CD-ROM
CD-ROM (Compact Disc,
read-only-memory) is an adaptation of the CD
that is designed to store computer data in the form of text and graphics, as
well as hi-fi stereo sound. The original data format standard was defined by
Philips and Sony in the 1983 Yellow Book. Other standards are used in
conjunction with it to define directory and file structures, including ISO
9660, HFS (Hierarchal File System, for Macintosh computers), and
Hybrid HFS-ISO. Format of the CD-ROM is the same as for audio CDs: a standard CD
is 120 mm (4.75 inches) in diameter and 1.2 mm (0.05 inches) thick and is
composed of a polycarbonate plastic substrate (underlayer - this is the main
body of the disc), one or more thin reflective metal (usually aluminum) layers,
and a lacquer coating.
The
Yellow Book specifications were so general that there was some fear in the
industry that multiple incompatible and proprietary formats would be created. In
order to prevent such an occurrence, representatives from industry leaders met
at the High Sierra Hotel in Lake Tahoe to collaborate on a common standard.
Nicknamed the High Sierra Format, this version was later modified to
become ISO 9660. Today, CD-ROMs are standardized and will work in any standard
CD-ROM drive. CD-ROM drives can also read audio compact discs for music,
although CD players cannot read CD-ROM discs.
Although
the disc media and the drives of the CD and CD-ROM are, in principle, the same,
there is a difference in the way data storage is organized. Two new sectors were
defined, Mode 1 for storing computer data and Mode 2 for compressed audio or
video/graphic data.
CD-ROM
Mode 1 is the mode used for CD-ROMs that carry data and applications only. In
order to access the thousands of data files that may be present on this type of
CD, precise addressing is necessary. Data is laid out in nearly the same way as
it is on audio disks: data is stored in sectors (the smallest separately
addressable block of information), which each hold 2,352 bytes of data, with an
additional number of bytes used for error detection and correction, as well as
control structures. For mode 1 CD-ROM data storage, the sectors are further
broken down, and 2,048 used for the expected data, while the other 304 bytes are
devoted to extra error detection and correction code, because CD-ROMs are not as
fault tolerant as audio CDs. There are 75 sectors per second on the disk, which
yields a disc capacity of 681,984,000 bytes (650MB) and a single speed transfer
rate of 150 KBps, with higher rates for faster CD-ROM drives. Drive speed is
expressed as multiples of the single speed transfer rate, as 2X, 4X, 6X, and so
on. Most drives support CD-ROM
XA (Extended Architecture) and Photo-CD (including multiple session
discs).
CD-ROM
Mode 2 is used for compressed audio/video information and uses only two layers
of error detection and correction, the same as the CD-DA. Therefore, all 2,336
bytes of data behind the sync and header bytes are for user data. Although the
sectors of CD-DA, CD-ROM Mode 1 and Mode 2 are the same size, the amount of data
that can be stored varies considerably because of the use of sync and header
bytes, error correction and detection. The Mode 2 format offers a flexible
method for storing graphics and video. It allows different kinds of data to be
mixed together, and became the basis for CD-ROM XA. Mode 2 can be read by normal
CD-ROM drives, in conjunction with the appropriate drivers.
The
CD-ROM, like other CD adaptations, has data encoded in a spiral track beginning
at the center and ending at the outermost edge of the disc. The spiral track
holds approximately 650 MB of data. That's about 5.5 billion bits. The distance
between two rows of pits, measured from the center of one track to the center of
the next track is referred to as track pitch. The track pitch can range from 1.5
to 1.7 microns, but in most cases is 1.6 microns.
Constant
Linear Velocity (CLV) is the principle by which data is read from a CD-ROM.
This principal states that the read head must interact with the data track at a
constant rate, whether it is accessing data from the inner or outermost portions
of the disc. This is affected by varying the rotation speed of the disc, from
500 rpm at the center, to 200 rpm at the outside. In a music CD, data is read
sequentially, so rotation speed is not an issue. The CD-ROM, on the other hand,
must read in random patterns, which necessitates constantly shifting rotation
speeds. Pauses in the read function are audible, and some of the faster drives
can be quite noisy because of it.