CHAPTER 13

Compact Disc and Drive Formats Chapter 13
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or 16 hours of mono sound can fit on one CD. The “differential” part of ADPCM refers to the fact
that it records the differences between one signal and the next (using only 4-bit numbers), which
reduces the total amount of data involved. ADPCM audio can be interleaved with video in CD-i (and
CD-ROM XA) applications.
The Yellow Book defines two CD-ROM sector structures, called Mode 1 and Mode 2. The Green Book
(CD-i) refines the Mode 2 sector definition by adding two forms, called Mode 2, Form 1 and Mode 2,
Form 2. The Mode 2, Form 1 sector definition uses ECC and allows for 2,048 bytes of data storage like
the Yellow Book Mode 1 sectors, but it rearranged things slightly to use the 8 formerly unused (blank
or 0) bytes as a subheader containing additional information about the sector. The Mode 2, Form 2
definition drops the ECC and allows 2,324 bytes for data. Without the ECC, only video or audio
information should be stored in Form 2 sectors because that type of information can tolerate minor
errors.
Media or titles available for CD-i include all manner of educational and training applications, games,
encyclopedias, karaoke, and movies. Note that CD-i discs can’t be “played” on PCs. In fact, because
the files are in an OS/9 file format, your PC won’t even be capable of seeing the files on the disc! Even
so, drivers have been written that can be installed and will enable viewing the files, and one enterprising
individual has even written a CD-i emulator called CD-iCE that emulates a CD-i player thus
enabling CD-i applications to be run. You can find out more about the CD-iCE emulator at
http://www.emuhq.com/cdi.
Today. the CD-i format is largely obsolete. The last revision of the standard was produced in May
1994. Philips sold off its entire consumer CD-i catalogue to Infogrames Multimedia in 1998, which
now owns the rights for virtually all consumer CD-i titles ever produced. Philips made a final run of
CD-i players in 1999, and it is doubtful any new ones will ever be produced. The legacy of CD-i lives
on in the other formats that use specifications originally devised for CD-i, such as the Mode 2, Form 1
and Form 2 sector structures later used in CD-XA and the MPEG-1 video format later used in the
White Book (CD-Video).
CD-ROM XA
CD-ROM XA originally was defined in 1989 by Philips, Sony, and Microsoft as a supplement to the
Yellow Book. CD-ROM XA brings some of the features originally defined in the Green Book (CD-i) to
the Yellow Book (CD-ROM) standard, especially for multimedia use. CD-ROM XA adds three main features
to the Yellow Book standard. The first consists of the CD-i–enhanced sector definitions (called
forms) for the Mode 2 sectors, and the second is a capability called interleaving (mixing audio and
video information). The third is ADPCM for compressed audio. The latest version of the CD-ROM XA
standard was released in May 1991.
Interleaving
CD-ROM XA drives can employ a technique known as interleaving. The specification calls for the
capability to encode on disc whether the data directly following an identification mark is graphics,
sound, or text. Graphics can include standard graphics pictures, animation, or full-motion video. In
addition, these blocks can be interleaved, or interspersed, with each other. For example, a frame of
video can start a track followed by a segment of audio, which would accompany the video, followed
by yet another frame of video. The drive picks up the audio and video sequentially, buffering the
information in memory and then sending it along to the PC for synchronization.
In short, the data is read off the disc in alternating pieces and then synchronized at playback so that
the result is a simultaneous presentation of the data. Without interleaving, the drive would have to
read and buffer the entire video track before it could read the audio track and synchronize the two for
playback.