CDROM is also called optical disk. An optical Disk storage system consist of rotating disk, which is coated with thin metal or other material that is highly reflective. Foccusing a laser beam or the surface of the spining disk which does data recording. The laser bam is turned on and off at a varying rate because of which tiny holes or pits are burnt into the metal burnt coating of the the disk alongthe tracks. In order to read the stored dataa less powerful laser beam is focussed on the surface. This beam isstrongly reflcted by the coated surface and weakly reflected by the pits producing pattern of on/off reflection that can be converted into electronic signals.
The storage density of optical disks is enormous, the storage cost is extermly low and the access time is relatively fast. one small inexpensive disk will able to replace 25 to 30 reels of magnetic tapes and any data on the disk can be accessed within a few milliseconds. Hence it is particularly suitable forb the archival storage of vast amount of data. Typical application are image processing, geological survey data, medical publishing indexes, historicalinformation files, etc. A serious short coming of current aviable optical disk system is thatd they are permananent storage device. Data once recorded cannot be erased and hence cannot be reused. The diameter of CDROM was 12 cm, weight 18gm and 1mm thick which has reduced to 51/4" in diameter. CDROMs are popular to store sound motion , video pictures , three dimensional pictures, graphics etc. New modification of CDs are WORM(write once read many), DVD(digital verstiale disc), RW(Read and write).
ADVANTAGE
- It is mass storage media for sound motion video, graphics, etc
- It is cheap storage media.
- The data can be stored for long time(20years)
- easy to exchange in a CD drive.
- It is reliable and economic media.
DISADVANTAGE
- Data access time is more in CDROM than hard disk
- ONce data is stored, they can't be changed.
- Only final refined data are stored in the CDROM.
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