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. 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.Today, CD-ROMs are standardized and will work in any standard CD-ROM drive.
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, namely CD-ROM Mode 1 and 2.
CD-ROM Mode 1
CD-ROM Mode 1 is the mode used for CD-ROMs that carry data and applications only .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.
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.
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. 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.
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