Didn’t it bring us the tears of joy? As history would have it the CD was the combined work of two great warriors of the tech industry. One was Philips and the other was Sony. Each company had introduced a working model of an optical disc in separate dates but later collaborated to produce the master piece we have become so acquainted to.
The basic design of a CD is some what like the diagram below.
A. A polycarbonate disc layer has the data encoded by using bumps.B. A reflective layer reflects the laser back.
C. A lacquer layer is used to prevent oxidation
D. Artwork is screen printed on the top of the disc.
E. A laser beam reads the polycarbonate disc, is reflected back, and read by the player.
The polycarbonate layer contains bumps as a single, continuous, extremely long spiral track of data. The reflective layer usually aluminum helps in reflecting the laser back. The aluminum is protected by a layer of lacquer to avoid chances of oxidation. At last the lacquer is covered by a design. The CD spirals start at the center and spread outwards.
This when taken into consideration allows CD’s to be smaller than 4.8 inches. The track is made by elongated bumps 0.5 microns wide a minimum of 0.83 microns long and 125 nanometers high and also each track is separated from one another by 1.6 microns. The picture below is a microscopic view of CD bumps.
To read data in such tiny scale requires high degree of precision. Looks like a job for laser man. The drive has three basic jobs to perform , first of is the spinning of the CD which is carried out by the drive motor . The drive motor rotates the disc at 200 to 500 rpm. The second task at hand is that of the laser and a lens system which is to focus and read the data of those very tiny bumps. The third job rests with the tracking mechanism that rotates the laser to mimic the spiral track. The tracking system has to be able to move the laser at micron resolutions.
When a CD is being read, a laser is shown through the polycarbonate layer and reflected off of the reflective material. The reflected laser light is in turn detected by an optical sensor which converts the received laser signal into electricity. Depending on whether the laser was focused on a bump or not, the electrical signal will be different because the reflected laser light will be different. The difference in the electrical signals is how a computer can read data off of the CD. This is how the CD works.Till the next time stay tuned.
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By Tahsin Uddin Mullic
North South University, Dhaka, Bangladesh
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The Aftermath Publications, Issue 3
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