Disc for storing digital information, abuot 650mb or 12cm/4.5 in across, mainly used for music, when it can have over an hour's playing time.The compact disc is made of alumiuim with a tr anparent platic coating; the metal disc underneath is etched by a beam with microscopicpits that carry a digital code representing the sounds.During playback, alaser beam reads the code and prodces signal that arre changed into near-exact replcas of the original sounds.
CD-ROM, or Compact-DiscRead-Onlymemory, is used to store written text, pictures, and video clips in addition to music. The disc are ideal for large works, such as catalogues and encyclopedias. CDs,or Compact-Disc Interactive, is a form of CD-ROM used with a computerized reader, which responds intelligently to the user`s instructions. Recordable CDs ,called ('write once, read many times'), are used as computer disc, but are as yet too expensive for home for home use.Video CDs,on sale since 1994, store an hour of video. High-density video disc, publicly demonstrated 1995,can hold full-length features. Erasable CDs, which can be erased andrecorded many times, are also used by the comuter indstry. These are coated a compond of cobalt anb gadolinium,whih alters the polarization of light falling on it.In the, the reflected from the is passed through polarizig filters and the changes in polarization are converted into electrical signals.