CHAPTER 13

714 Chapter 13 Optical Storage
The fundamental problem with nonstandard mixed-mode CDs is that if or when an audio player tries
to play the data track, the result is static that could conceivably damage speakers and possibly hearing
if the volume level has been turned up. Various manufacturers originally addressed this problem in
different ways, resulting in a number of confusing methods for creating these types of discs, some of
which still allowed the data tracks to be accidentally “played” on an audio player.
In 1995, Philips and Sony developed the CD EXTRA specification, as defined in the Blue Book standard.
CDs conforming to this specification usually are refered to as CD EXTRA (formerly called CD
Plus or CD Enhanced Music) discs and use the multisession technology defined in the CD-ROM XA
standard to separate the audio and data tracks. These are a form of stamped multisession disc. The
audio portion of the disc can consist of up to 98 standard Red Book audio tracks, whereas the data
track typically is composed of XA Mode 2 sectors and can contain video, song lyrics, still images, or
other multimedia content. Such discs can be identified by the CD EXTRA logo, which is the standard
CD-DA logo with a plus sign to the right. Often the logo or markings on the disc package are overlooked
or somewhat obscure, and you might not know that an audio CD contains this extra data
until you play it in a CD-ROM drive.
A CD EXTRA disc normally contains two sessions. Because audio CD players are only single-session
capable, they play only the audio session and ignore the additional session containing the data. A
CD-ROM drive in a PC, however, can see both sessions on the disc and access both the audio and data
tracks.
Note
Many artists have released audio CDs in the CD EXTRA format that include things such as lyrics, video, artist bio, photos,
and so on in data files on the disc. Tidal by Fiona Apple (released in 1996) was one of the first CD EXTRA discs from
Sony Music. There have been many CD EXTRA releases since then. For examples of other CD EXTRA discs, including current
releases, see http://www.cdextra.com.
CD-ROM File Systems
Manufacturers of early CD-ROM discs required their own custom software to read the discs. This is
because the Yellow Book specification for CD-ROM details only how data sectors—rather than audio
sectors—can be stored on a disc, and did not cover the file systems or deal with how data should be
stored in files and how these should be formatted for use by PCs with different operating systems.
Obviously, noninterchangeable file formats presented an obstacle to the industrywide compatiblity for
CD-ROM applications.
In 1985–1986, several companies got together and published the High Sierra file format specification,
which finally enabled CD-ROMs for PCs to be universally readable. That was the first industrystandard
CD-ROM file system that made CD-ROMs universally usable in PCs. Today several file
systems are used on CDs, including
■ High Sierra
■ ISO 9660 (based on High Sierra)
■ Joliet
■ UDF (Universal Disk Format)
■ Mac HFS (Hierarchical File Format)
■ Rock Ridge
Not all CD file system formats can be read by all operating systems. Table 13.17 shows the primary
file systems used and which operating systems support them.