13 Sep
2004
13 Sep
'04
10:21 a.m.
John Foust wrote:
I like this approach in theory. I think it has great
long-term
potential for data recovery and preservation for our hobby.
Certainly, if the original hardware can't reliably recover data stored
on disks or drums, this is a good way to go. The more information you
have, the better chance you hace.
Anyone care to give a thumbnail sketch of what
sampling rates
would be necessary to digitize the signals on recording media
(3 1/2", 5 1/4", 8" floppy, hard disks, drums, disk packs, etc.)
at sufficient rates to preserve the encoding for post-processing
in other tools?
I saw one reference online that said 3 1/2" and 5 1/4" floppies
were 250-500 kbit/s. If we were digitizing at a much higher
rate than the original simple circuitry that might've only
detected flux traversals, couldn't we recover more data?
Well, 300kb/s would suggest that if it went 10101010 then you'd have a
150kHz signal. Now, possibly it uses some kind of self-clocking or NRZ
code, so it may be 300kHz. I'd say that 8 bits of resolution would be
plenty, and perhaps four times the data rate, so a sample rate of 1.2MHz
might well be adequate. That would be well within the range of
(relatively) cheap A-D converters.
Obviously, the higher the resolution, the slower the conversion. Most
A-D converters (and I'm sure many, if not most of you know this) use
"successive approximation" to derive the correct value - get a D-A
converter and a comparator, then loop through "toggle the MSB, is it too
high or too low? Toggle the next bit, too high or too low?" all the way
down. You get "flash" converters for video (or you used to) which were
32 or 64 individual comparators and voltage references, and some
encoding logic to give you a binary output. Overkill really, but very,
very fast. If you found a fast 6-bit flash converter cheaply, that may
well be enough resolution for your track scanner. We're not going for
Dolby Digital quality here...
Gordon.