Videodisc, also known as laserdisc, thin circular platter that contains video and audio information similar to, but with greater storage capacity than, a compact disc. Videodiscs are used primarily in the entertainment industry to store and play movies and in education to contain information such as an encyclopedia.
The videodisc is made of clear plastic and is coated with a thin photosensitive material. Its diameter ranges from 12 to 30 cm (5 to 12 in). A videodisc contains a series of microscopic pits burned into it by a laser. Each pit represents the number 1, and each smooth area (untouched by the laser) represents the number 0. These 1s and 0s, called binary numbers, are used to store the information in a format compatible with the electronic circuitry of a computer. The sequence of microscopic pits is arranged in tracks that spiral into the center of the disc. These tracks are small enough to fit nearly 500 of them in the period at the end of this sentence. For mass duplication of a particular disc, a mold is made from a master disc, and subsequent discs are made from imprints of the master.
A finished videodisc can be placed in a laserdisc player designed to read the information encoded upon it. A low-power laser illuminates the disc as it spins, and the resulting reflected patterns made by the series of pits and smooth areas are focused onto a detector. The detector decodes the patterns into binary numbers, which are then translated by electronic circuitry into the original video and audio signals.
Schemes for encoding and compressing video signals, such as motion joint photographic expert group (JPEG) or motion picture expert group (MPEG and MPEG2), have enabled large amounts of video information to be stored on a single disc. Compression techniques identify a recurring series of numbers that represent a particular piece of information and replace that series with a single number to save space.
Compression technology, along with techniques that correct information when it is improperly stored or transferred from the disc, have made optical storage systems, such as videodiscs and compacts discs, useful for personal computer systems. This technology will allow the production of a new generation of optical storage media known as Digital Video Discs (DVD).
A DVD is a high-density compact disc encoded with MPEG2 video and audio information. These discs will initially contain about 136 minutes of video and audio information. The DVD will offer superior resolution for video information and will support both standard and high-definition television formats. Eventually, a higher-density version of the DVD will contain about nine hours of video and audio information.
The main drawback with optical storage media such as videodiscs is that new information may not overwrite information that has been burned into the disc. Magnetic tape, such as the kind used in VCRs, has the distinct advantage of being able to record and rerecord, but it lacks the resolution and durability of videodiscs. The videodisc is more resistant to heat and scratches, and produces higher-resolution images with better quality sound.