On 1/2/14 6:19 AM, Al Kossow wrote:
I haven't looked at how far back the original FM
decoder design on DECtape controllers goes. Doing
Manchester decoding per channel instead of using a common clock track would have helped
with head
skew issues.
In this little thought experiment, I had forgotten the clock speed is variable +/- 10%
because the
reels aren't constant speed. Clock recovery from the Manchester signal with that much
variation would
have been a pretty complicated circuit for avoiding some track skew, which can be fixed
with a few
shims on the head block (no fun to do on a TU56, if you've ever looked at the
instructions on how to
do it, but much cheaper).
With a microcontroller generating the clock track, it should be possible to vary the
frequency of the
clock track when recording it, which would get around the little problem that DEC made
later tapes shorter
so that you can't format as many blocks on them as you originally could. Back in the
day, you got around
the problem by putting your thumb on the reel as it got faster while you were formatting
it.
Of course, that got me thinking about putting tachs on the reel motors and redoing the H
bridges :-)
But that would be too much of a hack to the original hardware. What I'm doing is all
reversible.