Naked Science Forum
Non Life Sciences => Technology => Topic started by: neilep on 21/08/2009 12:01:15
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Dear Cd Peeps,
As a sheepy I of course luff CDs and I luff to keep them clean.
But I have been told that I must clean my cd thusly..
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In straight wiping movements from the inner to the outer edge.....and specifically I must NOT clean CDs in a traditional rotational wiping movement !
Why's that then ?
Thanks
Neil
xxxx
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When you clean the CD you actually always scratch it a bit.
A CD has a single track that starts in the middle, and spirals out to the edge.
When you clean it from the inside out the scratches go across the track at right angles and there's a small gap which the laser can't read.
The thing is, CDs have error correction codes- to slightly oversimplify, the information is written about 4 times around the disk, so if there's a small gap in the track, it can use the 2nd copy from further around, and if that's got it uses the 3rd copy, and if that's scratched it gets the 4th.
Basically CDs handle radial scratches really, really well; although even then if the CD is completely covered in dirt it can still fail.
No, the big problem is circumferential scratches.
What happens then is the track is completely gone where the scratch is for several inches, but you might think that's OK, it will use the 2nd or 3rd copy from further around.
Unfortunately, the CD reader loses the very thin track entirely, it jumps the track and lands on a different track, and the CD reader fails. If you clean it in the normal way you end up with too many circumferential scratches.
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I think you'll find that the data on a CD isn't duplicated and "written about 4 times around the disk". Rather, the data is interleaved so that logically consecutive data blocks don't physically follow each other on the media. This means that if the CD gets damaged or scratched, the damage or scratch is unlikely to run across logically consecutive data blocks, which for audio CD means that the missing data, or music fragment in this case, can be interpolated from the data either side.
For data CD-ROMs, the interleaving of the data blocks is combined with additional error correction techniques, as simple interpolation for pure data doesn't work, but the fact that you don't lose consecutive data blocks still helps things somewhat.
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I did say I over simplified it. What they actually do is not duplicate the data, but use an error correcting code- the data written to the disc has a degree of redundancy, they write some percentage of extra data overhead.
This extra data means that up to some percentage of lost data they can work out which bits were damaged and calculate what they would have been; they can do a perfect repair.
Interleaving is also used very heavily, if they didn't do that, then losing a section due to a scratch would wipe out the data as well as the data that allows them to repair damage! So they interleave the data with other data further around; about 1/4 of the way around the disc.
Here's a simple example of a data correcting code:
1 0 1 1 = 1
0 1 1 0 = 0
1 0 0 0 = 1
0 1 1 1 = 1
= = = =
0 0 1 0
What I've done on each row and column is if there's an even number of 1s I've put a 0, if odd, there's a 1.
What this means is that if you change any single number in the table from 1 to 0 or 0 to 1 (if there is an error), then, by comparing it with the overhead you can work out which one it is and fix it, by discovering which row and column is wrong and then flipping the bit. It even works if the single error is in the correcting code- in that case you'll get an apparent error in just a row or column.
More sophisticated error correcting codes can handle multiple errors.
But the error correcting code isn't the problem, with circumferential scratches the tiny laser dot reading the disk gets lost and ends up pointing at and reading entirely the wrong part of the disk!