CD implies Compact Disc. They are circular plates that are 12cmin diameter. Sony and Philips suggested the CD standard in 1980, and the invention was introduced to the US market in 1983.
CDs have the capacity to store up to 700 MB of data or 80 minutes of audio, and the data on them is stored as small notches on the disc and is decoded by a laser from an optic drive. The driver converts the notches – 1s and 0s – into a usable data.
The initial CDs were audio devices, which finally replaced audiotapes, which themselves replaced records. Audio CDs have the merit of allowing the user to jump to different tracks on the disc.
Also, CDs can be listened to multiple times without losing quality, unlike audio tapes that can start to lose quality after few times of listening to them. This is because the laser that interprets data on a CD does not apply pressure on the disc. While the play heads on a tape deck wear away slowly the magnetic strip on the tape.
By 1985, CDs became the most common software format. Because they could hold up far more data than floppy discs (700 MB compared to 1.4 MB). In 1988, the CD-R technology was invented allowing PC users to burn their CDs. However, the technology did not become popular until the late 1990s. A smaller CD, CD-3, is also available and is decipherable by tray-loading CD-ROM drives.
CDs are now increasingly being replaced by other forms of digital distribution and storage, with the result that audio CD sales rate in The US has dropped by 50% from their peak. However, they are still one of the major distribution methods for the music industry.