aek at bitsavers.org
Wed Feb 3 11:55:28 CST 2021
On 2/3/21 9:43 AM, Paul Koning via cctalk wrote:
> A nice benefit of capturing the raw waveforms and post-processing them is that you can do all sorts of very complex processing. If the media are nice and clean then simple processing is sufficient. If they are badly damaged, you may need more. If you do the processing in real time you may not know what all is needed. But if you do post-processing, you can add to the algorithms after data capture has been done, if what you have so far isn't yet good enough.
> I can imagine techniques like digital filtering, adaptive filters, maximum likelihood decoders, etc.
> In recovering data from tapes, with multiple tracks, people have often done this same sort of thing, full high bandwidth analog signal capture. You don't even need to know at the time what the data format is. If you think you know but you don't have it quite right, no matter, you just change the software and run another pass through the captured waveforms. No need to run the (possibly fragile) media through the machine again.
In the real world, this is fundamentally wrong.
You need to know you haven't captured garbage while the disk is still in the drive
and you want to minimize the time you spend dwelling on an individual track.
Magtape is a completely different issue where you attempt to get as much information as you can
before the tape sticks, tears off all of its oxide, or the head clogs.
Even if it doesn't stall, the motor can drag and goofs up the tape speed. This is an issue
for NRZI media.
We (CHM) have had amazing luck recovering 50+ year old 7, 9 and Whirlwind tapes with analog
recovery. See Len Shustek's talk at VCFW 2001 about the recovery software.
There is some hardware work going on to recover double-sided floppies using A/D channels
digitizing both sides of the disk simultainiously, and recovering 4-track pre-QIC cartridge
tapes the same way.
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