| Author: | Nate (guest: search) |
|---|---|---|
| Date: | Fri, Jan 06th, 2012 @ 14:36 ( . ) |
I assume you're using a histogram (frequency count) to determine timings for the 4 density options? If so, that's a reasonable approach. One thing you can use is that even protection schemes that used variable density within a track didn't use the scheme for every track. So you can just collect a single histogram for all tracks and you should see 4 peaks. For vmax, I think it didn't use all density values so you might just have 3 peaks. Another thing you can use is that you don't need all the peaks, just the distance (time delta) between two of them. Then you can fit this to the predicted other values, based on the divider circuit. If you have an RPM measurement, all of that is unnecessary. You could also perform the above testing once on a "normal" disk image, then use it as a stored drive profile. As long as its speed didn't vary too much, it could be used to analyze other protected images, even those with variable-density in a track. Finally, you can detect variable density easily by just looking for the shortest bit transitions in the track (a run of 1's or a sync, usually). Match those to your histogram to figure out which density setting it was. Now, run a window across your track, starting with that value and a prediction for where 0's will occur (no transition). Once you find that none of the samples in the window fits your profile, the density has changed. Find the shortest time between transitions in the window and fit to the histogram to see the new density selection. Repeat. I'm interested in hearing more about your findings on this. Thanks. |
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