An MP3 CD is a Compact Disc (usually a CD-R or CD-RW) that contains digital audio in the MP3 file format. Discs are written in the Yellow Book standard data format (used for CD-ROMs), as opposed to the Red Book standard audio format (used for CD-DA audio CDs).
MP3 files are supported by many modern CD players, including DVD players. In addition, some CD players are also capable of playing other formats, such as Ogg Vorbis and the proprietary Windows Media Audio and ATRAC.
Because of audio data compression, an MP3 CD does not have to spin all of the time, potentially saving battery power, however, decompressing the audio takes more processor time. The song is buffered in random-access memory, which also provides protection against skipping.
The number of songs that a disc can hold depends on how the songs are encoded and the length of the songs. A standard audio CD (74 minutes) can hold about 18 songs, a 650-MB data CD (equivalent to 74-minute audio CD) containing mid-quality (160-kb/s) audio files can hold approximately 9.5 hours of music or about 138 songs.
ID3 tags stored in MP3 files can be displayed by some players, and some players can search for MP3 files within directories on an MP3 CD. The sound quality of an MP3 CD is inferior to that of an audio CD, because MP3 compression is lossy.
There is no official standard for how the MP3 files on an MP3 CD are stored on discs. As such, the format expected by different players varies. This sometimes leads to incompatibilities and difficulty in playing discs, often because of filename length limits, sub-directory limits, number of files limits, and special character bugs.