DualDisc was an optical disc format combining a layer similar to CD on one side and a DVD layer on the other. It was widely introduced in 2005, after development by a group of record companies.
The discs are slightly thicker than a standard CD, and comprise a DVD layer of 0.6mm and a CD layer of only 0.9mm (less than the 1.1mm called for by the Red Book CD Specifications). The thinness of the CD layer meant less chance of the discs getting stuck in slot-loading players, but meant some CD players could not read the CD content. DualDiscs were not allowed to carry the CD logo, and many carried warnings about not all players being able to play them.
The DVD layer conforms to DVD specifications, and DualDiscs were allowed to use the DVD logo.
Record companies hoped the enhanced content would increase sales, and the extra content could include music videos, surround sound versions of the album, and documentaries.
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