How CDs work
By Marshall Brain
http://www.JewishWorldReview.com | (MCT) In 2007 we are celebrating an important event - it's the 25th anniversary for the CD, also known as the compact disc. In 25 years CDs have become ubiquitous, to the point where we take them completely for granted. But inside a CD you find both an amazing amount of technology.
To understand just how profound the effect of the CD has been, it is helpful to look back at the two reigning technologies in the early 1980s: The vinyl LP and the compact cassette. You may recall just how bizarre the vinyl LP was. An aficionado would have a massive turntable with little black and white markings along the edge so that he could calibrate the speed of the turntable with a strobe light. The turntable rested on big vibration-dampening pads to try to keep any spurious vibrations away from the turntable. The tip of the tone arm was measured with a sensitive scale to minimize its pressure in the record's groove. The LP itself was sprayed with an anti-static gun to cut back on dust. And even with all this technology, the first thing you heard from any vinyl disk was hiss. With the cassette the story was the same: the tape stretched, it flaked, it broke, it jammed.
Enter the compact disc. Because of its digital nature, its sound quality was amazing compared to vinyl or tape. It also eliminated hiss once and for all. Things like dust, fingerprints and small scratches didn't matter. The sound did not degrade over time as it did with vinyl or tape. And normal heat levels, as in a locked car on a hot summer day, didn't cause any problems. And there was an added bonus - over time, compact discs became incredibly inexpensive to produce. How inexpensive? So inexpensive that AOL could afford to mail out billions of them.
The thing that is so amazing about a CD is its simplicity. A CD is basically a mirror. The mirror is made out of a tough, clear plastic disc. A microscopic layer of aluminum is coated onto the plastic to create a mirror surface. To store the CD's data, the mirror has millions of tiny blemishes on it. When a laser hits the CD's aluminum surface, it either reflects cleanly off the mirror or scatters off a blemish. A computer inside the CD player interprets the reflections and scatterings as the ones and zeros of the computer binary code, and reassembles the ones and zeros into music (or data). That is the essence of a CD.
The process of making a music CD goes something like this. The songs of the CD are recorded using a microphone. The microphone picks up vibrations in the air. Those vibrations can then be digitally sampled. For a CD, a device called an analog-to-digital converter (ADC) samples the vibration pattern 44,000 times per second. Based on the intensity of the vibration pattern at the moment it is sampled, the ADC assigns a number between -32,767 and 32,768. Approximately 158 million of these ADC samples make up all the music on a 60-minute CD.
Now these 158 million numbers are turned into ones and zeros. Things like track information and error correcting codes are added. All the ones and zeros are placed end to end to formed a single microscopic spiral track that is about three miles long. This track is placed on a metal mold - a tiny bump on the metal represents a one, and a tiny flat area represents a zero.
To make a CD, liquid plastic injects into the mold and it picks up the pattern of microscopic bumps and flats. Once the plastic comes out of the mold, a layer of aluminum coats the bumps and flats on the plastic. The flats reflect a laser perfectly, while a bump scatters a laser beam. To read the CD, all the CD player has to do is read the ones and zeros, reassemble them into numbers and run the numbers through a digital-to-analog converter. Out comes music.
You can see why CDs are so inexpensive to produce. Each CD is nothing but a blob of liquid plastic injected into a mold, along with a milligram of aluminum. A coat of lacquer protects the aluminum and holds the CD label. There really is nothing to a CD.
In the early 1990s, music companies sold more than 440 million cassette tapes per year. Last year, the number had fallen to 700,000. That shows you just how thoroughly the CD has dominated the musical landscape. Of course, the days of the CD are now numbered - compact discs will be replaced by digital music players that use only the music's data, without any need for the plastic and aluminum.
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© 2007, How Stuff Works Inc. Distributed by McClatchy-Tribune Information Services.