Alternative to MIDI-CV revisited

[Tom May]

(Wow, I got quite an oscillation going by applying a small shock to
Don & Gene :-)

Don Tillman writes regarding the cap-discharge F-V converter:

>It's not too bad though; if you model it you'll see that it can come
>remarkably close over a range of a couple octaves.

I checked this out.  I've got a starting point which I determined by
matching the ideal vs. cap-discharge curves at three somewhat
arbitrary points between 82Hz (hmm) and 1000Hz to come up with some
circuit parameters.  The match is actually pretty terrible (except at
the three points where it is forced to be equal), but I'll let my
computer loose on it and see if it can't optimize it a bit.  Maybe
reduce the number of octaves I'm trying to match as well.  Just
looking at it, the -log curve looks more like the sum of *two*
decaying exponentials, but that would be a hard circuit to calibrate
in real life.

>Further, if you
>set a fixed time on each cycle to charge the cap you can extend the
>range a little further.

In a later message, Don said:

>The curve runs higher than it ought to be for lower pitches, lower
>than it ought to be for higher pitches, and very reasonable for a
>couple octaves in between.  If you use a fixed-time portion of the
>cycle to charge the cap, say 1/3 the period of the highest expected
>pitch, the high pitch end of the curve gets corrected up a little bit
>and you maybe get a three octave range.

That's too cryptic for me . . . are you talking about not charging the
cap all the way?  That would seem to have the opposite effect of what
is needed, so you must mean something else.  Maybe you mean using a
one-shot to supply an additional charging voltage (through a resistor)
while the cap is discharging, which would be just the same as
switching another resistance in series with the cap discharge resistor
at the start of each cycle to get a slower discharge for high
frequencies.  That would start to look more like my "sum of two
decaying exponentials" idea.

So what *do* you mean, Don?

Tom.