How Are Recordable CDs Burned?

October 18th, 2005


The following is from an email newletter I receive (reprinted with permission).

How are recordable CDs burned?

By Kim Komando

CDs are a great example of how fast technology moves. Ten years ago, CDs were high technology. I remember buying an expensive computer just to get a CD player (CD-ROM). Today, we burn CDs at the drop of a hat, to play in our cars. But it’s still good to know how this stuff works. The technology is pretty cool.

Digital information, including music, is stored in binary (1’s and 0’s). To represent all those 1’s and 0’s, a CD contains tiny spots that are highly reflective or poorly reflective. A laser is used to read the sequence of spots. The sequence can then be interpreted as sounds.

A CD-R disc has two important layers of material sandwiched between plastic. One is a layer of metal, typically aluminum. In front of that is the other layer, a special dye. The aluminum layer is highly reflective. The dye is mostly transparent, so overall the disc is completely reflective. But the dye can be changed by the laser in your CD-R drive.

The laser heats and burns tiny spots on the dye layer. The burned spots become nontransparent. They block light from reaching the aluminum layer. So a finished CD-R ends up with both highly reflective and poorly reflective spots. These are the 1’s and 0’s, respectively.

Your CD-R drive uses a strong laser to burn a disc’s dye layer. Typically, CD-Rs have a second, weaker laser, used for playing. It is too weak to affect the dye’s transparency. CD-ROMs also have only a weak laser.

The chemicals used for the dye layer eventually degrade, ruining the disc. Disc manufacturers use various dye formulas, some sturdier than others. But the cheapest last only a couple years. And price tag aside, it’s difficult to discern the quality of CD-R brands. I always buy name brands.

Rewritable discs use a layer of crystallized material instead of a dye. A CD-RW drive’s laser melts tiny spots of the layer. The spots cool too fast to re-crystallize; that makes them opaque. Those spots are 0’s, because they do not reflect light. Crystallized spots, which are transparent, are 1’s.

When data is erased on a CD-RW, the spots are melted again. But they’re heated to a lower temperature and cool slowly enough to re-crystallize.

Commercially produced discs like software or music albums (and my books) are not burned. They use tiny bumps or dips to represent 1’s and 0’s. The dips and bumps are molded directly into a disc’s plastic. The sequence is then coated by a layer of metal, usually aluminum. These discs can last for decades.

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2 Responses to “How Are Recordable CDs Burned?”

  1. [link]Tony Says:

    I didn’t know CD-Rs only last a couple of years. That’s disconcerting, since many people use them as a permanent snapshot records of their data and the manufacturers don’t usually list an expiry date.

  2. [link]Chad Cloman Says:

    There is controversy on that particular point. Manufacturers claim differently, but some tests hold up the claim that CD-Rs don’t last more than a few years without data loss. I always buy brand-name CD-Rs for long-term storage, and I keep the important stuff on my hard drive as a backup. I use the off-brand CD-Rs for short-term storage (mainly transferring from one computer to another)–but it’s almost easier these days to use USB keys instead of floppies/CDs.

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