[HN Gopher] Demystifying DVDs
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Demystifying DVDs
Author : boltzmann-brain
Score : 68 points
Date : 2025-12-29 03:25 UTC (2 days ago)
(HTM) web link (hiddenpalace.org)
(TXT) w3m dump (hiddenpalace.org)
| boltzmann-brain wrote:
| 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.
| enoent wrote:
| 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.
| compsciphd wrote:
| 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.
| bri3d wrote:
| You are both correct and the article discusses it accurately:
|
| > Then you have 2048 bytes of user data, scrambled for the
| reasons mentioned before. The best way to look at the sector as
| a whole is to think of each sector as 12 "rows" consisting of
| 172 bytes each. After each 172-byte row is 10 bytes of ECC data
| called Parity Inner (PI), which is based on Reed-Solomon and
| applied to both the header and scrambled user data per row
| within the sector itself. Then, after the user data and parity
| inner data, is the 4-byte EDC, which is calculated over the
| unscrambled user data only. Then, finally, Parity Outer (PO) is
| another form of ECC that is applied by "column" that spans over
| an entire block of multiple sectors stacked horizontally, or in
| other words, a group of 16 sectors. Altogether, this adds up to
| 2366 bytes of recorded sector data.
| userbinator wrote:
| 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
| sandreas wrote:
| I did some research in dumping damaged dvds lately because I had
| some little treasures laying around that were just unusable.
|
| I had good success using DVD Decrypter (hardly available, I got
| my copy years ago) ignoring bad sectors in the settings,
| Redumper[1] and, if DVD is only scratched very badly using car-
| anti-scratch / polish chemicals with a lot of patience [2] (some
| hints: Toothpaste does not work unless it is whitening
| toothpaste, don't use tools like a drill, will make the problems
| even worse, prefer the slow process of polishing by hand).
|
| 1: https://github.com/superg/redumper
|
| 2: https://www.youtube.com/watch?v=AN8ET7axaNk
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