What is a CD?

 A compact disc is a thin, circular disc of metal and plastic about 12cm (just over 4.5 inches) in diameter. It's actually made of three layers. Most of a CD is made from a tough, brittle plastic called polycarbonate. Sandwiched in the middle there is a thin layer of aluminum. Finally, on top of the aluminum, is a protective layer of plastic and lacquer. The first thing you notice about a CD is that it is shiny on one side and dull on the other. The dull side usually has a label on it telling you what's on the CD; the shiny side is the important part. It's shiny so that a laser beam can bounce off the disc and read the information stored on it. sony cfd s70

How CDs use optical laser technology

Until CDs were invented, music was typically stored on vinyl (plastic) LP (long-playing) records and cassette tapes. LPs scratched easily, while tapes could stretch and distort and sometimes snapped or seized up entirely. Both of these ways of storing music were primitive compared to CDs. LPs were played on turntables with a moving arm that bounced along a groove in the plastic, reading back the music as it went. Record players (or gramophones, as they were sometimes known) used mechanical technology for recording and playing back sound: the moving arm turned the bumps in the plastic into sounds you could hear. Cassette tapes (used in such things as the original Sony Walkmans) worked a different way. They stored sounds using magnetic technology. When you put a cassette into your Walkman, a small electric motor dragged the tape past a little electromagnet. The electromagnet detected the pattern of magnetism on the tape and an electronic circuit changed this back into the sounds that fizzed and popped in your headphones.

With the invention of CDs, people finally had a more reliable way of collecting music. CD players are neither mechanical nor magnetic but optical: they use flashing laser lights to record and read back information from the shiny metal discs. One of the main problems with LPs and cassettes was the physical contact between the player and the record or tape being played, which gradually wore out. In a CD player, the only thing that touches the CD is a beam of light: the laser beam bounces harmlessly off the surface of the CD, so the disc itself should (in theory) never wear out. Another advantage is that the CD player can move its laser quickly to any part of the disc, so you can instantly flip from track to track or from one part of a movie to another.

How CDs are recorded and played back

Note: In the explanations that follow, I'm deliberately going to simplify how CDs store music as patterns of zeros and ones. It's much more complex than I'm going to make it seem, and it's beyond the scope of an introductory article like this, but I will briefly describe what really happens at the very end.

LP records stored music as bumps on the surface of plastic, while cassettes stored it using patterns of magnetism. These are called analog technologies, because the sound is stored as a continuously varying pattern (of bumps in the plastic of a record or fluctuations in the magnetism on a cassette tape). In a CD, music (or other information) is stored digitally (as a long string of numbers). After the music has been recorded, it is converted into numbers by a process called sampling. Almost 50,000 times a second (44,100 to be exact), a piece of electronic equipment measures the sound, turns the measurement into a number, and stores it in binary format (as a pattern of zeros and ones). The sampling process turns a CD track lasting several minutes into a string of millions of zeros and ones. This is the information stored on your CD. In other words, there is no music on a CD at all—just a huge long list of numbers.