Description

This article is from the CD-Recordable FAQ, by Andy McFadden (fadden@fadden.com) with numerous contributions by others.

2-11] How does an audio CD player know to skip data tracks?

(1999/04/11)

There are subcode flags in the Q channel for each track:

Data
If set, the track contains data; if not, the track contains audio.
Digital Copy Permitted
Used by SCMS. Set to allow copies, clear to prevent them.
Four-Channel Audio
The Red Book standard allows four-channel audio, though very few
discs have ever been made that use it.
Pre-Emphasis
Set if the audio was recorded with pre-emphasis.

The last two are rarely used.

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