How to repair "holes" in reflective surface

[FredThompson]

I've run into this with audio discs more than DVDs but the concept will probably apply to both.

Discs are read by laser light passing through the data layer and reflecting back from a silvery surface, right?

What happens when that silvery stuff is gone? No data, right?

OK, Alcohol 120% will allow creation of a disc image that includes these holes which can then be loaded into its virtual drive.

However, that won't allow recovery of the data layer. Assuming the data layer is ok, are there any tricks to replacing small areas of the reflective surface? I'm thinking about the little "pinholes" that sometimes happen, not large scratches. Is there some form of silver paint pen or something like that which will work?

[Atamido]

That hole in the silvery stuff is the missing data. Since data is actually recorded on the silvery stuff, filling it in with silver paint would be like filling it with zeros. Well, really it would be random data since it wouldn't dry perfectly smooth and there is the 3 layers of ECC that would be screwed.

[FredThompson]

Yeah, I pretty much figured it's gone. Gonna try Alcohol and see if the resultant image can be ripped yielding a filler of nothing. Hope this works.

[FredThompson]

Well, that actually worked, at least for an audio disc. I made an Alcohol image with all the options (sub-codes, etc.) and it mounted just fine. Audiograbber was happy with it. I'm sure there are gaps but it's better than nothing. Wonder how it would do for DVDs...

[Soulhunter]

Since data is actually recorded on the silvery stuff, filling it in with silver paint would be like filling it with zeros.
Really ??? I thought the data is saved by "melting" holes in a plastic layer between the silver-back and a clear-plastic-front... So that light gets trough the plastic and gets reflected from the silver backside... The "melted" arrears eat less light, so more gets reflected, not melted arrears eat more light so the phase is changing (I/O)... Everything completely wrong ???

Bye

[FredThompson]

IIRC, the data layer is pressed. CDR and CDRW are done with phase-change dies. There is no melting. The silver is right against the data layer and is only a few microns thick. If the silver is scratched off, the data portion is probably destroyed.

[Soulhunter]

CDR and CDRW are done with phase-change dies. There is no melting.
Good to know !!! But how is the phase-change done... :confused:

Anyhow, THX for the info... ;)

Bye

[Kedirekin]

On a pressed CD, the thicker front plastic portion of the disk is "pressed" from a master, giving the plastic pits and lands. Aluminum is then applied to the plastic via vapor deposition, giving the CD its reflective surface. A lacquer is then applied to the back to protect the aluminum.

In truth, all of a pressed CD is reflective, but the optics works because the pits (or the lands, don't know which) are are out of focus and thus diffuse the laser, which results in less light hitting the optical pickup.

I suppose theoretically one could repair a CD that had lost it's aluminum as long as the pits and lands weren't damaged, but it's very unlikely that the pits and lands would survive an event violent enough to remove the aluminum. In any case, I think it would require something much more precise that painting the back of the CD with quicksilver.

As FredThompson points out, CDRs are completely different. They emulate pits by changing the reflectivity through the dye layer, rather than by gouging out (or burning) pits in the substrate. Again, theoretically you could repair a CDR that lost it's mirror surface as long as the dye layer wasn't damaged, but that is extremely unlikely.

[FredThompson]

There are a couple of places that will do repair to the reflective layers but it's very expensive. As I noted above, Alcohol's disk image method is good enough to get a read although there will be data loss.