The Making Of A CD Disc
The CD disc is made up of polycarbonate plastic measuring 0.047 in thickness and weighing 16 – 20 grams. To make the CD disc reflective it will have a thin layer of aluminum on the bottom of the disc followed by a protective film of lacquer applied directly over the reflected layer of aluminum. The CD disc holds its data by using tiny indentations which are called pits and lands that are put into a spiral track on the disc. The CD is read by using an infrared laser that goes through the bottom of the disc and picks up the polycarbonate layer with the information on the disc.
By using the different heights of the pits and lands the laser will read the difference in the way the light is reflected allowing it to measure the change in intensity with a photodiode. The photodiode will read the data from the CD disc. The pits and lands on the disc are not like the binary data with zeros and ones. Inverted encoding is used instead which means the change from a land to pit or a pit to land will represent a one and no changes represent a series of zeros. This technique of encoding was created to be used for digital audio CD’s and is now the standard recording format for all CD’s and CD-Rom’s. The CD disc is used for many applications and can be duplicated from one to one thousand or more copies. Many musicians use the CD duplication process since it is a cost effective way to distribute their music.