Compact Disc

Compact Disc, or CD, optical disk on which sound is recorded in a digital format by assigning numerical values to measurements of the sound. The sound can then be reproduced with high-fidelity (see Sound Recording and Reproduction). The compact disc is an improvement over its analog, or continuous recording predecessor, the vinyl phonograph disk (record, LP). Sound is recorded on vinyl records by creating grooves on the record, which are then read by a stylus, or phonograph needle. The compact disc is the most popular medium for recorded music and has almost entirely replaced the record.

The principle behind all sound storage and reproduction systems is similar: The vibrations of air that correspond to sound waves are converted into electrical signals that can be recorded onto a storage and playback medium. The most common storage and playback mediums are mechanical (the phonograph), magnetic (cassette tapes), or optical (the compact disc). The optical medium, however, represents a technological advancement over mechanical and magnetic media for a number of reasons: It provides more memory capacity than the analog tape or phonograph in a relatively small size, it reproduces sound with higher-fidelity, and its method of operation involves no physical contact between the device used to read information from the disk and the disk itself—eliminating wear to the optical disk.

Compact discs are made using sophisticated digital sound recording and manufacturing systems. The first step in recording sound on a CD is for the recording system to sample the analog sound at specified intervals. The recording system then electronically transforms these sampled audio signals into a digital, or discrete numerical, format. The next step is for the digital information to be physically transferred to the optical disk. To do this, a special master optical disk that has a light-sensitive base layer is exposed to pulses of light from a laser as the disk spins. The pulses of laser light encode the digital information onto the disk. Once the disk has had the necessary information recorded on it, it is immersed in a chemical that etches the areas exposed to the laser beam, leaving microscopic pits in the surface of the CD that represent the 0s and 1s of the digitally converted sound.

To read information from a compact disc, CD players use a low-intensity laser scanner that reflects light off of the surface of the disk. The intensity of the light reflected back from the surface of the CD depends upon whether the light strikes a flat spot, which corresponds to a 0, or a pit, which corresponds to a 1. Special circuitry in the CD player converts the variations in intensity of the reflected light into electrical signals. These signals can then be amplified and reproduced by standard stereo equipment.

The Sony and Philips Corporations developed the compact disc in the early 1980s. The CD quickly gained popularity, rapidly replacing vinyl records as the preferred medium for musical recordings. The same technology that is used in the compact disc to store sound digitally is also used to store text and images in an optical storage system called the CD-ROM (Compact Disc Read-Only Memory). New variations on CD technology allow users themselves to store information on special types of CDs. Users can record information on a blank compact disc-recordable (CD-R) or compact disc-rewriteable (CD-RW) . Once recorded, a CD-R cannot be changed, but a CD-RW can be erased and re-written multiple times.