loss of quality, so there are those who say not to copy that work and that it is "clone".

Pioneer, to prevent your device from being used in pirate work, you have not entered the digitized information. Therefore, it is only possible to remove copies of films or programs using analog cables, but quality is lost both in image and sound. In any case, the digital signal includes the devices and it is not surprising to develop a confidential industry dedicated to the adaptation of these devices and the manufacture of high quality "clones".