The Compact Disc.

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The Compact Disc

   The compact disc if by far one of the most revolutionary mediums to have been invented in recent times. It has completely changed society in a numbers of ways and has benefited everyone. In this coursework I aim to discover how a CD is produced, written on and how it is read inside a CD-ROM. I will also be looking at the difference between a CD-R and a CD-RW. The average CD-R or RW can hold about 6 billion bits of binary data. This is about 780 megabytes of data, and at 2000 characters per page an average CD can store up to 275,000 pages of text. A CD can also hold about 74 minutes of audible music as it samples at 44.1kHz.

    The first aspect I chose to look at was how a CD is produced at a manufacturing plant and what materials and components go into making a CD.

   A CD is made up of a number of components. Its base material is polycarbonate; it is what makes the CD strong and provides a surface for other layers to be applied to. A reflective layer is then applied to the surface of the polycarbonate using a process called sputtering. This is a shiny layer that is used to bounce the laser beam back to its original source. This means that this layer must have a very high integrity so that it remains in shape and not break apart. This reflective layer is usually made up of silver, but on occasions it is made up of gold or platinum. With a CD-R and CD-RW there is an extra photosensitive layer, this layer is very sensitive to different frequencies. This allows for a CD recorder to imprint the bumps and pits onto the CD. Each pit is approximately 0.5 microns wide and 0.83 microns to 3.56 microns long. After this process a clear lacquer coating is applied to prevent oxidation and to seal in the reflective layer. This thin layer gives no protection form scratches or any other harm that might come to the CD.

The picture above shows the layers that are involved in producing a CD. A complete CD is normally 1.2mm thick. This is very thin as there are six or seven layers that make up the CD.

   

After discovering what a CD was made from, I then wanted to look at how a CD is read in a CD-ROM drive. This was very interesting as there are a number of things that have to happen in order for the CD-ROM to read the CD.

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The model below shows how the CD-ROM scans a CD.

   The laser beam that is sent out from the laser inside the CD-ROM travels through a grating. This diffraction grating separates the colours of incident light. This is done to generate tracking beams, which can be sent through a prism and up onto the CD.

 

   The polarizing prism in a CD is known as a Wollaston Prism. The Wollaston prism is made up from two right triangle prisms with perpendicular optic axis. Two beams enter this prism known as the O-ray ...

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