Demystifying Dvds

So I've recovered a lot of damaged DVDs and I think in my research it showed that DVDs also do ECC across larger than the 2048 data blocks (maybe 16 of them?)

So when I used ddrescue, I would read in that block size (instead of just 2048) as if I would get lucky and get a good read (or enough signal that ECC could repair it on the large block).

This was very effective at recovering DVDs with repeated reads vs when I had previously done it with 2048 byte reads only I would end up with 2048 byte reads scattered all over (which if ECC is done on 16x2k 32k byte block size, means there was a lot of data I was leaving on the floor that should have been recovered on those reads).

Ddrescue was also good for this in the sense that if I was trying to recover a DVD (video) from multiple damaged DVDs, as long as they were not damaged in the same location, i was able to fill in the blanks.

Perhaps you can correct me about the 16 block mechanism, perhaps it was just random that it worked and my understanding at the time was wrong.

[delayed]

I have a burned CD with some bad sectors that I'd like to recover (it's not a game, it's a rendered animation from 1999… and unfortunately, it's zipped to save 0.1% or so); this makes me want to go hunting for a couple hundred CD-ROM drives to save it…

The whole article is about the heroic efforts to dump a DVD that has bad sectors by using a combination of different methods that ultimately yielded a fully read disc.

Someday I hope we see the equivalent of something like greaseweasel for optical media where someone can collect an image of the disc itself that can later be postprocessed using software to extract data.

Which drives and parameters for the READ BUF SCSI command yielded the expected 2366 bytes per sector? I imagine that it was combined with seeks to each sector before reading from the buffer (as it would be harder to isolate multiple sectors data in cache?).

It seems like it was a follow-up from previous bruteforce efforts, which include a spreadsheet with various results, but it would help to have some conclusions on which were best: http://forum.redump.org/topic/51851/dumping-dvds-raw-an-ongo...

Also, couldn't find any source/download for DiscImageMender.

Extremely interesting read. I need to go back over it again in detail on my computer not just my phone while holding my baby.

A key theme in a future fiction I am writing (slowly) is that all digital data has been lost and the time we are in now is known as a digital dark age where little is known about our society and culture. Resurrecting an archeologically discovered DVD is a key plot point I am working through. That it will be the first insight into our time in over a millennium. Other conflicting interests will be finally succeeding at re-introducing corn at commercial scale after all hope had been lost and past attempts at re-germinating from the frozen seed bank had failed for hundreds of years. It's a work in progress.

As someone interested in preservation, the question that comes to mind is what modern/maintained tools are able to use all this information to make good dumps.

I was going to post: Isn't this information about 15 years too late? But I can see by the comments, it's not.

I've done something similar a long time ago; using raw read commands, reversing the descrambler output, and then statistical accumulation on the actual bitstream. By showing the output in real-time on a bad-sector you can actually see the signal appearing above the noise.

It's strange to see no mention of cleaning the drives themselves, although maybe it was implicit --- if you have a pile of old drives sitting around, chances are they're not going to be perfectly clean. A tiny bit of dirt on the lens can have a huge effect on the read signal, especially on a marginal disc.

Related article from 18 years ago: https://news.ycombinator.com/item?id=21242273