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CD+G is an official CD format, developed by the same company who developed the other CD formats as well, Philips.
CD+G is essentially a basic audio CD, but in addition of regular PCM audio, it also contains graphics data, stored on the subchannel data area, which is typically used by CD players to control the playback of the CD.
CD+G discs can be played with regular audio CD players and they behave exactly like normal audio CDs when played in one. However, when played with a player that supports CD+G format, the graphical data representation can be shown while the disc is playing. This graphical signal is usually outputted to a regular TV set.
Most common use for CD+G format is with karaoke players, that use the audio tracks for the music and the graphical data for the lyrics.