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Consider just how audio, video, and computer files are stored on a CD. Although these three types of information are different, they are stored in the same way: digitally. But what does that word really mean?
Programmers, technotypes, and hardware jockeys use the word digital when they're talking about binary, the language that's used by computers around the world. Unlike the imprecise languages that are spoken and written by mere mortals, binary data is built from only two values — 0 and 1. These values are often referred to as off and on, respectively, as shown in Figure 1. (In fact, a computer is only a huge collection of switches, but that's another story.) Therefore, computer files and digital music are a long line of 0s and 1s. If you sat next to a light switch for 100 years and flipped it off and on in the proper sequence, you would have the visual version of a digital song from a CD (and a bad headache along with incredibly sore fingers).
Figure 1: All that information is just a big bunch of 0s and 1s.
Now that you're privy to the binary master plan, you can see how the absence and presence of light perfectly represent binary data — things in a room are either dark or bright. The geniuses who developed CD technology took this concept one step further! They had the great idea of using a laser beam to read the binary data that's stored on a disc, and that's where pits — microscopic indentations arranged in a single, tiny groove that spirals around the disc— in the aluminum layer take center stage.
Figure 2 shows how the binary data is read: When the laser beam hits a pit on the surface of the CD, the beam scatters, so most of it isn't reflected (hence, darkness, which. in this case. stands for a 0 in binary data). If the laser beam hits one of the flat surfaces — they're called lands, by the way — the beam is cleanly reflected back, and the drive senses the reflected light. (This is a 1 in binary format.) And, ladies and gentlemen, that is why the bottom of a CD shines like a mirror; the rainbow effect is caused by the microscopic groove that runs across the surface. Naturally, the process of reading data from a disc happens very fast, but that's really all there is to it.
Figure 2: The surface of a CD, as read by your friendly neighborhood laser beam.
Essentially, DVD technology works the same way — with a difference or two. A DVD-ROM disc can hold the approximate equivalent of seven CDs, and Figure 3 shows why: The pits on a DVD-ROM are much smaller and are packed closer together on the surface of the disc, and the drive uses a much more powerful laser beam to read them. DVDs can also have multiple reflective layers, so data can be stored on both sides.
Figure 3: Compared to a CD, you don't find much elbow room on a DVD-ROM.
 | Believe it or not, the DVD specification standard provides for two-sided DVD-ROM discs that have two layers on each side, for more than 27 CDs' worth of storage space on a single DVD-ROM! However, these discs are so hard to manufacture that they're on the endangered species list, and you may never see one. |
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