1

I have 3 copies of a CD-ROM that came with a book, and all of them came cracked.

By the third time, I thought that I could probably recover the information using these 3 different discs and completing the missing bits from one another. It seems that just the beginning of the CD cotains information, maybe a few Megabytes, so I could try to do this.

I know it's extremely difficult to do by hand given the amount of bits, but it could be useful to later automate with electronics, knowing exactly how a CD-ROM looks like microscopically, given that a standard CD/DVD drive doesn't seem to give such a low level access but instead just returns errors.

So, how much magnification is required to see the lands and pits, the tracks, and all of the actual structure that would otherwise be seen by the laser of a CD/DVD drive?

Would 100x would be enough, or would I need something more like 600x or 1000x to see an area that contains 650-700 Megabytes?

Or do you know some way to make a CD/DVD drive return every readable bit as well as indicate the unreadable bits so I can then complete the data, even when the drive doesn't seem to detect a CD?

alt.126
  • 171

2 Answers2

6

A 700 MB CD-ROM will have 360,000 sectors, each of which have 2,352 bytes, each of which have 8 bits. So that's 6,773,760,000 1's and 0's you'll be reading with your microscope. If you can read and record 1 bit per second without error for 12 hours a day, you'll be able to get the data off your disk in about 430 years. A microscope that could discern 500 nanometer wide pits wouldn't be exceptionally hard (below is a picture taken at 100X magnification), so it's your call if you think you would be okay staring at images like that for the next 430 years or if you'd prefer a little more fidelity.

Community
  • 1
Aron Foster
  • 475
4

This is a very bad idea.

First off, you're going to have to replicate each of the lands and pits onto a new CD. Not easy.

Secondly, let's say you are trying to recover just 1MB off of a CD by recording each land and pit you see. If you record per bit, you'll be reading upwards of 8 million bits. Now, let's say you can do 2 bits a second. That would be over 46 days of solid work. No eating, drinking, or sleeping. You spend 46 solid days of your life recording each little bit off of a CD (1111 hours). Recovering a full CD (700 MB) would take over 177.5 years.

Third, data forensics of the sort already exist. As long as the cracks aren't near each other, a system could theoretically recover data from all three drives. It would be expensive and slow, but it would be possible. (NB: Another caveat of this is that all three disks must be the exact same. Literal clones of each other. No changes to any files in any way, shape, or form).

Fourth, you'll most likely be able to contact the book's publisher or just download the resources online. They're everywhere.

To answer your other question, it looks like you can see CD bumps/grooves at 1000x, so you'll probably be able to do data recovery somewhere around 1000x.

Forcing a CD to read "unrecoverable" bytes is not really possible.

pits and lands

The long distance between the land and a pit will be recorded as all zeros, or corrupt weird data. Not going to work.

Chindraba
  • 2,058
Kaz Wolfe
  • 701