Chapter 10: CD-ROMs and DVDs^205
CD-ROM formats. On CD-ROM Mode 1 discs, the CIRC method used is calledLayered
EDC/ECC (error-detection code/error-correcting code), which determines if an error has oc-
curred in a data block and corrects it. This error detection and correction method requires
the use of additional bytes at the end of the sector. EDC uses 4 bytes, ECC uses 276 bytes,
and between them are 8 bytes of unused space.
A Mode 1 sector provides 2,048 bytes of user data. This area can be divided into
blocks of 512, 1024, and 2048 bytes each, but a CD-ROM typically has the same block
length throughout. A block cannot be bigger than a sector, which is also the smallest
addressable unit on the CD-ROM. CD-ROM Mode 2 sectors do not use additional error
detection and correction, which leaves the bytes behind the sync and header bytes
(2,336 bytes) as user bytes.
CD-ROM Extended Architecture (CD-ROM XA)
TheRedBookCD-DAandtheYellowBookCD-ROMformatssoonprovedtoorestricting
to producers, so Philips, Sony, and the Microsoft Corporation combined to develop the
CD-ROM Extended Architecture, or CD-ROM XA format. The CD-ROM XA format is an
extension of the Yellow Book format standard.
CD-ROM XA discs can mix CD-ROM Mode 1 and Mode 2 formats to store computer
data, compressed audio, graphics, and video content. CD-ROM XA does not use addi-
tional EDC/ECC capabilities, so the user gains the 288 bytes used in CD-ROM Mode 1
formats for this purpose. This formatinterleaves, meaning that it mixes different types of
data together in different mode formats on the same CD, allowing music, data, program-
ming, and graphics to share a single CD.
CD-ROM XA discs require a drive certified for the CD-ROM XA format. Because they
usually contain compressed audio and video, these devices include hardware decoders
that decompress the data as it is read.
CD-Interactive (CD-I)
In 1986, the demand and rapid growth of multimedia were the catalysts leading to the
creation of the CD-Interactive or CD-I format. CD-I discs contained text, graphics, audio,
and video on a single disc format. Special hardware was used to connect CD-I players to
television screens for output. CD-I, like the CD-ROM XA, is a derivative of the Yellow
Book, but the CD-I used a proprietary and unique formatting.
Bridge CD
The term bridge CD refers to discs that support extensions of the CD-ROM XA format,
defined in what is known as the White Book. These discs are called bridge CDs because
they bridge the CD-ROM XA and the CD-I formats and can be used for either. Using the
White Book specification, CD-I discs will work in CD-ROM XA drives, and CD-ROM XA
discs will work in CD-I drives. Examples of a bridge CD is the Kodak Photo CD and the
Video CD format.