paulkoning at comcast.net
Wed Feb 3 12:18:43 CST 2021
> On Feb 3, 2021, at 12:55 PM, Al Kossow via cctalk <cctalk at classiccmp.org> wrote:
> 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.
Fair enough, but that means your real time processing needs to be sufficient to know where the tracks are. And if the media are in bad shape, you may in fact want to capture each track N times at slight offsets from the nominal position, then do signal processing to recover the data as best you can.
Come to think of it, the techique of reading 1/2 inch tape with 36 track MR heads is somewhat similar: you get multiple readings of the same nominal data track and can use the additional data to help with recovery.
> 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.
I will, thanks for the pointer. I expect to need this in the not too distant future (for reading 1/2 inch 10 track tapes).
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