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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

2 Answers 2

up vote 12 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
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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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Yes, this is also true for DVDs. Basically DVDs storage is very similar but employs a smaller wavelength/higher frequency LASER. Thus it can read smaller markings aka more data per surface area. It also does a few other things like multiple layers, variable focus on the laser to read multiple layer etc etc, but the basic method is the same. –  Hennes Aug 3 at 22:39

A CD-R disc cannot be used again, but it might be possible to delete existing data.

The disc comes out of the box reflective, but then your drive burns the solid thin chemical dye layer below the plastic and "roughens" it so it becomes slightly transparent. This makes the light disperse when a laser hits it, thus it counts as a zero essentially since the laser gets little light back. There is no way to fix this burned area and make it reflective again. The only way to wipe the data, is to turn 01011010 into 00000000.

Asus makes software called E-Hammer that may work with non-Asus drives:

E-Hammer allows you to permanently delete the data on writable disc (CD-R and DVD-R) formats. Once the data is deleted, it cannot be recovered or restored, and the optical drive will not be able to read the disc. This ensures data security, eliminates the hassle of scratching your old discs, and conforms with our commitment to environmental protection.

If there were software that would just make the laser burn 0's across the disk and ignore the formatting and toc etc., the data is gone. DD in Linux can probably do it.

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The data might be gone, but then how can new data be written after that? –  Arjan Aug 3 at 20:33
    
It cant be used again. The disc comes out of the box reflective, but then your drive burns the solid thin chemical dye layer below the plastic and "roughens" it so it becomes slightly transparent. This makes the light disperse when a laser hits it, thus it counts as a zero essentially since the laser gets little light back. There is no way to fix this burned area and make it reflective again. The only way to wipe the data, is to turn 01011010... into 00000000... –  Rick Aug 3 at 20:46
    
"It cant be used again." Exactly. Makes me wonder how this answers the question then. (Unless you intended to write "The disc cannot be used again, but it might be possible to erase existing data. [...]") –  Arjan Aug 9 at 13:38
    
(I edited your post to make clear it's not about rewriting.) –  Arjan Aug 10 at 11:45

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