Key2Audio

key2audio is a copy restriction system for Audio CDs, developed by Sony DADC. The system gained notoriety after it was discovered that one can effectively disable the system by tracing the outer edge of a CD with a felt-tip marker.

The system works by applying a bogus data track onto the disc during glass master manufacturing. Since computers are designed to read data before music, this prevents playback on a PC and thereby prevents ripping as well as ordinary listening using a computer. CD players ignore any non-audio tracks on a disc, and can therefore play the CD unimpeded - Enhanced CDs also take advantage of this factor.

Currently there are two major versions of the key2Audio protection: the original key2Audio, and key2AudioXS.

key2audio products are highly compatible because the audio session is not modified. Since additional data sessions are used, the CD is not Red Book compliant and should not use the "Compact Disc" logo. As with any passive DRM technology some players are unable to play the non-standard disc.

Since optical computer CD/DVD drives are also capable of reading Red Book audio CDs, a lot of player manufacturers use these same CD/DVD ROM drives instead of specialized Audio CD / Video DVD only drives in order to reduce costs.

DVD players and car stereo systems seem to have most problems with incompatibilities of key2Audio protected CDs.

A protected CD still achieves a maximum recording time of 77 minutes and supports full ISRC, UPC and CD-Text capabilities.

Early versions of this copy protection were infamous for being easily circumvented with a marker pen, by drawing a circle on the rim of the visible edge of audio data.[1]

It appears that key2Audio protected disc contains three sessions written on the disc. Two of them contain small data tracks, and one contains the audio tracks. The regular CD players pick the audio tracks outright, while the computer CD and DVD players get confused by the corrupted TOC information. CD players and drives that do not support multisession discs are immune against this confusion.

...
Wikipedia