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CD+G

An audio compact disc format that contains graphics data in addition to the audio data. A CD+G disc can be played on a regular audio CD player, but when played on a CD+G-compatible player, can output a graphics signal (typically, the CD+G player is hooked up to a television set or a computer monitor). After an earlier life supporting video games, CD+G is being used for CDs for karaoke systems, with the graphics used to display song lyrics.

A compact disc contains two kinds of data: Content data, which is used to store audio, computer software, etc., and subchannel data (or metadata), which is normally used by the CD player to help control the disc. In each sector of a CD there are 2,352 bytes of content data and 96 bytes of subchannel data.

Each of the 96 subchannel data bytes can be thought of as being divided into 8 bits. Each of these bits corresponds to a separate stream of information. These streams are called “channels,” and are labeled starting with the letter P, so:

Channel
P, Q, R, S, T, U, V, W

carries bit
7, 6, 5, 4, 3, 2, 1, 0

Channels P and Q on a regular audio CD are used to assist the CD player in tracking the current location on the disc, and to provide the timing information for the time display on the CD player. The CD+G format utilizes channels R through W to store 16-color (4-bit) graphics for a display that is 300×216 pixels in size.

The videogame consoles Sega CD, Sega Saturn, Commodore Amiga CD32, and the Atari Jaguar CD (which was an attachment to the Atari Jaguar) also played CD+G format CDs.

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