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How A Compact Disc Works

This tutorial explores how a laser beam is focused onto the surface of a spinning compact disc, and how variations between pits and lands on the disc surface affect how light is either scattered by the disc surface or reflected back into a detector.

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The compact disc, although appearing to be stationary in the tutorial, should actually be envisioned as rotating at high speed. A beam is emitted by the laser and directed onto a single track on the disc by the prism/beamsplitter. As the disc rotates, the beam encounters a series of pits and landings that determine whether the beam is reflected back into the detector (from a landing) or scattered (a pit).

The disc is rotated by a drive system that regulates the speed of rotation so that data is read at the same speed regardless of the laser detector's position. This means that rotation speed decreases as the laser detector slowly traverses from the center to the periphery of the disc in order to maintain a constant rate of data acquisition.

Light from the laser beam must penetrate a thin protective layer of plastic (not illustrated) on the disc before striking the reflective coating that contains pits and landings. As the disc rotates, light reflected from landings on the disk strikes the photo sensor producing a series of electrical pulses that are coordinated with a timing circuit to generate a stream of 1s and 0s that produce the binary code of information on the disc. At the bottom of the tutorial, a simulated bit reader continuously streams the binary code as it is retrieved from the disc.

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