How do re-writeable CDs (CD-RW) work?

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Offline Simulated

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How do re-writeable CDs (CD-RW) work?
« on: 17/01/2008 23:14:25 »
Them are wierd lol how?
« Last Edit: 12/02/2008 11:47:27 by chris »

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Offline Simulated

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Re: How do re-writeable CDs (CD-RW) work?
« Reply #1 on: 10/02/2008 21:52:07 »
Anyone Know? Just wondering

Thanks

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Offline DoctorBeaver

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Re: How do re-writeable CDs (CD-RW) work?
« Reply #2 on: 10/02/2008 22:06:32 »
A normal CD uses microscopic bumps to store data. The surface of the CD is a mirror, and the bumps disrupt the mirror's perfect surface. The laser that reads the CD can detect the difference between a perfect mirror and an imperfection caused by a bump because of the difference in reflectivity. By interpreting "perfect mirror" as a "1," and "bump" as a "0," it is easy to store digital information on a CD. The bumps on a CD are moulded into the plastic when it is manufactured, so they are permanent.

To create a writeable CD (CD-R), you need to modify the surface of a CD so you can burn data onto it, turning it into a CD-R. There are no bumps on a CD-R. A clear dye layer covers the CD's mirror. A write laser heats up the dye layer enough to make it opaque. The read laser in a CD player senses the difference between clear dye and opaque dye the same way it senses bumps -- it picks up on the difference in reflectivity.

To create a rewriteable CD (CD-RW), you need a dye layer that can be changed back and forth between opaque and transparent.
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Offline Simulated

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Re: How do re-writeable CDs (CD-RW) work?
« Reply #3 on: 10/02/2008 22:10:38 »
Ah dude that's amazing

so same with dvd's that are r and rw?

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Offline DoctorBeaver

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Re: How do re-writeable CDs (CD-RW) work?
« Reply #4 on: 10/02/2008 22:27:32 »
AFAIK
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Offline techmind

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Re: How do re-writeable CDs (CD-RW) work?
« Reply #5 on: 12/02/2008 10:22:06 »
I understood that "RW" discs use a phase-change material rather than a dye. If the material is heated and cooled quickly it sets into an amorphous (glassy) state, whereas if it is heated and cooled slowly it crystalises as it sets. The two states have different reflectivities. Obviously "quickly" and "slowly" is relative - CD-RW burning is pretty fast, and during recording the two states will be achieved by slightly different laser-pulse profiles.

Unfortunately the difference in reflectivity of the two "RW" phase-change states is rather less than that of regular (or dye "R") CD/DVDs, which is the main reason why older or deteriorating drives tend not to play back RW discs.
"It has been said that the primary function of schools is to impart enough facts to make children stop asking questions. Some, with whom the schools do not succeed, become scientists." - Schmidt-Nielsen "Memoirs of a curious scientist"

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Offline DoctorBeaver

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Re: How do re-writeable CDs (CD-RW) work?
« Reply #6 on: 12/02/2008 11:35:09 »
techmind - that may well be true. I got my info from 1 of those "How it works" sites, so I cannot guarantee its accuracy.
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