CD

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These days, CDs or compact discs are just about everywhere. Conventionally, they are used as repositories of music, data or computer software, and have become the means for distributing large amounts of information in a well packaged form. They aren’t just easy to use or produce but easy to create too, if you have a computer and a CD-writer drive. It can store 74 minutes of music.

You will find that a CD is a very simple device made of plastic, about 1.2 mm thick. A CD is predominately an injection-molded piece of clear polycarbonate plastic. Upon this layer of plastic is placed a thin protective layer of reflective aluminum, to cover the bumps there. On the acrylic is printed the label.

A CD is composed of one spiral track of data that circles from the inside of the disc to the outside of the disc. Since the spiral track begins at the center, it stands to reason that the CD can be smaller than 12 cm. The track is made up of elongated bumps, each 0.5 microns wide, a minimum of 0.83 microns long and 125 nanometers high. The dimensions of the bumps are so very small that they make the spiral track look very, very long.

It is the job of the CD player to find and read the data stored in the form of bumps on the CD. We have already mentioned that these bumps are incredibly small. Considering this, the CD player is very precise gadgetry. Its drive consists of a precisely-controlled drive motor to spin the disc, a laser and a lens system focus to read the bumps, and a tracking mechanism that moves the laser assembly so that the latter follows the spiral track. This system should move the laser at micron resolutions.

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