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Looks to me like this is just FSK (https://en.wikipedia.org/wiki/Frequency-shift_keying), and if so that's great because it means you can use techniques more sophisticated than the ones in that post, which in turn means that you have a chance to go after more 'unreadable' discs.
> To store a "0" data bit, there will only be one peak in an 8us window; to store a "1" data bit there will be two peaks. Looks to me like this is just FSK ( https://en.wikipedia.org/wiki/Frequency-shift_keying ), and if so that's great because it means you can use techniques more sophisticated than the ones in that post, which in turn means that you have a chance to go after more 'unreadable' discs.
I'd be interested to see the results of mounting a much smaller head (eg. a hard drive head) to the floppy drive, and then creating a full 2D image of the disk surface magnetics. That should be able to read disks where the disk surface has stretched or warped, and the tracks are no longer perfectly circular. I think there's also a reasonable chance such a method could also be used to recover data on a disc that has b…
A more-clever technique would write two sectors in a row with the same sector number but different data in each. The program loader would rapidly demand that sector twice, getting different data as the disk passed under the read head. A copy program, however, would only copy each sector once and be missing the data from the sector with the duplicate number.
> To store a "0" data bit, there will only be one peak in an 8us window; to store a "1" data bit there will be two peaks. Looks to me like this is just FSK ( https://en.wikipedia.org/wiki/Frequency-shift_keying ), and if so that's great because it means you can use techniques more sophisticated than the ones in that post, which in turn means that you have a chance to go after more 'unreadable' discs.
Do you have any recommendations for any particular techniques or software? I'd gladly try them.
Here's some references: https://www.rfcafe.com/references/articles/wj-tech-notes/fsk...
http://edge.rit.edu/content/P09141/public/FSK.pdf
There's lots of matlab code floating around out there, if you're willing to try gnuradio that'll work (for example: https://nccgroup.github.io/RFTM/fsk_receiver.html), and this looks promising: http://www.whence.com/minimodem/
ETA: Audacity might be able to do it too : https://www.youtube.com/watch?v=tKNNnbgoGdI
Unlike HDDs, floppy drive heads do actually rub on the surface of the media in normal operation. This is notable because repeated "scrubbing" in an attempt to statistically recover data[1] can make things much worse, especially if the media surface (or worse, the head) is already damaged.
The attempt that bore some fruit was to instead locate the start of the sector data, and continually look at the next 8us of the analog stream. If the voltage appears to generally drift in one direction, then it's a "0" bit, or if the voltage peaks one way and then the other, it's a "1" bit.
More distinctly: if the signal difference between two samples 8us apart is approximately the same, it's a 1; if there's a large difference ("large" being something suitable and AGC-ish like a running average or similar), it's a 0.
[1] For optical media this is not a big concern, and I once wrote some code to recover degraded CD/DVD(+/-R) sectors by disabling the ECC of the drive and reading the raw data many times; it was surprisingly effective and interesting to graph the results as they were being obtained in realtime and see the bits literally rising out of the noise.
Oh wow, I did not realize that floppy discs used the same exact encoding that was used on tape drives of the era, e.g. the TI-99/4A [1]. The wave file in the article is the exact same sound I'd listen to as a child, loading programs from cassettes. Total coincidence -- I recently have been recovering games from my old TI-99/4A cassettes, also using an oscilloscope (the same model in fact; for no particularly good rea…
The common IBM PC and related formats use "double density" MFM: https://en.wikipedia.org/wiki/Modified_frequency_modulation
Doubtful there’s any treasures to be found here, but reading this article is giving me some inspiration to spend some time on this project again.
It could be that the disc read head is more sensitive, or able to hover closer to the disc surface Unlike HDDs, floppy drive heads do actually rub on the surface of the media in normal operation. This is notable because repeated "scrubbing" in an attempt to statistically recover data[1] can make things much worse, especially if the media surface (or worse, the head) is already damaged. The attempt that bore some frui…