VIDEO DISC TECHNOLOGY
Video-recorded discs have never enjoyed the popularity of videotape cassettes for several reasons, among which are the fact that (with the exception of advanced DVD types) it is not possible to record on them at home; even though they are cheaper to produce in bulk, they have tended to be more expensive in the shops than tape; and software availability is not as good as for tape. On the credit side they give much better, more detailed pictures than everyday videotape formats, and they do not wear because their play process is a contact-free one. Because of their picture and sound replay quality video disc players find their best market among home-cinema enthusiasts, many of whom import discs from the USA (where there is a wider choice of films and a different censorship scheme) and take advantage of the multistandard capability of most LDV players and many TV sets.
LASERVISION
Laservision is the longest-established optical video disc system, having started in 1972 with Philips. It offers virtually full-broadcast bandwidth, and is the only home video system capable of handling an analogue colour signal en-suite as it were: conventional videocas- sette formats require the chroma subcarrier to be frequency-shifted in a colour-under system, and low-band ones (standard-VHS and Video8) relimitedtoabout2.5MHzluminancebandwidth.ForLaser- vision the vision and sound information is encoded on the disc in the form of a series of tiny pits in its surface, which is then aluminised and sealed with a coat of plastic. Ordinary video discs are 30 cm (12 inches) in diameter, and double-sided. The signal-bearing pits are arranged in a continuous spiral, starting near the disc centre and finishing at the outside edge. They are very closely spaced at around 600 lines per mm, giving a total track length of about 34 km, 21 miles. The readout system is optical, and the pick-up sensor does not touch the disc at all – it depends on reflections from the pitted surface. Thus there is no wear or deterioration of the disc, even in still-frame. Since the optical system focuses on the subsurface pits, dust, fingerprints and (within reason) superficial damage to the disc surface have no effect on reproduction quality. The specifications and characteristics of the Laservision system are given in Table 20.1.
