Enhanced CD is an umbrella term that covers several different types of disc formats that all do basically the same thing: they allow a CD that contains both audio and computer data to play on either a CD audio player or a CD-ROM drive. They are included here under their own heading, because each of them uses a combination of at least two other colored books, and because this type of disc, in its many forms, could become the replacement for the familiar Red Book audio disc.
Most CD Audio discs are not full; most of them contain only about 60 minutes of music, leaving about 14 minutes worth of space at the outer edge of the disc empty. In the early days of CD manufacturing, this space was the hardest part of the disc to mold reliably, but advances in manufacturing have largely eliminated this problem. Meanwhile, producers of audio CDs and their recording artists were increasingly looking for a way to “enhance” their musical offerings by including other types of data: liner notes, video clips, graphics, photographs, artist bios. For consumers with ordinary CD audio players, this information should not interfere with the normal performance of a CD audio disc; but the ever-increasing number of consumers with both a computer and CD-ROM drive and a CD audio player could view the data on their computers. The format choices for such a disc, until recently, were two: Mixed Mode or CD-i Ready.
Mixed Mode discs consist of a track of CD-ROM (Yellow Book) data followed by up to 98 tracks of CD Audio (Red Book) data. (This something of a misnomer, because Red Book audio is not precisely a Mode - although it is sometimes mistakenly referred to as Mode 0.) By definition, the format requires that the CD-ROM data be in the first track only. If one of these discs were played on a CD audio player, the first, data track would produce effects ranging from silence to full-volume static, depending on the brand of CD audio player used. These types of discs were used for some early multimedia applications. The development of sound cards enabled multimedia CD-ROM developers to include sound files in the same track as the rest of the application, eliminating the need to place audio data in a separate track. There were some audio discs recorded and marketed in this form, but they required an extra label, and sometimes music stores displayed them in a separate section, to avoid the complaints of customers who might get an earful of static when they tried to play track one.
A CDI ready disc is a standard audio disc with some extra features. The discs play CD audio on a normal CD audio player, but you can only access the special features by using a CD-i player. The Red Book specification defines index points in the subcode of each track that instruct the CD audio player to skip to specific points in the track. Usually, the track only uses index points 0 and 1. Index 0 marks the beginning of a track; index 1 marks the beginning of the music in a track. Between these points are two or three seconds of silence about 12 blank sectors. On live albums, the sound may be continuous, but the index points are still there to allow the CD Audio player to locate tracks. An audio CD player skips the first two-second gap between index 0 and index 1 at the beginning of the disc and starts playing the music on the first track. It plays the remaining gaps one for each track (song) on the disc between each track. You can extend the length of this gap to at least three minutes (182 seconds) and place CD-i data in this area. The data could include lyrics, liner notes, biographical information about the performer, or graphics. Although a CD audio player plays only the music, a CD-i player also reads the CD-i Ready disc's information in the pregap and displays it on screen. The player displays this information in one of two ways. The information pertinent to each track might be loaded into the CD-i player's memory before the audio starts to play. The information then appears while the music plays from the disc. Lyrics or graphics related to the song, for example, might appear on the screen. Alternatively, the disc also can be read in the standard CD-i method, playing interleaved audio and visual information from the data in the pregap. An example of this method might include audio and video data of an interview with the performer.
Hidden Track or CD-ROM Ready
There are several companies currently offering their own flavor of what is in essence a CD-i Ready disc for CD-ROM. Hidden Track, Track Zero, i-trax, and Audio Vision discs, to name a few, are essentially audio discs with “hidden” data in an extended pregap area. The extended pregap could be as long as 30 or 40 minutes, or longer. Many hidden track discs are shared hybrid CD-ROM applications that run on Mac or Windows platforms, with the third element of Red Book audio. The disc plays exactly like a regular CD audio disc on an audio player. On an MPC 2-compliant system, one has access to liner notes, Quicktime video, and graphics, as well as to the audio tracks themselves.
The advantages to “CD-ROM Ready” are many. The data tracks enjoy prime locations near the hub of the disc, ensuring quick access and smooth playback of video and audio sequences. These types of discs will play on most CD Audio players and both multisession and non-multisession CD-ROM drives, using existing drivers. They can be mass produced using existing systems. The disadvantages are that some CD Audio players may try to play the disc starting at index point 0 of the first track, and some drives may “back up” into this area, producing the dreaded full-volume static. Some CD Audio disc producers feel that this method, because it exploits a non-defined “loophole” in the Red Book specification, does not preserve the “purity” of the Red Book standard.
Stamped Multisession, CD Plus, or “Blue Book”
The Orange Book specification for CD-R allows a CD-R disc to be recorded in more than one session, rather than “disc-at-once," or as a single volume. The multiple sessions and tracks on the CD-Recordable disc can be recorded as entirely separate volumes, or so that each separate session contained “pointers” to link the information in earlier sessions to later sessions. Depending on the recording method, the resulting disc could appear as a logical whole, or as discrete sessions. These discrete or linked sessions could be Yellow Book or Red Book, in any order. Thus, it is possible to create a disc that places audio data in the first track of the first session of the disc, where it will be recognized and played by CD Audio players, and CD-ROM data in the second session of the disc, where it will only accessible by CD-ROM drives controlled by personal computers.
In May of 1995, Philips announced a specification to define “stamped multisession” discs. A subset of that specification, designed expressly for stamped multisession discs limited to two sessions (one music, one data), has been released as a new color book (Blue Book) and called CD Plus. As obvious and elementary as it may seem to use multisession for enhanced audio applications, there are some stumbling blocks. First and foremost is the fact that replicators of CDs are not yet equipped to mass-produce “stamped multisession” discs with any kind of reliability. The software used to create glass masters for the various disc formats -- Red Book, Yellow Book, etc. -- does not as yet include support for multisession. Equipment that verifies masters and performs quality testing for finished discs is not set up to verify stamped multisession discs.
The advantage to CD Plus is that it is a single, defined, licensed standard supported by Philips, Sony, Microsoft, and Apple. It will play on CD Audio players with no possibility of producing static, and on many newer computers with CD-ROM drives. The disadvantages are that there are many existing CD-ROM drives -- approximately 40% of the installed base -- that are not capable of playing multisession discs. Even existing multisession drives may require a driver upgrade in order to use these discs. CD-ROM data is stored at the outer edge of the disc -- not prime real estate for fast, error-free playback.