In 2000, one of the biggest news stories was the rise of Napster and similar file-sharing programs. With these programs, you could get an MP3 version of just about any song you want without shelling out a dime. The record companies were fairly upset over this turn of events, and understandably so: They weren’t making any money off the distribution of their product to millions of people. An external writable CD drive, also called a CD burner: With this type of drive, you can take music or data files from your computer and make your own CDs.
But there was money to be made on the “Napster revolution,” as electronics manufacturers and retailers soon discovered. In 1999, 2000 and early 2001, sales of CD burners and blank CD-Recordable discs skyrocketed. Suddenly it was feasible for the average person to gather songs and make their own CDs, and music-mix makers everywhere wanted to get their hands on the means of production. Today, writable CD drives (CD burners) are standard equipment in new PCs, and more and more audio enthusiasts are adding separate CD burners to their stereo systems.
In less than five years, CDs have eclipsed cassette tapes as the mix medium of choice. In this edition of HowStuffWorks, you’ll find out how CD burners encode songs and other information onto blank discs. We’ll also look at CD re-writable technology, see how the data files are put together and find out how you can make your own music mixes with a CD burner. CD Basics A CD has a long, spiraled data track. If you were to unwind this track, it would extend out 3. 5 miles (5 km). If you’ve read How CDs Work, you understand the basic idea of CD technology.
CDs store music and other files in digital form — that is, the information on the disc is represented by a series of 1s and 0s (see How Analog and Digital Recording Works for more information). In conventional CDs, these 1s and 0s are represented by millions of tiny bumps and flat areas on the disc’s reflective surface. The bumps and flats are arranged in a continuous track that measures about 0. 5 microns (millionths of a meter) across and 3. 5 miles (5 km) long. 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 an optical sensor on the laser assembly. The CD player interprets this as a 1. When the beam passes over a bump, the light is bounced away from the optical sensor. The CD player recognizes this as a 0. A CD player guides a small laser along the CD’s data track. In conventional CDs, the flat areas, or lands, reflect the light back to the laser assembly, while the bumps deflect the light so it does not bounce back. The bumps are arranged in a spiral path, starting at the center of the disc.
The CD player spins the disc while the laser assembly moves outward from the center of the CD. At a steady speed, the bumps move past any point at the outer edge of the CD more rapidly than they move past any point nearer the CD’s center. In order to keep the bumps moving past the laser at a constant rate, the player must slow the spinning speed of the disc as the laser assembly moves outward. The CD player spins the disc while moving the laser assembly outward from the middle. To keep the laser scanning the data track at a constant speed, the player must slow the disc as the assembly moves outward.
At its heart, this is all there is to a CD player. The execution of this idea is fairly complicated, because the pattern of the spiral must be encoded and read with incredible precision, but the basic process is pretty simple. In the next section, you’ll find out how data is recorded on CDs, both by professional equipment and the home CD burner. Light Write In the last section, we saw that conventional CDs store digital data as a pattern of bumps and flat areas, arranged in a long spiral track.
The CD fabrication machine uses a high-powered laser to etch the bump pattern into photoresist material coated onto a glass plate. Through an elaborate imprinting process, this pattern is pressed onto acrylic discs. The discs are then coated with aluminum (or another metal) to create the readable reflective surface. Finally, the disc is coated with a transparent plastic layer that protects the reflective metal from nicks, scratches and debris. The different layers of a conventional CD As you can see, this is a fairly complex, delicate operation, involving many steps and several different materials.
Like most complex manufacturing processes (from newspaper printing to television assembly), conventional CD manufacturing isn’t practical for home use. It’s only feasible for manufacturers who produce hundreds, thousands or millions of CD copies. Consequently, conventional CDs have remained a “read only” storage medium for the average consumer, like LPs or conventional DVDs. To audiophiles accustomed to recordable cassettes, as well as computer users who were fed up with the limited memory capacity of floppy disks, this limitation seemed like a major drawback of CD technology.
In the early ’90s, more and more consumers and professionals were looking for a way to make their own CD-quality digital recordings. In response to this demand, 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 concentrated light of a particular frequency and intensity, the dye turns opaque: It darkens to the point that light can’t pass through. A CD-R doesn’t have the same bumps and lands as a conventional CD. Instead, the disc has a dye layer underneath a smooth, reflective surface. On a blank CD-R disc, the dye layer is completely translucent, so all light reflects. The write laser darkens the spots where the bumps would be in a conventional CD, forming non-reflecting areas.
By selectively darkening particular points along the CD track, and leaving other areas of dye translucent, you can create a digital pattern that a standard CD player can read. 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. So, even though the CD-R disc doesn’t have any bumps pressed into it at all, it behaves just like a standard disc. A CD burner’s job, of course, is to “burn” the digital pattern onto a blank CD.
In the next section, we’ll look inside a burner to see how it accomplishes this task Feel the Burn In the last section, we saw that CD burners darken microscopic areas of CD-R discs to record a digital pattern of reflective and non-reflective areas that can be read by a standard CD player. Since the data must be accurately encoded on such a small scale, the burning system must be extremely precise. Still, the basic process at work is quite simple. The CD burner has a moving laser assembly, just like an ordinary CD player. But in addition to the standard “read laser,” it has a “write laser. The write laser is more powerful than the read laser, so it interacts with the disc differently: It alters the surface instead of just bouncing light off it. Read lasers are not intense enough to darken the dye material, so simply playing a CD-R in a CD drive will not destroy any encoded information. The laser assembly inside a CD burner The write laser moves in exactly the same way as the read laser: It moves outward while the disc spins. The bottom plastic layer has grooves pre-pressed into it, to guide the laser along the correct path.
By calibrating the rate of spin with the movement of the laser assembly, the burner keeps the laser running along the track at a constant rate of speed. To record the data, the burner simply turns the laser writer on and off in synch with the pattern of 1s and 0s. The laser darkens the material to encode a 0 and leaves it translucent to encode a 1. The machinery in a CD burner looks pretty much the same as the machinery in any CD player. There is a mechanism that spins the disc and another mechanism that slides the laser assembly. Most CD burners can create CDs at multiple speeds.
At 1x speed, the CD spins at about the same rate as it does when the player is reading it. This means it would take you about 60 minutes to record 60 minutes of music. At 2x speed, it would take you about half an hour to record 60 minutes, and so on. For faster burning speeds, you need more advanced laser-control systems and a faster connection between the computer and the burner. You also need a blank disc that is designed to record information at this speed. 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 around $20. The main drawback of the format is that you can’t reuse the discs. Once you’ve burned in the digital pattern, it can’t be erased and re-written. In the mid ’90s, electronics manufacturers introduced a new CD format that addressed this problem. In the next section, we’ll look at these CD-Re-writable discs, commonly called CD-RWs, to see how they differ from standard CD-R discs.
