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How CDs Work
Source: "How CDs Work", HowStuffWorks (http://www.howstuffworks.com), by Marshall Brain. HowStuffWorks, Inc., 2002

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. 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.

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.

REPLICATION
The CD fabrication machine uses a high-powered laser to etch the bump pattern into photoresist material coated onto a glass plate. Through an elaborate imprinting process, this pattern is pressed onto acrylic discs. The discs are then coated with aluminum (or another metal) to create the readable reflective surface. Finally, the disc is coated with a transparent plastic layer that protects the reflective metal from nicks, scratches and debris.

DUPLICATION
An alternative sort of CD that could be encoded in a few easy steps. CD-recordable discs, or CD-Rs, don't have any bumps or flat areas at all. Instead, they have a smooth reflective metal layer, which rests on top of a layer of photosensitive dye.

When the disc is blank, the dye is translucent: Light can shine through and reflect off the metal surface. But when you heat the dye layer with concentrated light of a particular frequency and intensity, the dye turns opaque: It darkens to the point that light can't pass through.

By selectively darkening particular points along the CD track, and leaving other areas of dye translucent, you can create a digital pattern that a standard CD player can read. The light from the player's laser beam will only bounce back to the sensor when the dye is left translucent, in the same way that it will only bounce back from the flat areas of a conventional CD. So, even though the CD-R disc doesn't have any bumps pressed into it at all, it behaves just like a standard disc.

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