A compact disc (or CD) is an optical disc used to store digital data, originally developed for storing digital audio.
A standard compact disc, often known as an audio CD to differentiate it from later variants, stores audio data in a format compliant with the red book standard. An audio CD consists of several stereo tracks stored using 16-bit PCM coding at a sampling rate of 44.1 kHz. Standard compact discs have a diameter of 120mm, though 80mm versions exist in circular and "business-card" forms. The 120mm discs can hold 74 minutes of audio, and versions holding 80 or even 90 minutes have been introduced. The 80mm discs are used as "CD-singles" or novelty "business-card CDs". They hold about 20 minutes of audio.
Red Book is the standard for audio CDs (Compact Disc Digital Audio system, or CDDA). It is named after one of a set of colour-bound books that contain the technical specifications for all CD and CD-ROM formats. The Red Book was released by Sony and Philips in 1980, but the idea of the CD is older. In the 1960s James T. Russell had the idea to use light for recording and replaying music. So he invented in 1970 an optical digital television recording and playback machine, but the world did not jump on. In 1975 representatives of Philips visited Russell at his lab. They discounted his invention but they put million of dollars in development of the CD and presented it together with Sony in 1980. So James T. Russell was the original inventor of the idea of the CD.
Another interesting thing is, that there are many stories about the diameter of the compact disc. Some of them tell that Beethoven 9th symphony was the main reason for making the CD able to hold 74 minutes of audio data. Kees Immink tells in his paper “The Compact Disc Story” that the diameter of the CD was based on size of the compact cassette.
The first CD player was called Sony CDP-101. It was presented on the 1st October 1982 and was able to play audio CDs. The price was 625 US dollar.