Introduced in 1994, the idea behind enhanced CDs was to offer the music buyer some extra bonus features using space on the disc that was unfilled by the music. This might take the form of music videos (similar to the earlier idea of CD Video), interviews, wallpapers, pictures, lyrics, or links to an artists website.
Rather confusingly, there were three different ways on combining the different content on the disc. Prior to 1996, enhanced CDs were created in one of two ways. One was to have the multimedia content in the first track, but this meant that track 1 needed to be skipped when played in a standard CD audio player or else the multimedia track could cause an unpleasant noise to be heard. A subsequent solution was to have the multimedia content in the pre-gap before track 1 (rather like the way a CD-i Ready disc worked). Both of these methods were known as ‘mixed-mode’, combining Red Book Digital Audio and Yellow Book CD-ROM information in a single ‘session’ on the same disc.
To get around the problems of mixed-mode discs, Philips and Sony released the Blue Book standards in 1995, and worked with Apple, Microsoft and the Recording Industry Association of America (RIAA) to develop them. This new standard specified multisession technology that used two sessions on the disc, the first one containing audio tracks, and the second one containing data that is invisible to CD audio players. In 1996, most enhanced CDs used mixed-mode with the multimedia in the pre-gap, but by 1997 most were using Blue Book standards. The only downside to the newer multisession technology was that CD-ROM drives made before 1996 might not recognise them if they were not multisession capable.
Enhanced CDs can usually be recognised the official ‘Enhanced CD’ logo recommended by the RIAA. They may also have the CD EXTRA logo (previously known as CD Plus), which was trademarked by Sony for use on Blue Book discs.
Later technologies like DVD-Audio or DualDisc tried to perform the same function of offering the music album with bonus material such as videos, but later albums often came instead with the videos on a separate DVD-Video.