Audio compact discs carry sound information recorded as digital data. Most audio CD players use an uncompressed data format capable of producing high-quality sound. The majority of music CDs come in a format called WAV, which is also used for some sound files in personal computers. In addition, many CD players can handle other formats that allow extended playback time.
The audio on a CD is represented by a long stream of numbers. The CD player retrieves these numbers from the disc and reconstructs the original sound waves from the data. The numbers are recorded digitally. As long as the disc has no damage, the numbers are always the same, and the music that comes from them is a close replica of the original.
WAV files have data in 16-bit chunks recorded at a rate of 44.1 kHz, chosen to capture the highest frequencies audible to humans, about 20 kHz for an average young adult.
Compressed Data Formats
The WAV files used on most music CDs carry every piece of audio data the way it was recorded. Other common sound file formats, such as WMA and MP3, can be heavily compressed in a mathematical process that squeezes more minutes of recorded sound onto a CD at the cost of reduced sound quality. These formats are sometimes used for recording audiobooks, allowing for the hours needed for book narration without requiring multiple discs.
Most modern CD players have sophisticated electronics that play discs containing these compressed formats as well as WAV. However, professionally recorded CDs almost universally use the WAV format. Some of the oldest CD players that made before the other formats became common may not play MP3s or WMAs at all.
Red Book Standard
Music CD publishers organize data on music CDs in accordance with the Red Book standard, a technical document developed by Sony and Philips in 1980 that specifies standards for CDs, with music CDs having different standards from those used for photo albums or computer documents. A few record labels, including Sony and EMI, have in the past produced CDs that do not completely conform to the Red Book standard; in some instances, these CDs have extra or altered data intended to foil music piracy. Some types of CD players, including portable models or in-dash units for cars, may not be able to play these copy-controlled discs.