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The Compact Disc

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Informattion Technology Coursework The Compact Disc Introduction The concept of the CD is based on the well-known 31/2-inch floppy disk. It is just a more complex way of storing information on a mobile piece of plastic. Today the CD has branched into all areas of modern life. Compact discs are used in music, videos, computer games and software. In this project I hope to uncover how the CD works including the mechanisms of CD drives and focus on the use of the laser in the system, all from a physicist's perspective. I also wish explain the practical benefits of the CD and why it has become so widely distributed in such a short time, and just how it has done so much for us. There seems to be a bright future for this format of data storage. It does not look as though it may die out with the LP and cassette, especially with crystal quality sound, more advance 'readers' and 'players', home recordable CD's and DVD's it looks as though there's no stopping this 12 centimetres of plastic. But with MP3 and Mini discs on the horizon, the CD may not be able to do as much for us as we thought. About the CD James T Russell gave birth to the concept of information reproduced without physical contact in the late 1960's. After many years of experimenting the first compact discs were tested successfully in the late 1970's. However they were not very similar in appearance to the CD's we are familiar with today. ...read more.


When they interfere on the returning journeys the two rays in effect cancel each other out, as there path difference will be precisely half a wavelength. In the same way waves can emphasize each other, as well as cancel each other out. In the diagram to the right you will see that in the second circumstance at the bottom, the waves return 'in-step' or 'in phase' with each other, (i.e. no path difference) combining to make a larger amplitude wave, with the same wavelength that is reflected. The first situation is what is known as destructive interference, and the lower is called constructive interference. In the system this constructive interference will occur when they two rays reflect off of the same surface level, either a bump or 'land'. Thus creating the perfect means for identifying between the two heights. There for, the basis for reading the disc all lies in light intensity. As I said this all relies on the height of the bump on the disc, which is one quarter of the wavelength of the laser. However, the laser passes through the polycarbonate plastic before it meets the information layer of pits, during which distance the wavelength will alter, as it will be slowed down due to the change in medium. To find out the new wavelength of the laser beam we must know the refractive index of the plastic. This can be done by using Snel's law. As you can see from the diagram on the left the angle of incidence (i) is 27� and the resultant angle is 17�. Snel's law states that (refractive index) ? = sin(i) / sin(r) ...read more.


Hardware Installations ? Computer Console Gaming This vast usage of the CD can be based on its production costs. Compared to a book the cost per 'bit' of information is considerably cheaper. Also, the CD's size is another reason for its success. Whole libraries can be dramatically reduced in size, if their books are replaced with compact discs as the average book contains in computer terms 3kbytes of data per page compared to the CD's 750Mbytes of information space. That's equivalent to approximately 250'000 book pages! However the CD is still being improved upon. DVD's (Digital Video/Versatile Discs) with are the same size as CD's are being transformed to contain up to nine hours of impeccable quality visual and audio information, by enhancing by MPEG2 format. CD's have also been squashed down into the latest in personal music technology called the minidisc. As it less than half the size of a standard CD, yet with the same capacity it is more fragile and is encased in another layer of plastic that makes it similar in appearance to the 31/2inch floppy disc. The minidisc has the advantage of the incredible ease of recording and re-recording music from other music sources and CD's, plus the benefit of even more compact size. It seems the CD has given birth to a wide range of inventions that seize the advantage of non contact reading, that means the products are much more durable. The CD has become one of the most inspiring and productive pieces of modern technology that we have ever known and it will continue to evolve and grow in our world filled with information. ...read more.

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