EMZETT.Compact Disc (CD)
In short: Optical disc, 12 cm across, for music, developed by Philips and Sony and on the market since 1982. An audio CD holds about 74 to 80 minutes of music.
In more detail: A CD stores information as tiny pits and smooth lands along a spiral track. A laser scans the track, a photodiode measures the reflected light, and that yields the bits. Audio CDs follow the “Red Book” standard: 16 bits per sample, 44.1 kHz, two channels (stereo).
In Depth
Structure and how it works
The disc consists of clear polycarbonate, a very thin reflective aluminium layer and a protective lacquer. It is read from below through the polycarbonate with a laser of about 780 nm (infrared). The track starts at the inside and runs outwards; the disc spins faster near the centre so that the track passes the laser at constant speed.
Variants
The pressed CD (CD-DA) is made in a factory. With CD-R a stronger laser burns marks into a dye layer once; CD-RW uses an alloy that can be switched repeatedly. Burned CDs are more sensitive to light and ageing than pressed ones. For data instead of music there is the CD-ROM.
Care and faults
Scratches on the reading side disturb the laser; scratches on the label side are worse because the reflective layer sits almost directly under the lacquer. Error correction (Reed-Solomon) compensates small defects. Clean from the centre outwards, not in circles. The CD has been replaced as the main medium by DVD, downloads and streaming, but remains popular with collectors, audio dramas and albums.
See also: CD-ROM, DVD, Optical drives