Shoutbox

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Originally posted by WDZ
http://www.cdrfaq.org/faq07.html#S7-12

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If you scratched the top (label) side of a CD-R, and it no longer works, your disc is toast. (If you scratched it, and it still works, copy the data off while you still can.)

If you scratched the bottom side, then all you've done is etch the polycarbonate (plastic), and it can be repaired like any other CD. A common misconception is that the data is on the bottom, but if you examine it carefully you will see that the data is beneath the label. The laser reads the data through the polycarbonate layer, and if the layer is scratched the laser will refract onto the wrong part of the disc.

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That FAQ is about CD-Rs, but I assume the same applies to all CDs...

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Sorry, but that faq contradicts itself (although it states somewhat true things (but not completely!), it states it in the wrong way or not exact enough and misconceptions may occur)...

Also, while they talk about the top part in that paragraph, the last sentence is not about the 'top' part, but about the 'bottom' part:
The laser reads the data through the polycarbonate layer (=bottom protection layer), and if the layer is scratched the laser will refract onto the wrong part of the disc.

But, it does NOT matter what side you scratch. Scratching the top side deep enough will render the disc useless as you also damage the aluminium reflection layer or even the protection/data layer which contains the physical data (pits/falls) on the disc.

Scratching the bottom side deep enough (scratching thru the polycarbonate protection layer) will do exactly the same: you will damage the pits and falls and maybe even the aluminium reflection layer. Although this polycarbonate protection layer is thicker than the top 'label' layer.

Another pointer: If you scratched the bottom side, then all you've done is etch the polycarbonate (plastic), and it can be repaired like any other CD.

This is NOT always true, only superficial scratched can be repeared, deep scratches will permanently damage any CD just as well...

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1) sticky or printed Label (red part in the graphic)
2) Acrylic layer (top protection layer) = 200 µm (micometer)
3) Aluminium reflection layer (makes the laser bounce and detect) = 0.1 µm (micometer)
4) Polycarbonate plastic layer (bottom protection layer) = 1.2 mm or 1200 µm, this layer also contains the actuall pits and falls which represent the physical data.
So yes, the bottom layer does contain the data (although the pits and falls are much closer to the top than to the bottom).

The aluminium layer above that is only to bounce back the laser beam and does NOT contain the actuall data per-say; the physical data (pits and falls) is formed by the bottom protection layer!

pits and falls are 125 nm tick = 0.125 µm = 0.000125 mm

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Originally posted by John Anderton

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Originally posted by Patchou
Yes, it applied to all CD and DVD formats. Scratching the top of your disk makes them definitively unreadable.

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Thats what i was going to say ..... take a old cd and try to scratch off the label and silver foil ..... you will get a transparent peice of plastic which is good for no other use than playing frisbee if you live in an apartment on the 6th floor or something
It has happened to me

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This is not because you scracthed of the label or because "the data is beneath the label". This happened because while scratching it off, you made very deep scratches; scratching right thru the top protection layer (which is relative very thin compared to the bottom protection/data layer).

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Originally posted by John Anderton
Some of my dvd's are scratched from the bottom but the inner layer which keeps the data and the label are perfectly ok.

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How do you know this? You examined every micro-inch of the datalayer of the CD under a microscope?

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Originally posted by John Anderton
The disc isnt always readable. How do you fix it Cause imo it cant be ....

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You (can try to) fix it like you fix any other CD. Though it is never sure if it will repear every damage.