Searching a music database for the missing album information in a music CD disc. Old, as well as many new, music CDs do not contain artist name, album and song titles, but MP3, AAC and other compressed audio formats include fields called "tags" for this data (see ID3 tag). When a CD is played in the computer or is being ripped to a compressed format, the media player software queries an online database such as CDDB (www.gracenote.com), freedb (www.freedb.org) or MusicBrainz (www.musicbrainz.org) to retrieve this information.

Why Identify the Disc?
When a song is played, the actual title can be displayed instead of track 01, track 02, etc. If the CD is being ripped, the titles are automatically entered into the meta-data fields of the MP3 or other compressed format. Without this service, users have to enter the titles manually for every track in the album. However, for MP3 collections that were never tagged or improperly tagged when the CDs were ripped, there are music databases and tagger programs that add the correct meta-data (see music file identification).

Only Track Number and Time
When they designed the CD in the early 1980s, they did not set aside a few thousand bytes for descriptive data among the 650 million available. An appalling lack of vision in retrospect, meta-data fields were added many years later (see CD Text), but are not mandatory and not always used.

The only meta-data guaranteed to be in a music CD are a table of contents (TOC) with track numbers and lengths, precisely what has been displayed on a consumer CD player since day one. Although not impossible, it is unusual to have two CDs with the same number of tracks and lengths in the same order, and it is this table of contents data that identifies a music CD. See CDDB, freedb, ripping and music search.