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Side by side: an ordinary CD, a recordable CD, and a rewritable DVD.

How CD burners work

by Chris Woodford. Last updated: September 30, 2014.

Learning to read is one of the most useful and important things you'll ever do, but it only works if there are things to read—in other words, if you want to read things, generally you need to be able to write them too. When compact discs first became popular in the 1980s, they were sold purely as read-only audio compact discs (CD-DA, ones you could play music from but not record onto). It wasn't long before computer companies realized they could use CDs to distribute software (programs) very cheaply, and ordinary computer users soon saw that CDs would be even better if you could write music and data on them as well as just read from them. That's how recordable CDs (CD-Rs) came to be developed, but the snag was that they could only be written on once; you couldn't erase and reuse them. Soon enough, though, the computer whizzkids developed rewritable CDs (CD-RWs) that you could erase and rewrite any number of times. All three types of CDs (ordinary read-only CDs, CD-Rs, and CD-RWs) work in a different way. Let's take a closer look and find out how!

Photo: Audio compact discs have dwindled in popularity since the development of MP3 players and downloadable music, but CD-Rs and CD-ROMs are still widely used for storing computer data, and DVDs remain hugely popular for playing and recording home movies. If you look from the shiny side, you'll see that different types of CD/DVD are recognizably different. Ordinary CDs look silver (right); recordable CDs look greenish-yellow (bottom), and rewritable CD/DVDs tend to be more blueish (top).

How does an ordinary CD work?

An ordinary CD is a sandwich of plastic, aluminum, and polycarbonate.

If you've read our main article on CDs and DVDs, you'll know the basic idea already. The shiny, reflective layer inside a CD is covered with little depressions (pits) and flat areas (lands) arranged in a tight spiral. As the CD spins, a laser and a photocell (light detector) scan across the surface reading the pits and lands and converting them into strings of binary zeros and ones, which an electronic circuit then turns back into music or data (computer information).

Virtually all mass-produced discs work this way, including the CDs you buy in record stores, the DVDs you borrow from video libraries, and the CD-ROMs containing free software you find taped to the fronts of computer magazines. They're manufactured in their millions in factories using dozens of pressing machines. Each machine is mounted with a reverse-version of the disc (with the pits turned into bumps) and is pressed against blank plastic discs to produce zillions of exact copies. Once a disc is pressed, it's coated with a thin aluminum layer (so it will reflect laser light), covered with protective polycarbonate and lacquer, and the label is printed on top.

Illustration: An ordinary CD is a sandwich of plastic (in which bumps have been pressed by a master disc), reflective aluminum, and protective polycarbonate plastic.

How does a recordable CD (CD-R) work?

How a CD-R stores data with areas of burned and unburned dye.

In theory, then, if you wanted to make ordinary CDs in your own home, you'd need to install a huge and expensive CD-pressing machine. Fortunately, you don't need to do this—and that's because recordable CDs (CD-Rs) work in a completely different way. This time, there are no pits and lands imprinted on plastic. Instead, in between the protective polycarbonate and the reflective aluminum, there's a layer of dye. Normally the dye is translucent: laser light zooming into the disk from a CD player will pass straight through it, hit the reflective aluminum, and bounce straight back down again.

So far so good, but how do we store information on a compact disc like this? A CD-R writer has a higher-powered laser than normal, which generates heat when it strikes the disc, burning the dye and making a tiny black spot. Later, when a CD reader aims its laser at that spot, the light is completely absorbed and doesn't reflect back. This indicates that a zero ("0") is stored on the disc at that point. In places where the dye is unburned, the laser light reflects straight back again, indicating that a "1" is stored on the disc. See where this is going? By creating areas of burned black dots, and other places where the dye is left alone, a CD-R writer creates a pattern of binary zeros and ones that can be used to store information. That, incidentally, is why CD-R writers are often called CD burners—they literally do burn. Unfortunately, once the dye is burned it's permanently transformed: you can't change it back again. And that's why you can only write a CD-R disc once.

Illustration: With a CD-R, binary information is stored as burned areas (0) and unburned areas (1) in the dye layer sandwiched between the protective polycarbonate and the reflective aluminum.

How does a rewritable CD (CD-RW) work?

How a CD-RW stores data with areas of amorphous and crystalline metal alloy.

Let's say you're charged with the task of developing a type of compact disc that can be written to or erased over and over again. Clearly you can't use either of the methods we've discussed so far (the pits and lands method from read-only audio CDs or the burned-dye method used in CD-Rs). What you really need is a CD made from a substance that can easily be converted back and forth between two different forms, so it can be used to store a pattern of zeros and ones, then erased and used to store a different pattern later on if necessary.

Most of us learned in school that the atoms (or molecules) in solids, liquids, and gases arrange themselves in different positions, with atoms in solids tightly locked together. Some solid materials are more complex than this: their atoms (or molecules) can be arranged in two or more different ways called solid phases. (Solid carbon, for example, can exist in very different phases that include graphite and diamond.) That's just what we need to make a CD-RW disc.

Instead of having a layer of dye, a CD-RW has a layer of metallic alloy that can exist in two different solid forms and change back and forth between them. It's called a phase-change or phase-shift material. Sometimes it's crystalline, with its atoms/molecules arranged in orderly ways, so it's translucent and light can pass straight through it; other times, its atoms/molecules are jumbled up in a much more random and disorderly form called an amorphous solid, which is opaque and blocks light. When a CD-RW laser hits this material, it changes tiny little areas of it back and forth between the crystalline and amorphous forms. When it creates a crystalline area, it's making part of the CD reflective and effectively writing a one ("1"); when it makes an amorphous area, it's making the CD non-reflective and writing a zero ("0"). Because this process can be repeated any number of times, you can write and rewrite a CD-RW pretty much as many times as you like!

Illustration (above): With a CD-RW, binary information is stored as areas of metal alloy that are either crystalline or amorphous. Crystalline areas have a regular structure that lets light pass through to the aluminum area and reflect back down again, thus storing ones. Amorphous areas have a random structure that scatters incoming laser light, so it can't reflect back, thus storing zeros. A CD-rewriter can change the metal alloy on the CD from one form to the other and back again, which is why this kind of disc can be erased and rewritten many times over.

The read/write laser head from a typical CD writer/burner. The read-only laser/photocell read head from a typical CD player.
Photo: A CD/DVD writer/rewriter (left) has a much more sophisticated laser read/write head than an ordinary CD/DVD player (right). Depending on the type of player, the read/write head needs to be able to read ordinary CDs and DVDs, recordable discs, and rewritable discs—so it really needs to be capable of several quite different reading and writing operations.

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Text copyright © Chris Woodford 2010. All rights reserved. Full copyright notice and terms of use.

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Woodford, Chris. (2010) CD writers. Retrieved from http://www.explainthatstuff.com/how-cd-writers-work.html. [Accessed (Insert date here)]

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