From a 8/25/04 press release:
DualDisc was officially launched today by a consortium of record labels including EMI Music, Sony BMG Music Entertainment, Universal Music Group, Warner Music Group and 5.1 Entertainment Group/Silverline Records.
DualDisc is a two-sided disc made up of a CD on one side and a DVD on the other. In addition to a full album on the CD side, the DVD side provides the full album in enhanced sound (such as surround sound and/or DVD-Audio or LPCM stereo), and also includes a wide range of special features such as music videos, interviews, photo galleries, web links, concert footage and lyrics.
Although DualDisc creates a new music experience, it does not require new equipment. DualDisc is compatible with nearly any device that can currently play a CD or a DVD-whether a home stereo system, portable disc player, car stereo, game console or PC.
More info is available at High Fidelity Review’s DualDisc – The Hybrid CD/DVD Disc Part 1 and Part 2, including the patent war that may be looming. I don’t understand how they can say that DualDisc requires no new equipment, but it may have DVD-Audio content; many DVD players don’t include the technology required to play DVD-A.