31–40 of 55 posts
Another great restoration in all its technical glory is one of the flight computers from Apollo[0]
Earlier quoted context omitted.
Great that you're working on something similar! How are you finding those tools for handling degraded tape waveforms? I keep bouncing back and forth between hacking up my own vs. wanting to find some existing tool that has some clever math formula.
Both those I linked seem to work fine, though the tapes I'm recovering from are mostly in good condition. The Python one is a bit more robust than the one included with ti99sim, but much much slower and picky about DC offset. (Note that both of these also decode the TI's frame format; but it should be easy to pull out the core waveform decoder of either.)
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…
Some drives (like those for Atari 8-bits) were intelligent, so software didn't have direct control over the recording parameters. So publishers would create custom authoring drives that could write bad sectors on command; the program loader would check for a bad sector and refuse to load if it wasn't present. A more-clever technique would write two sectors in a row with the same sector number but different data in ea…
Hey @dang, this account has made many good comments since then.
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…
Some drives (like those for Atari 8-bits) were intelligent, so software didn't have direct control over the recording parameters. So publishers would create custom authoring drives that could write bad sectors on command; the program loader would check for a bad sector and refuse to load if it wasn't present. A more-clever technique would write two sectors in a row with the same sector number but different data in ea…
This is also done in some optical disc copy protections, although the identically numbered sectors are far apart, causing seeks from the end to return different data than seeks from the beginning of the media.
Good thing I bought it cheap from China.
Interesting they think it was written to after the dent. I'd think it'd error during write.
So, the operating system could verify the write by reading it back, but I don't think the BBC Micro disc ROMs typically did that for file writes -- only for formats.
> 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.
https://colab.research.google.com/drive/1Zvb6bQfC3thc5-39exp...
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…
How did you do this? Is there some SCSI/ATAPI command for this?
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…
It's just FM, on floppies known as "single density" mode: https://en.wikipedia.org/wiki/Frequency_modulation The common IBM PC and related formats use "double density" MFM: https://en.wikipedia.org/wiki/Modified_frequency_modulation
[edit] it actually IS mfm after all https://wiki.amigaos.net/wiki/Amiga_Floppy_Boot_Process_and_...