CD Basics: The Path
The bumps are arranged in a spiral path, starting at the center of the disc. The CD player spins the disc while the laser assembly moves outward from the center of the CD. At a steady speed, the bumps move past any point at the outer edge of the CD more rapidly than they move past any point nearer the CD's center. In order to keep the bumps moving past the laser at a constant rate, the player must slow the spinning speed of the disc as the laser assembly moves outward.
At its heart, this is all there is to a CD player. The execution of this idea is fairly complicated, because the pattern of the spiral must be encoded and read with incredible precision, but the basic process is pretty simple.
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The CD player spins the disc while moving the laser assembly outward from the middle. To keep the laser scanning the data track at a constant speed, the player must slow the disc as the assembly moves outward.
In the next section, you'll find out how data is recorded on CDs, both by professional equipment and the home CD burner.