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Monday, April 16, 2001
On Hardware

Store data but scratch-me-not
By Jasjot Singh Narula

PROBABLY perpetual greed in the human mind was the inspiration that led to the invention of CD-ROM (Compact Disk- Read Only Memory) — the greed to have more storage capacity. Today compact disks (CDs) have become an integral part of computer users’ life, spurred on further by low cost storage for distributing large quantities of information, i.e. computer programs, graphics, and databases in reliable packages.

A CD is a small plastic disk with a metallic surface on which information, especially high quality sound, is recorded whereas the CD-ROM is a non-volatile optical data storage medium using the same physical format as audio CD, readable by a computer CD-ROM drives.

CD-ROMs are digital disks from which data can be read by a special laser. The laser reads binary data as 0s and 1s by distinguishing between microscopic "lands" and "pits" dots on the surface of the disk.

CDs are easily available in different formats like audio/video or computer CD and are widely accepted because they are cheaper and efficient. A major drawback is they are "read only" means once the data is encoded onto a CD it cannot be changed.


The CD was a brainchild of James.T.Russell, a Washington-born scientist. He was a keen music lover and aware of the poor sound quality of vinyl phonograph records. From there he got an idea to develop a system that would record and replay sound without physical contact between hardware. After some years of struggle he finally managed to invent the first digital-to-optical recording playback system and also found a way to record onto a photosensitive platter in tiny bits of light and dark.

Through ’70s, Russell continued to refine CD-ROM. The computer industry adopted this medium as a source of information storage in mid-80s.

Storing data on CD

A CD is a piece of round-shape plastic made from polycarbonate layer with a diameter of 12 cm with 1.2 mm in thickness.

Mass production of CDs is a complicated task. A master CD is used to form a layer of polycarbonate on to the other CDs, which are stamped with the "pits" and "lands" on the spiral track. Once a clear piece of polycarbonate is formed, a thin reflective aluminium layer (which is what makes the disk shine) is spitted onto the disk, covering the pits. Then a thin acrylic layer is sprayed over the aluminium to protect it.

Initially the compact disk is totally flat. The data stored on a CD is in the form of 1s and 0s, where 1s are the "pits" that represent the portion burnt by the laser onto the master CD and 0s represents "land" or flat aluminium surface. Once the laser-beam passes through the polycarbonate layer, it reflects light from the aluminium layer (the ‘pits’ reflect light differently than the ‘land’) and hits optoelectronic device that detect the change in light or change in reflectivity.

A CD is a digital storage medium and as compared to a hard disk or a floppy drive, they are enduring but improper care can destroy them. So always use a CD-cover or a jewel case to sheath them.