Computers will play any kind of CD, but standalone CD players such as in a car or a Hi-Fi will generally only play audio CDs.
A Compact Disc (also known as a CD) is an optical disc used to store digital data. It was originally developed to store sound recordings exclusively, but later it also allowed the preservation of other types of data. Audio CDs have been commercially available since October 1982. They remain the standard physical storage medium for audio. although sales of commercial CDs have been falling for some years while digital downloads (for storage on hard drives or flash-based music players) have been increasing.
Standard CDs have a diameter of 120 mm and can hold up to 80 minutes of uncompressed audio (700 MB of data). The Mini CD has various diameters ranging from 60 to 80 mm. Mini CDs are sometimes used for CD singles or device drivers, storing up to 24 minutes of audio.
The vast majority of audio CDs are encoded according to the Red Book Standard. The Red Book standard is a 16-bit, 44100 Hz PCM stereo stream of audio. This is very similar to (though not directly comparable with) stereo WAV and AIFF files encoded at 16-bit, 44100 Hz. Audio CD quality sounds identical to those formats. However, because the data is a stream with a TOC (Table of Contents), rather than a set of self-contained files, most operating systems can't open the audio for editing in the same way that a WAV or AIFF file can be opened.
The basic Red Book specifications state that:
- Maximum playing time is 79.8 minutes.
- Minimum duration for a track is 4 seconds (including 2-second pause).
- Maximum number of tracks is 99.
- Maximum number of index points (subdivisions of a track) is 99 with no maximum time limit.
CD-audio and Audacity
Audacity cannot read CD-audio directly. In order to edit audio from a CD using Audacity, the audio data must first be extracted from the CD and converted to an audio file format that Audacity can read. This process is known as ripping. Audacity cannot rip CD-audio, so other software must be used. See How to import CDs in the Audacity Manual.
Just as Audacity cannot read CD-audio, it also cannot be used to create audio CDs directly. Audio CDs are created by the reverse process of ripping, where software burning applications convert audio files into a data stream on the CD. This process also requires the computer to have a recordable CD drive in order to make the CD. See How to burn CDs in the Audacity Manual for details.
If you need more playing time than the normal 74 or 78 minutes (for example, to try and accommodate a C90 cassette or two LPs onto one CD) some CD burners will let you "overburn" into the blank CD space so as to extend the playing time by a further few minutes, so giving you the possibility of up to 80 minutes playing time on a 650 MB disc or up to 86 minutes on a 700 MB disc. Overburning (if your burning software and burner supports it) is always done using Disc at Once (DAO) mode in which the tracks are burnt continuously without turning the laser off.
It is also theoretically possible to overburn using "90 minute" (790 MB) or "99 minute" (870 MB) CD-R discs. However there is no guarantee whatsoever that your CD burner will accept such CD-R discs, or that your CD player will play anything other than a Red Book Standard 650 MB disc burned with 74 minutes of audio.