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I know its kind of a dead / dieing technology but i was wandering what stops a cd-r from be rewritten like a cd-rw.

Is it a sort of firm-ware on the disc essentialy in place to make people buy more cd's ?

Or is there a more technical reason, ie. cd's are store data magnetically (correct ?) Does a cd-r have less magnetism so once its been written to thats it ? (sorry if thats completely wrong, i have no idea the way cd's work..)

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CD's work much like the vinyl records of old. They don't rely on magnets, the laser actually etches pits and grooves into the CD data layer, which represent's 0's and 1's. –  Kruug Jan 8 '13 at 21:46
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Howstuffworks.com: howstuffworks.com/cd-burner5.htm –  user142485 Jan 8 '13 at 21:53
    
@Kruug: Mmmh. In that case, why is a CD-RW rewritable? Curious –  Ariane Jan 8 '13 at 21:53
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It does not etch holes. It changes the dye. Holes/bumps are only for pressed CDs. –  Hennes Jan 8 '13 at 21:56
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1 Answer

up vote 8 down vote accepted

Data on a regular CD is stored by making bumps or pits (hole) on the CD medium. This disturbs the reflection of light. A 0 or a 1 is determined by either reflection or non-reflection of light.

A CD-R works somewhat the same. It has a reflective surface, but this surface is covered by a dye. You can use a relative high intensity LASER to heat up the dye layer and turn it opaque. Afterwards the CD-R can be read in the same was as a normal CD. Light is either reflected or not.

CD-RW's work in the same way, but use a paint which can be turned opaque or not depending on the heat of the LASER.


Unlike the dye in CD-RW's, the Dye in a CD-R can not be reverted to its previous state. This is what prevents rewriting to an already written CD-R.

Nitpick exception: Rewriting with the same image or a image with only the right bit changed is technically possible, but I know of no software which would allow it. And that really is an edge-case.

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o: There's a difference between "CD" and "CD-R"? –  Ariane Jan 8 '13 at 21:56
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Yes. CD as in the silver, factury pressed CDs. And CD-R for the recordable ones which are not silver but come in several colours (using Azo as dye for blue coloured CD-Rs, Cyan for Cyanine using CD-Rs and green for pthalocyanine based CD-Rs). –  Hennes Jan 8 '13 at 22:00
    
Good answer, though doesn't exactly answer the question of what prevents it from being re-written (ie, once burned the dye cannot be reset by the burner as with CDRW). –  techturtle Jan 8 '13 at 22:29
    
You are right. I should make it explicit so it does answer rather than explain. –  Hennes Jan 8 '13 at 22:30
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Presumably it is only some "good hygene" in the writing program that keeps a cd-r from being overwritten. A "security erase" feature could turn the disk entirely to 1's or something like that. –  ddyer Aug 30 '13 at 22:37
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