Having a thickness of a mere 1.2 millimeters and a diameter of 12 centimeters, a CD weighs only 28 grams but has the capacity to store 74 minutes of music recording or 783 megabytes data on its 5 kilometers long spiral track. Compact disc mainly comprises of three layers. The topmost layer is made of acrylic, followed by aluminum, and the bottom layer of polycarbonate. To record data on the CD, pits and bumps are created on it using a laser beam. The pit or bump is the 0 in digital language and the flat area, without the indentation is called land, which reads 1 in the computer language.
When the CD is placed in the CD drive, the pits and lands are read and decoded using the computer processes. This bottom layer is transparent, passing through which a laser beam ‘reads’ 0.5-micron broad pits recorded on the aluminum layer. These pits, as said before, are indicative of binary digits 0s and 1s and each 0 and 1, in turn, is indicative of the connected data (viz. audio, video). Various combinations of these digits are used to encode the electronic data digitally.
When a CD is inserted in the disc drive, the following sequence of events takes place: The laser beam strikes the aluminum layer of CD penetrating through the transparent polycarbonate layer and is reflected towards the photodetector situated beneath the CD. (This process takes place many times in a second.) While the CD is being read, it rotates with the help of an electric motor. When reading the outer edge, it rotates at 200 revolutions per minute and 500 when reading the inner edge. Since the pit is not flat, it does not reflect laser much whereas the flat surface reflects laser more.
The photodetector detects changes in the reflected beam and constantly passes on information to an electronic circuit. It is the job of this circuit to convert the digital data into electronic signals. The reflection of intense light is decoded by the cell as land or 1, while the reflection of dim light is understood as a pit or 0. These are processed and decoded to form sound signals that can be amplified for reproductions on loudspeakers and visual signals that can be relayed on TV.
The use of pits and bumps on the CD, also explains why scratches and cuts on the surface of the CD hamper the reading and display of information. Often you must have tried playing a CD with scratches, and it would show interrupted sound or data. This happens because the pits and lands are tiny, and a single small scratch can disrupt many of them at once. The laser beam cannot be reflected, and the data is erased from that part.
CDs were initially used only for sound recordings. With the advancement in technology, pictures and videos also came to be recorded on CDs. The experts have revolutionized the field of data storage by applying this simple principle in DVDs in addition to the CD. In a way, the credit for this revolution belongs to James T. Russell, who developed the first workable digital CD device in the late 1960s.