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The laser disc

One of the important uses of laser technology is in the development of the laser disc system. Since the amount of information that may be carried on a laser disc is enormous: one video disc can carry as much information as a whole set of the Encyclopaedia Britannica. The original laser discs were superficially similar to a record, in that it was a 30 cm diameter disc with a spiral track running round it; there the similarity ends, however.


On the laser disc the track is only about 1 mm wide and is made of a series of tiny pits, each pit some 0.16 mm deep and of varying length (Figure 1). These pits are scanned by a fine laser beam only 0.9 mm in diameter. The reflected light from the flat part of the disc is detected by a photodiode and this modulated beam is converted into a television picture. The disc is given a thin metallised coating and is then protected by a layer of plastic through which the laser light can pass.

With a constant angular velocity (CAV) disc about 54 000 television frames may be carried, with some 28 x 109 bits of information per side!

However the modern CD runs at a varying speed so that the pits pass the laser at a constant linear speed. This means that the motor that spins the disc must run faster when the laser is reading from sections of the track nearer the centre of the disc than it does nearer to the outside edge.



The CD and the DVD.

The compact disc has now been developed and is widely used and the advent of the DVD (Digital Video Disc) with finer tracks enables even more information to be stored on small discs. For example a CD will hold up to 750 Mb of information, at present the 5250 pages of the schoolphysics.co.uk site occupies a mere 500 Mb!

Figure 2 shows a cross section through a CD. The depth of the pits has been very much exaggerated.

The wavelength of the light from the laser determines the smallest size of the pits and so effectively limits the amount of information that can be stored of the disc. At present this wavelength is 780 nm but with the development of lasers emitting blue light the pit sizes could be made smaller thus increasing the storage capability of the disc.

The pits are 0.83 mm long on a CD and only 0.40 mm long on a DVD.

When the laser beam passes over the edge of a pit the laser light is reflected from both the bottom of the pit and flat surface around them. Interference occurs between the incident and reflected beams. The depth of the pits is chosen to be one quarter of the wavelength of the light so that the light reflected from the base of the pit is out of phase with the light reflected from the surrounding surface and so the detector records a very low intensity. This corresponds to a zero in binary.

When the laser is over a flat surface the reflected beam is strong and this corresponds to a 1 in binary.

Notice that the depth of the pits given at the beginning of the section appears to be less than of a wavelength. This is because the wavelength of the laser light is reduced when passing through the polycarbonate plastic layer.

The information is recorded on the CD as a digital signal expressed in binary. The original analogue signal is sampled over 40 000 times a second and a value for each sample stored as a binary number.

An ANALOGUE SIGNAL is one where the waveform in the information follows the original waveform exactly at all times

A DIGITAL SIGNAL is one where the original waveform is sampled at regular intervals and a number given to the value of the disturbance at each of these points.

Binary numbers are used for these sampled values.

Binary numbers use base 2 instead of the more common decimal base 10. On the binary scale using 8 bits the decimal number 7 is 00000111, 12 is 00001100, 25 is 00011001, 87 is 01010111 and so on. This means that 87 would be a zero followed by a one, then a zero, another one, another zero and finally three ones.

The big advantage of the digital signal over the analogue one is its purity and lack of background interference or 'noise'.

Imagine that we sample the analogue signal at the point where the value is 87 but where there is a 3% noise overlaying the pure signal. This would give a distortion to the analogue value. However in the digital version the individual bits are either 1 or 0. A 2% distortion of the value 0 would still be 0 and a 2% distortion of 1 would give 1. You could think of the ones and noughts as being like small cups containing water which are either empty of full. A little water ion the bottom of one cup is treated as an empty cup and an almost full cup is treated as one that is completely full.

Some people actually still prefer the 'rougher' sound produced by the analogue recordings on vinyl records to the purer reproduction of a CD. They say that it makes the recording more real.

 

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© Keith Gibbs