Recover Data From a Bad Disc, Restore Damaged CD/DVD [GUIDE]
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By the time most people learn that optical media are not bullet-proof, it's too late. :(
The internet is full of horror stories about "lost data" and "dead discs", and most often the blame is put in the wrong place (on the format itself, instead of the lack of testing or use of poor quality media). Understand that much of the time, it's the DVD hardware at fault, your disc isn't really bad. Consider trying a better quality DVD drive, known for its reading quality, such as a Pioneer ROM/burner or a BTC burner. Many burners, such as Samsung/Sony, LiteOn and LG, are not the greatest at reading discs. What To Do When You Find a Bad Disc When you run across bad discs, there are two things the must be done:
How to Recover / What Software to Use Use one of several programs to try and extract data.
What Next? In the future, use only good media, and test the discs if the data is important. :) |
I think there's more you could add to this FAQ. I have used two disc scratch removal products. One is a handheld repairer. You crank the handle, and a felt band rubs across the disc as it turns. The disc must be wetted first to reduce friction.
Another type is a Philips motorized repairer. It has 3 discs, from rough, to polishing, to cleaning. You use them in order. I practice though, I just use the rough disc and it works with one 2minute session. Another tidbit I could add. Why do they always say to clean discs from the center outwards? Well for CD's I can say for sure, it's how the error correction is layed out on the disc. I can explain it very simply as follows: if you store data, add an extra digit which is 1 if there's an even number of 1's in the data. So 1111 adds 1, 1110 adds 0. Then if there's ever the wrong number of 1's, you know there's a problem. Great, this is called parity and detects an error. Now arrange the data in a two dimensional table. Add the parity for each row and column. Now, not only can you detect the error, but you can pinpoint the exact point at which it ocurred. To fix the error, just flip the bit. Good data: 11 1 01 0 -- 0 1 Bad bit: 01 1 01 0 -- 0 1 You can see that row 1 and column 1 have the wrong parity, thus bit (1,1) should be flipped. Yep, that's basically how error correction works. You can read more about Hamming codes. There are other codes and it's pretty cool how they work. The one on CD's is called Reed Solomon code. There are codes to detect insertions and deletions, bits flipping in just one direction, and other variations. Satellite transmissions and cell phones use other types. You can read about Turbo Codes which are close to theoretically ideal (best protection per extra data), but also fast to decode. Anyhow, the error correcting codes are stored circumfrentially, so scratches from the hub radially only affect several bits, which is easily correctable. I believe up to 1mm scratch is correctable, I'd have to look it up. |
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It's basically a really shoddy hand-crank version ($50) of the professional RTI DiscChek devices ($3k+). Want to buy a DiscChek? They're on Amazon: http://www.amazon.com/gp/product/B00...SIN=B001J5TH1S There's just really no comparison. I know that's quite costly for casual home users, but $50 for the cheap crank is a bit of a kick to the guts, too. You'd generally be better off locating a local video service or DVD rental store, which will charge a few dollars to clean your discs. A local rental place charge $3 per cleaning, and even a dozen discs is cheaper than than the crank -- and with better results. Quote:
And there is still a risk that you'll just make the disc worse -- something that almost never happens from an RTI DiscChek. Quote:
Thanks for posting. :) |
I have to agree with you in a way, it's obvious that my hand crank device leaves new scratches in a specific pattern. My motorized device is better in this respect. However, it has saved some discs for me. If a disc still isn't made better, I just take it to a video rental place who will fix it. Some people use their power tools with a buffing attachment. These generally make a much smoother looking surface.
As for DVD error correcting codes, you are probably referring to the C1 and C2 error rates. You can read highly technical tests of these types on www.cdr-info.com. C1 errors are ok and are normal. C2 errors on data disks are bad. If beyond a certain point, you've lost data. I can't find any good references right now, but I had a book on this. I believe they are based on Reed-Solomon codes. These are specified by (n,k), which means the number of bits it takes in total and the number of bits it protects. A (255,223) RS code protects 223 bits with the addition of 32 bits, totalling 255 bits. A cd typically has 2352 byte blocks which protect 2048 bytes of data. The RS code has the least amount of overhead for what it protects, for that type of code. You can theoretically protect data with almost no overhead, this is what Shannon's Theorem states. We have come quite close to this limit and it's used practically today in satellite transmissions and cell phones. Quite amazing really. |
ps could you comment on my entry to the disc labelling FAQ? I put a lot of research into that. Short answer, disc labels are actually OK and the weight doesn't make it wobble. The problem lies in the tension added by the label; if too stiff it pulls the disc back towards the center which makes it wobble.
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Thanks, you have significant knowledge! I wouldn't necessarily dismiss 'peanut butter' as a type of abrasive. In the food industry, one term oft-discussed is 'mouth-feel' which actually relates to texture. It's quite possible that even peanut butter has a well-specified granular size. I can't quote you the exact micron-dimensions, but it's quite likely to be within a typical 'spec' for ground products. After all, all grinding is done by the same processes. This is only an opinion however. I've seen specs for example for solder paste which is very highly controlled. This is an example of 'happenstance' which gives you a product of specific granularity which is useful for polishing. Such coincidences are the boom of DIY processes. Very clever in fact, so don't dismiss out of hand!
Look up 'Jerri Elsworth' on YouTube, she is one my my favorite DIY'ers (featured in 'MAKE Magazine', a source of DIY projects, for those who like to discover 'how things work'.) |
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