CS 121 Lecture Notes - Lecture 39: Hard Disk Drive, Constant Linear Velocity, Photodetector
28 Jul 2020
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Document Summary
Reading a cd consists of converting lands and pits to digital information (zeros and ones). The fundamental element for reading a cd is a low power laser, which emits infrared radiation and is focused towards the bottom of the cd. The light passes through the polycarbonate layer and hits the aluminium layer. If the beam hits a hole (pit), the percentage of reflected light is very small. On the other hand, if the beam hits a flat area (land), a large percentage of the light is reflected. The reflected light radiation is directed towards a photodetector which, depending on the intensity of the received light, can easily detect whether a land or a pit has been focused. The transformation of lands and pits into digital values does not follow a direct correspondence. In other words, a land does not mean a digital value "0", and a pit does not mean a digital value.
