| CD and DVD Technology |
| CDs, CD Burners, and DVDs In 2000, one of the biggest news stories was the rise of Napster: it was possible to get an MP3 version of just about any song you wanted for free. It recently became feasible for the average person to gather songs and make their own CDs. Old-fashioned vinyl records stored analog information. Compact Discs (CDs) store music in digital form: the information on the disc is represented by 1s and 0s. These 1s and 0s are represented by millions of tiny bumps and flat areas on the disc's reflective surface. 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 a sensor. When the beam passes over a bump, the light is bounced away from the optical sensor. The bumps are arranged in a spiral path, starting at the center of the disc. The CD player spins the disc while the laser moves outward from the center of the CD. A CD has a continuous track that measures about 0.5 microns (millionths of a meter) across and 5 km long The CD fabrication machine uses a high-powered laser to etch the bump pattern into material coated onto a glass plate - this pattern is pressed onto acrylic discs. The discs are then coated with aluminum to create the reflective surface. Like most complex manufacturing processes, conventional CD manufacturing isn't practical for home use. Electronics manufacturers introduced 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 a special laser, the dye turns opaque: It darkens to the point that light can't pass through. 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. The main advantage of CD-R discs is that they work in almost all CD players and CD-ROMS, which are among the most prevalent media players today. In addition to this wide compatibility, CD-Rs are relatively inexpensive. You can pick up a stack of 20 CD-Rs for less than $20. CD-RW discs have taken the idea of writable CDs a step further, building in an erase function so you can record over old data you don't need anymore. A DVD is very similar to a CD, but it has a much larger data capacity. A standard DVD holds about seven times more data than a CD does. A DVD can also be used to store almost eight hours of CD-quality music per side. DVDs use bumps similar to those used by CDs, but that are much smaller. To increase the storage capacity even more, a DVD can have up to four layers, two on each side. The laser that reads the disc can actually focus on the second layer through the first layer. The data track of a DVD is incredibly tiny -- just 740 nanometers separate one track from the next (a nanometer is a billionth of a meter). If you could lift the data track off a single layer of a DVD, and stretch it out into a straight line, it would be 12 kilometers long! That means that a double-sided, double-layer DVD would have 48 kilometers of data! When movies are put onto DVDs, they are encoded in MPEG-2 format and then stored on the disc. MPEG is a way of compressing the size of a video file. Compression is done by eliminating redundant or irrelevant data. Each frame can be encoded in one of two ways: � All of the information of a frame is stored = no compression. � Some of the information for a frame is taken from a frame that comes before or after = compression. DVDs can provide far better sound quality than CDs. |