There are eight subcode channels on the disk.
Important. The data is distributed uniformly across the entire CD, and each channel can hold a total of about 4MB.
The P subcode channel:
Identifies the start of a track, but is usually ignored in favor of the Q channel.
The Q subcode channel:
Includes useful information, which can be read and written on many recorders. The user data area contains three types of subcode-Q data:
- position information,
- media catalog number (MCN),
- ISRC code.
Other forms are found in the lead-in, and are used to enable multisession and describe the disc TOC (table of contents).
The position information is used by audio CD players to display the current time, and has track/index information. This can be controlled when doing Disc-At-Once recording.
The ISRC (International Standard Recording Code)
is used by the recording industry. It states the country of origin, owner, year of issue, and serial number of tracks, and may be different for each track. It's optional; many CDs don't use this. The media catalog number is similar, but is constant per disc. Note these are different from the UPC codes.
The R-W subcode channels
are used for text and graphics in certain applications, such as CD+G (CD w/graphics, supported by SegaCD among others). A new use has been devised by Philips, called ITTS. It enables properly equipped players to display text and graphics on Red Book audio discs. The most recent result of this technology is "CD-Text", which provides a way to embed disc and track data on a standard audio CD.