COMPACT DISK AND DISC RECORDABLE

COMPACT DISK

CD Rom stand for Compact disc read only memory. These discs are cecoming popular now a days especially for use with microcomputers. The name implies to its application. Once inserted into the CD Rom driver, the text, video images and so on can be read into Ram for processing or display. However the data on the disc are fixed and cannot be altered this is in contrast to read/write capability of magnetic discs. CD Roms have tremendus storage capacity. The capacity of a single CD Rom is upto 680 Mbytes wquvalent to over 600 normal size books each of 500 pages or about 470 DS/HD diskettes. Magnetic disks store data in concentric tracks each of which is divided into sectors. The sectors on the inside tracks hold the same amount of information as those on the outside tracks. Although this wastes magnetic disk space, it optimized the speed at which data can be written and retrieved because a magnetic disk spins at a constant speed. In contras, CD Roms store data in a single track that spirals from the centre to the outside edge. The ultra thin track spirals around the disk thoudands of times. Data are recorded on the Roms reflective surface in the form of pits and lands. The pits or depressions are tiny reflective bumps that have been burned in with a laser beam. The pits are flat areas separating the pits. A land reflects the laser light into the sensor and a pit scatters the light. A spot that reflects the laser beam into the sensor is interpreted as a 1, and the absence of a reflection is interpreted as a 0. Together they record read only binary information that can be interpreted by the computer as text, audio, images and so on. Once the data have been recorded, a protective coating is applied to the reflective surface. The speed at which a given CD Rom spins depends upon the phsical location of the data being read. The data passes over the moveable laser detector at the same rate, no matter where the data are read. Therefore the CD Rom must spin more quickly when accessing data near the centre. Even though with the changing speed of the disk, reading data from an optical medium is a relatively simple matter, writing data is a different matter. The fact that you cannot write data to a CD Rom does not mean that this storage medium is not useful. Infact many applications rely on huge volumes of data that that rarely changes. For example, dictionaries, encyclopedias, medical, legal and other professional referance libraries, software packages, music and video all require termendous amount of data that you would not normally want to alter.

COMPACT DISC RECORDABLE

For large quantities, CD Rom disks can be produced by manufacturers with expensive duplication equipment. Nearly all the PCs have CR Rom drives. This rapid and universal acceptance of CD Rom has given rise to an exciting new technology CD R, compact disk recordable. A CD R disk is functionally equvalent to a pre recorded CD Rom. A locally developed CD R will paly in any CD Rom drive.