Graphics and multimedia:Compact discs

Compact discs

Compact discs are now widely used for music recordings. As explained in Chapter 3, compact discs store information in a digital form, which means that, unlike ordinary ana­logue recordings, there is no degradation of the signal. If the reproduction is less than perfect, then the fault lies with the other analogue parts of the system - such as the microphones and the loudspeakers - rather than the com­ pact discs themselves.

Being digital, compact discs clearly have an important role in the digital world of IT. For besides audio recordings, they can be used to store computer data and text, as well as video images in digital form. Interactive video systems based upon compact disc are likely to be much more significant in the future than IV systems based on videodisc.

Laser light represents a binary 1by a short pulse of light from a laser gun, and a binary 0 by the absence of a pulse. To record information on a compact disc, very powerful pulses of laser light are used, which burn tiny pits in its surface. As the disc rotates, and the laser gun works its way like a gramophone stylus across the surface, a tightly-packed spiral of tracks consisting of thousands of tiny pits is built up.

On playback, the process works in reverse. A low­ powered laser beam scans the tracks of the rotating disc, being reflected back by the silvered surface. The pits on the surface are 114 of the wavelength of the laser light in depth, so light reflected from the bottom of a pit is exactly out of phase with light reflected from the surface. It therefore interferes with it, largely cancelling it out. The returning beam strikes a photoelectric cell, which converts the light to electric pulses. The less intense light from a pit produces a smaller pulse than light reflected from a spot on the surface which doesn't contain a pit. In this way the original pattern of 1s and Os is reproduced.

As with videodisc, there is no physical contact between the surface of the disc and the playback mechanism, and so no wear is caused by repeated playings, and grease or dust normally has no affect. Unlike videodisc, a compact disc is only 5.25" across, the same as an ordinary floppy disk.

Compact disc for multimedia applications

The huge capacity of compact discs means that they can store large amounts of computer data, text, images, and sound. Encyclopaedias, for example, are now being pub­lished in compact disc form. This not only makes them very compact, it also means that, since they can be linked to the power of the computer, all entries on a selected topic can be rapidly retrieved and displayed on the screen.

Another development is CD-V, short for 'Compact Disc- Video', which will be used for high-quality sound and video. One obvious application for CD-Vis pop videos.

There are also interesting possibilities of computer-con­ trolled multimedia presentations using compact disc sys­tems. These could form a low-cost alternative to IV systems.

Philips, for example, has been working for a number of years on CD-I, which stands for 'Compact Disc - Interac­tive', and is bringing out a mass-market CD-I player in 1992. (A professional system is already available.) This will cost a fraction of the price of a videodisc player, and will use comparatively inexpensive compact discs. Quite a number of CD-I discs are currently being produced ready for the launch, covering topics as diverse as photography, golf, and language learning.

CD-I aims to take full advantage of the enormous capacity of compact discs for storing images and sounds as well as text, and combine this with the power of the computer to develop a new publishing medium. This will revolutionize encyclopaedias, dictionaries, training materials, and so on, which ideally contain visual, aural, and textual components. A single disc is able to hold a complete English-language dictionary, including the words in audio and some accompanying pictures in (still) video.

Like interactive video, the computer software will allow the user to access any part of the disc by means of a system

of menus or questions, and if necessary will conduct him through the disc as part of a training package, with ques­tions, feedback, and branching.

Unlike interactive video, the pictures are digitized, and have to be read from the disc and processed by the computer before they can be displayed on the screen. The Philips machine will have two video processors, which means that it can display simultaneously two different signals from the disc, e.g. text and real pictures. So it can be used in much the same way as interactive video, at a fraction of the (hardware) price.

Two competing multimedia compact disc systems that have already reached the market are Intel's Digital Video Interactive (DVI) system and Commodore's CDTV (Com­modore Dynamic Total Vision). At the moment, Philips CD-I seems to be technically superior, but things change rapidly in the highly competitive world of computing and multimedia.