A CD has a long, spiraled data track. If you were to unwind this track, it would extend out 3.5 miles (5 km).
If you've read How CDs Work, you understand the basic idea of CD technology. CDs store music and other files in digital form -- that is, the information on the disc is represented by a series of 1s and 0s (see How Analog and Digital Recording Works for more information). In conventional CDs, these 1s and 0s are represented by millions of tiny bumps and flat areas on the disc's reflective surface. The bumps and flats are arranged in a continuous track that measures about 0.5 microns (millionths of a meter) across and 3.5 miles (5 km) long.
To read this information, the CD player passes a laser beam over the track. When the laser passes over a flat area in the track, the beam is reflected directly to an optical sensor on the laser assembly. The CD player interprets this as a 1. When the beam passes over a bump, the light is bounced away from the optical sensor. The CD player recognizes this as a 0.
A CD player guides a small laser along the CD's data track. In conventional CDs, the flat areas, or lands, reflect the light back to the laser assembly; the bumps deflect the light so it does not bounce back.
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.
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.
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.
In the next section, you'll find out how data is recorded on CDs, both by professional equipment and the home CD burner.
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