Videodisc and compact disc technologies

Abstract

The majority of optical discs (videodiscs and compact discs) are based on the reflective optical disc (a rigid plastic disc). Both are recorded and read by lasers. In the case of videodiscs the data are stored as an analogue signal (analogue signals are based on the variations that occur in a continuous electronic signal such as occurs on casette tapes and videotapes). The most successful videodisc format was developed by Philips and is often referred to as a laservision disc. Videodiscs (laservision discs, laserdiscs) are 30 cm (12 inches) in diameter. The storage capacity of a videodisc is impressive, enabling a resource of approximately 55 000 still frames per side or 36 min of moving video (PAL, see below). Two digital sound tracks can also be incorporated into the videodisc. For historical and political reasons, three separate television (broadcast) formats have arisen in different parts of the world. Phase alternation line (PAL, used in the UK and the majority of Europe, plays at 25 frames per second), National Television Standard Committee (NTSC. used in the USA and Japan, 30 frames per second) and sequential colour with memory (SECAM and developed in France, not frequently encountered. 25 frames per second). This can lead to incompatibility but dual format (PAL/ NTSC) videodisc players are now widely available. Access times are rapid and usually less than one second. The laserdisc is now regaining popularity as a high-quality medium for movies and music videos and Karaoke systems are based on laserdisc jukeboxes.