The 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 a 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.
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 a 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.
CD-ROM Data Storage
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
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
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.
Data Encoding and Reading
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.
Bibliography
Year of publication: 2005
Page title: What is CD-ROM? - Definition from WhatIs.com
Website Name: WhatIs.com
Publisher: Margaret Rouse
URL: http://whatis.techtarget.com/definition/CD-ROM
Access date: November 12, 2016
No hay comentarios.:
Publicar un comentario