In 1994, the big issue for enhanced CD developers was where to place the interactive data or Yellow Book on the disc. Several developers had put the multimedia on the front, or first track, of the enhanced audio CD. These discs were generally called mixed-mode enhanced CDs. Mixed-mode reflects the fact that a CD Red Book audio track and CD-ROM Yellow Book track share space on the same disc. The Freedom Sessions from Canadian singer Sarah McLachlan is an example.

The technology to develop mixed-mode discs was not ideal. A mixed-mode CD required you to manually skip over the first track of interactive data when playing the disc in a regular audio CD player, otherwise you'd get no sound at all or, worse yet, a nasty noise which may of harmed your speakers. The disc only started to act like a regular audio CD from the second track on. Mixed-mode enhanced CDs were commonly stickered with an alert like SKIP TRACK ONE OR ELSE. You shouldn't see any mixed-mode discs dated beyond 1995. But kudos to early enhanced CD developers, until 1995-1996, it was the only way you could get large amounts of audio and CD-ROM data on the same disc.

Next came the "PRE-GAP SOLUTION"

In 1995-1996 developers were able to place the CD-ROM portion of the enhanced disc in the pre-gap area just before the first track of audio. With this method you did not have to manually skip the first track when playing the disc in your CD player, because the interactive data in the pre-gap area is generally unnoticed by the audio CD player.

Yet there were drawbacks. You can back into the CD-ROM data on the disc if you press rewind long enough on your CD player, which will get you to that nasty noise again (although before you get to the noise, you're likely to hear a record-ed message telling you to stop). Additionally, most older CD players won't play pre-gap discs and only a few developers were able to create pre-gap discs. So, to create the discs, you needed to develop your own solution or make arrangements with a developer who had proprietary pre-gap technology.

In 1996 Apple Computer, Microsoft, Sony and Philips decided there needed to be a standard so that consumers would be able to play their ECDs on their CD-ROM drives without problems (hopefully...)!

Now there is "MULTISESSION SOLUTION/BLUEBOOK"

You've may of heard of another form of enhanced CD, CD Plus. CD Plus was believed to be almost 100 percent compatible with existing audio CD players, something pre-gap and track zero (mixed-mode) discs were not able to do. CD Plus which is now called CD-Extra is favored by record labels and artists who prefer a solution which doesn't require you to make any adjustments when playing the disc in your audio CD player. You play the CD Plus or Enhanced CD the same way you play a regular audio CD with exactly the same result. And you don't need the loud warning sticker plastered on the front of your favorite CD! But do look for the Enhanced CD logo...you don't want to miss the multimedia that has been added to your CD!

Technically speaking, CD Plus uses Stamped Multisession technology, which allows two separate sessions to be placed, or pressed, on the same disc. The first session for audio, the second session for ROM data . The audio CD player then automatically reads the first portion of the compact disc that is regular audio. The audio CD player never gets to the interactive data (which now follows the audio session), virtually eliminating the chance for CD Plus discs to play improperly in a regular audio CD player.