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Motor spins CD at a variable rate so that the portion under the detector
always moves at the same speed. (Inner tacks rotation slower.)
Laser projects concentrated light beam at CD
Laser penetrates a protective plastic layer and strikes a shiny
reflective layer on the bottom of the disk.
Light is reflected from a land and sent via a prism to a light sensing
diode, or scattered when it strikes a pit (tiny depression).
Each pulse striking the light sensing diode generates an electric
voltage, which are matched to a timing circuit to produce the binary code.
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