Dvd's and Cds for the Computer

It's amazing when you think about it: you can store a movie several hours long on a shiny piece of plastic no bigger than your hand – on a DVD. CD’s are one of the most popular ways of storing music and computer data. In the mid-1990s, CDs evolved into digital video/versatile discs .

Throughout this article, we will be mentioning CD’s, but almost everything about CDs also holds true for DVDs – except a DVD hold’s approx. 6-7 times as much data than a CD.

A compact disc is a thin, circular disc of metal and plastic about 12cm (just over 4½ 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 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.

Until CDs were invented, music was typically stored on 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 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. To put it another way, CDs are based on the technology of Star Wars! 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.

Records stored music as bumps & grooves 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. CD’s & DVD’s turn 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. In other words, there is no music on a CD at all—just a huge long list of numbers.

CDs are made from an original "master" disc. The master is "burned" with a laser beam that etches bumps (called pits) into its surface. A bump represents the number zero, so every time the laser burns a bump into the disc, a zero is stored there. The lack of a bump (which is a flat, unburned area on the disc, called a land) represents the number one. Thus, the laser can store all the information sampled from the original track of music by burning some areas (to represent zeros) and leaving other areas unburned (to represent ones). Although you can't see it, the disc holds this information in a tight, continuous spiral of about 3-5 billion pits. If you could unwrap the spiral and lay it in a straight line, it would stretch for about 6 km (almost 10 miles)! Each pit occupies an area about two millionths of a millionth of a square metre. That's pretty small !

Once the master disc has been made, it is used to stamp out millions of plastic duplicates—the CDs that you buy and put into your music player or computer. In a CD-making factory, the master CD is recorded by a laser beam burning information into the surface of a disc. In your home, you play CDs back in almost exactly the opposite way.