Multi & Mixed
Mixing it up
The new High Sierra format called for data identifying features, both audio and data could be encoded on the same CD since the VTOC could clearly tell the drive the type and location of data on the CD. Early CD-ROM games took advantage of this capability to put standard Redbook audio on the same disc as the software. The standard audio tracks could even be played back in a regular CD player, but you had to be careful to not attempt to play the data track.
In the new High Sierra format, all of the data has to be recorded to the disc at one time (in a single session). You could not go back and add more information to an existing layout; you had to create an entirely new disc layout. As a cost cutting measure for manufacturing, the CD-ROM XA (Extended Architecture) was created to add the ability to record information into a CD format in multiple sessions. This is done by creating a second, or additional table of contents on the disc that the player could use to recognize the location of all of the data.
CD-ROM drives that are not multi-session compliant will only recognize the original data that is on the disc, and will not be aware that any other data was added later on. A CD-XA drive should have no problem locating the multiple VTOCs created from multiple copy sessions.
CD-XA also introduced "data interleaving." Most graphical data, such as movie files contain program and audio data. The new CD-XA allowed for "identification marks" that would allow the drive to determine what type of data it was about to read (graphics, audio, program, etc…) and handle it accordingly.
Data interleaving is what makes movie playback directly from compact disc possible. It allows the drive to handle the different audio and image information on the fly. In this manner, the audio that accompanies a specific frame of video would be reproduced in the correct manner: video, audio, video, audio. To assure correct audio/video synchronization without data interleaving, the entire audio and video tracks would need to be buffered to main memory before being played back.
Improvements in the control hardware used to read the data and present it to the computer has reduced the amount of control data required. This opened up more of the disc capacity for storage of user information. True CD-XA compatibility requires that a special audio compression called "Adaptive Differential Pulse Code Modulation", or ADPCM be used. CD-ROM drives require a special decoding circuit to correctly interpret data stored in this form. As of right now, all drives should be fully CD-XA compatible , however, this was not entirely true of some low cost manufacturers back when CD-XA first appeared since many omitted the ADPCM decoder from their designs. It's still possible that certain generic brand drives produced today might have trouble reading certain disc formats as well, due to cost cutting factors in manufacturing.