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TrueDisc Error Correction for Disc Burning?
An anonymous reader asks: "Macintouch has a link to a new piece of software — TrueDisc — which claims to make data burned to record-able discs more reliable. More specifically it uses interleaved redundant cells to rebuild data should part of the disc be scratched. On the developer's blog they say they plan to create an open-source implementation of the TrueDisc system, now that it is not going to be included in the Blu-ray/HD-DVD standards. Have any of you used this software before, and what alternatives are already available?"
There's already good parity software available. Parchive will create redundant data that can be burned on the same disc or a separate one. You can create up to 100% redundant data so even if the original disc is lost you can completely restore the files. The software is free and open source. The windows version is called quickpar.
what alternatives are already available?
RAR compression has an option for redundancy. You set what % you want to be able to recover if it becomes corrupted.
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Then your overhead is 100%. They promise an overhead of 14%.
There are much better error correction schemes than "duplicate the data" -- look up Reed-Solomon.
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In Mode 1 CD-ROM, for every 2048 bytes of data, there's 276 bytes of Reed-Solomon error correcting code and 4 bytes of error detection. Considering we're talking bytes, that's pretty reliable, and as you know, a single scratch often doesn't render a CD totally useless. Usually, a CD has to look like an ice skating rink after an hour of skating for it to fail miserably, and light scratches, even in high numbers, are generally not a factor.
So what the hell? Why is this even necessary, unless you're using a Mode 2 CD (and then, Mode 2 is usually used for videos/streaming data, which requires a more sequential read, where adding ECC would defeat the purpose).
Waste of money.
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Maybe because losing the exact same bit position in two different thirds of the disc is somewhat likely? In some scenarios (uniform degradation of the disc), you only need about sqrt(n_of_blocks) random corrupted blocks before an unfortunate collision of error is likely, by the birthday paradox. On a CD with 360,000 blocks of 2048 bytes, sqrt(360000) = 600, so about 0.167% of corruption and RAID 5 fails.
Get some Brasso (brass polish) and a soft, lint free cloth and you are in business. Really. You just polish the surface so it's all even and thus reflects light equally. If you are nervous about using Brasso, there's a number of products designed just for this purpose, though they are way more expensive and Brasso does just as good a job.
Either way the point is that with error correction as it is now, it's not hard to fix a CD if needed.
Music CD's already include error correction bytes embedded in each frame of data, so I assume this technology does the same sort of thing for data CD's/DVD's/bluray's/etc..
On music CD's, there's one error correction byte for every three bytes of data. That's a lot more space-efficient than just burning your data twice.....
Music CD's have piss poor error correction, by data standards. CD-ROM and DVD-ROM (which includes the video variant, since it's an application of DVD-ROM) have much more robust error correction. There is more error detection (and correction) per block on a CD-ROM (consequently, less for data) than on a music CD. Music CDs have the additional advantage of not needing to be precise; it can try to guess (interpolate) the missing data it runs into, or, at worse, skip (which may or may not be noticable). Can't do that with a spreadsheet.
Burning your data twice also has the advantage of being able to separate the copies (to different physical locations). Error correction technologies aren't going to help if you CDs and DVDs are roasted in a fire; the extra copy you made and put into storage elsewhere will still be safe.
> In general the ECC on DVD is going to prevent you from getting bad data; it's extremely unlikely that you're going to be able to successfully read an ECC block with (say) some bits flipped.
I burn my movie collection on DVD with par2 blocks and md5sum files. When I verify them, with some disks in some drives I get data errors. So I have seen in practice that you sometimes get silently corrupted data. My NEC-3500A burner is starting to get old, and doesn't read as well as it used to, I guess.
Par2 is slow to generate, but worth it. I have actually recovered files from slightly bad disks thanks to my par2 files.
For a disc with lots of small files, par2 would suck because it doesn't know about directories, for one thing. dvdisaster is like par2 for iso images. I don't think it's set up so that you can keep the error correction files on the same disc as the data, though.
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From article: "Since the TrueDisc format is open and the master copies stored by TrueDisc are located in the standard ISO 9660 filesystem"
That pretty much fucks up anyone's day when they wanna burn a UDF DVD doesn't it? ISO9660 doesn't support files greater than 4GB, you can only have 8 directories deep (until the 1999 spec but I always had a hell of a time reading this stuff on anything but XP), stupid filename restrictions (and then do you use Joliet or RockRidge or whatever to fix it or not?)..
This $89 (or $52 intro price) TrueDisc sounds rather similar to the open source dvdisaster. It builds Reed-Solomon error correction data from CD or DVD iso images, which can be either augmented to the image and burned on the same disc, or stored separately. It's somewhat similar to par2/quickpar, but dvdisaster is more specialized for CDs and DVDs.
So far all the comments I've read are way off the mark about what is interesting about TrueDisk. Yes it's true that TrueDisk is just yet another error correction scheme. What is slick about it is it's high usability. This comes from two things
1) It writes the correction bits to a separate partition from the "regular" bits. As a result, the primary partition looks exactly like a regular CD. put it in any computer, even one not equipped with the TrueDisk Software and it can be read normally.
2) The amount of the redundancy is automatically chosen. It just uses any left over space when it finalizes the CD.
As a result the operation of TrueDisk is pretty much transparent. You only need to invoke the truedisk software to read a disk that has been corrupted. Uncorrepted disks can be read normally. So You won't lose your data if you don't have the software or the company goes out of bussiness and it stops working on newer OS's. (All you would lose without the software is the ability to recover from the redundant bits. ).
In comparison to PAR or RAR, you are not compressing the data so it's faster. Now I note that if you compress and then add redundancy you could potentially have higher redundancy for a given amount of data on a fixed CD size. So there could be some theoretical advantages to RAR and PAR. However, those PAR/RAR disks cannot be read in-place (they have to be expanded) nor in "real time" (say if you are playing video). They are very slow to write. They can't be read on any computer without the same verison of par/rar. And if you do lose bits beyond the point of recovery the compressed bits will span a much greater extent in the data space--you might even lose the entire CD with PAR/RAR. So you can see that TrueDisk has usability advantages even if it's redundancy is less and it's uncompressed.
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I agree, DVDisaster is quite nice, I've my main copies of backups, then a separate disc with error correction files with copies being held on a hard drive and eventually tape, other hard drives and any other medium I can...at least for the really important backups. My ogg collection I'm not too worried about (as I can always re-rip the song/album should corruption occur)