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.

Compact discs (CDs), which may be found in over 25 million American homes, not to mention backpacks and automobiles, first entered popular culture in the 1980s. But their history goes back to the 1960s, when an inventor named James Russell decided to create an alternative to his scratched and warped phonograph records—a system that could record, store, and replay music without ever wearing out. The result was the compact disc (CD). Made from 1.2 mm of polycarbonate plastic, the disc is coated with a much thinner aluminum layer that is then protected with a film of lacquer. The lacquer layer can be printed with a label. CDs are typically 120 mm in diameter, and can store about 74 minutes of music. There are also discs that can store 80, 90, 99, and 100 minutes of music, but they are not as compatible with various stereos and computers as the 74–minute size. The information on a standard CD is contained on the polycarbonate layer, as a single spiral track of pits, starting at the inside of the disk and circling its way to the outside. This information is read by shining light from a 780 nm wavelength semiconductor laser through the bottom of the polycarbonate layer. The light from the laser follows the spiral track of pits, and is then reflected off either the pit or the aluminum layer. Because the CD is read through the bottom of the disc, each pit looks like a bump to the laser. Information is read as the laser moves over the bumps (where no light will be reflected) and the areas that have no bumps, also known as land (where the laser light will be reflected off the aluminum). The changes in reflectivity are interpreted by a part of the compact disc player known as the detector. It is the job of the detector to convert the information collected by the laser into the music that was originally recorded onto the disc. This invention brought 22 patents to James Russell, who today says he working on an even better system for recording and playing back music.

In computer science, acronym for compact disc read-only memory, a rigid plastic disk that stores a large amount of data through the use of laser optics technology. Because they store data optically, CD-ROMs have a much higher memory capacity than computer disks that store data magnetically. However, CD-ROM drives, the devices used to access information on CD-ROMs, can only read information from the disc, not write to it.

The underside of the plastic CD-ROM disk is coated with a very thin layer of aluminum that reflects light. Data is written to the CD-ROM by burning microscopic pits into the reflective surface of the disk with a powerful laser. The data is in digital form, with pits representing a value of 1 and flat spots, called land, representing a value of 0. Once data is written to a CD-ROM, it cannot be erased or changed, and this is the reason it is termed read-only memory. Data is read from a CD-ROM with a low power laser contained in the drive that bounces light—usually infrared—off of the reflective surface of the disk and back to a photodetector. The pits in the reflective layer of the disk scatter light, while the land portions of the disk reflect the laser light efficiently to the photodetector. The photodetector then converts these light and dark spots to electrical impulses corresponding to 1s and 0s. Electronics and software interpret this data and accurately access the information contained on the CD-ROM.
CD-ROMs can store large amounts of data and so are popular for storing databases and multimedia material. The most common format of CD-ROM holds approximately 630 megabytes (see Byte). By comparison, a regular floppy disk holds approximately 1.44 megabytes.
CD-ROMs and Audio CDs are almost exactly alike in structure and data format. The difference between the two lies in the device used to read the data—either a CD-ROM player or a compact disc (CD) player. CD-ROM players are used almost exclusively as computer components or peripherals. They may be either internal (indicating they fit into a computer’s housing) or external (indicating they have their own housing and are connected to the computer via an external port).
Both types of players spin the discs to access data as they read the data with a laser device. CD-ROM players only spin the disc to access a sector of data and copy it into main memory for use by the computer, while audio CDs spin throughout the time that the audio recording is read out, directly feeding the signal to an audio amplifier.
The most important distinguishing feature among CD-ROM players is their speed, which indicates how fast they can read data from the disc. A single-speed CD-ROM player reads 150,000 bytes of data per second. Double-speed (2X), triple-speed (3X), quadruple-speed (4X), six-times speed (6X), and eight-times speed (8x) CD-ROM players are also widely available.
Other important characteristics of CD-ROM players are seek time and data transfer rate. The seek time (also called the access time) measures how long it takes for the laser to access a particular segment of data. A typical CD-ROM takes about a third of a second to access data, as compared to a typical hard drive, which takes about 10 milliseconds (thousandths of a second) to access data. The data transfer rate measures how quickly data is transferred from the disk media to the computer’s main memory.
The computer industry also manufactures blank, recordable compact discs, called CD-Rs (compact disc-recordables), that users can record data onto for one-time, permanent storage using CD-R drives. Compact disc-rewriteables (CD-RWs) are similar to CD-Rs, but can be erased and rewritten multiple times. Another technology that allows the user to write to a compact disc is the magneto-optical (MO) disk, which combines magnetic and optical data storage. Users can record, erase, and save data to these disks any number of times using special MO drives.