Friday, October 14, 2005


A friend and I were talking about a CD which was damaged and the conversation expanded to the general mechanics of the technology. I knew very little, and later did a goggle and the longtime computer techie Kim Komando's web site popped up with this simple and beautiful explanation. So read and learn.

"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 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.
Thanks to Kim Komando...

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