Q. What is Compact Disk ROM?

Both audio CD and CD-ROM (compact disk read-only memory) share similar technology. Main difference is that CD-ROM players are more rugged and have error correction devices to ensure that data are appropriately transferred from disk to computer. Both types of disk are made the same way. Disk is created from a resin like polycarbonate. Digitally recorded information (either music or computer data) is imprinted as a sequence of microscopic pits on surface of polycarbonate. After that Pitted surface is coated with a highly reflective surface frequently aluminium. Shiny surface is protected against scratches and dust by a topcoat of clear acrylic. Lastly a label can be silk-screened on acrylic.

Figure: The CD Surface and Operation

Information is retrieved from a CD or CD-ROM by a low-powered laser housed in an optical-disk player or a drive unit. The laser shines via clear polycarbonate whereas a motor spins disk past it (Figure above). Intensity of reflected light of laser alters as it encounters a pit. Specially if laser beam falls on a pit light scatters and a low intensity is reflected back to source.  Areas between pits are termed as lands. A land is a flat surface that reflects back at a higher intensity. Change between lands and pits is detected by photo sensor and converted in a digital signal. Sensor tests surface at regular intervals. Beginning or end of a pit signifies a 1 when no change in elevation takes place between intervals then a 0 is recorded. Data on CD-ROM are arranged like sequence of blocks.