Technology And Physics Behind The Compact Disc

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Good morning students and teachers, today I will be talking to you about the technology and physics behind the Compact disc otherwise known as the CD
Everybody knows that the CD is slowly fading out and being replaced by a digital world where files and streams are the main methods of music delivery. But the CD is not yet done.
What is a CD? A CD is a thin circular disc comprised of metal and plastic, the CD is about 12cm in diameter and is usually made up of 3 layers, it is capable of storing up to 74 minutes of audio but usually only has 50 minutes stored on it to make sure it is at optimal function, the digital data that can be stored on a CD is; 44 100 samples per second, therefore, you can store 682 MB on a CD that is capable of 74
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Lacquer is used rather than the polycarbonate due to the positioning of the metal film.
The metal layer of the CD is the layer that contains the data. CD data is represented as tiny indentations known as "pits”, these pits are encoded in a spiral track that if unrolled from it spiral would be around 5.5 km long, the spiral is moulded into the top of the polycarbonate layer, these are microscopic and usually expressed in the measurement of a micron µm which is 1 millionth of a meter. The areas between pits are known as "lands". The pits are even smaller than the lands and are approximately 100 nm deep by 500 µm wide, and vary from 850 µm to 3.5 µm in length. The measurement of the pit represents the binary information of the data stored on the disc.
The optical system employs a highly coherent light source and the pits are made approximately a quarter wavelength deep. The readout beam axis is nominally aligned to be perpendicular to the disc plane. When there are no pit-land edges in the spot, all of the reflected beam will share the same phase. The phase of the reflected beam will, however, change by 180 degrees when the spot moves from pit to land or vice versa. When the optical spot traverses a pit-land edge the magnitude of the beam reflected back into the sensor will momentarily drop almost to zero...The reflected beam then consists of two portions, equal in magnitude but opposite in phase.
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