Quote:
Originally posted by blackcheck
i don't understand how you can compute this by timing, could
you explain ?
should be easy to calculate the length of one 'ring' with timing,but the angle ?
yeah, well i guess it's my lack of knowledge about cds...
how is a single sector arranged on the cd (physically) ?
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Sectors are arranged in a continuous groove (rather like a record) but instead of a constant angular speed (45 rpm) it is a constant linear speed. This means that there are more sectors in an outer loop of the spiral than on an inner spiral.
I worked out once that each sector takes a linear distance of about 14mm (this is not exactly true since adjacent sectors are combined to improve the error correction.
Imagine two sectors, sector 1 on the inner loop of the spiral and a sector on an outer loop (say sector 200000). If the disk drive is told to read sector 1 and then sector 200000 then there will be a delay between the data being returned for each sector. This delay will be made up of several components.
1. The operating system overhead.
2. The time for the drive to seek from the inner to the outer sector.
3. The time for the sector 200000 to rotate under the read head.
Now (1) can be ruled out since it will be fairly constant for each read (and very short anyway) and will cancel out if we are looking for timing differences.
(2) will be fairly constant and will depend upon the drive.
(3) will depend upon a few factors, one being the angular distance between the start and end sector. The other being the rotational speed of the drive. We can assume that the rotational speed is constant and will cancel out.
By making several measurements between sector 1-200000, 1-200001, 1-200002 etc. it is possible to work out which outer sector has the smallest angular distance to sector 1. This is what I believe CD-Cops measures.
One flaw with CD-Cops was that it always measured between the same sectors. If you measure these timings and then 'play them back' on another copied disk then you could fool the protection into believing that the disk was an original.