Alternative to MIDI-CV revisited

[Tom May]

I was thinking about the frequency-to-volts/octave conversion problem,
wondering whether it would be possible to use the exponential decay of
a cap (dis)charge somehow, when I happened to find the following in
the archives while looking for something completely different.  On
Thu, 8 Feb 1996 14:08:14 -0800 (a long time ago indeed), Don Tillman
<don at till.com> was discussing frequency-to-voltage converters with
Gene Stopp and wrote the following:

>There's a better way.  Start the beginning of each cycle of the input
>signal with a cap charged to a reference voltage.  Discharge that cap
>exponentially with a simple resistive load.  S&H that voltage at the
>end of the input cycle.  You now have an volts-per-octave F-to-V.
>Simple, all analog.  (No, I haven't built it yet.)

Wait, this won't work at all.  I could work out the math, but consider
the two extreme cases: at infinite frequency the cap will not
discharge at all so you will get the reference voltage.  What you
really want is positive infinity.  At zero frequency the cap will
discharge completely and you will get zero volts when what you really
want is negative infinity.  This scheme compresses the entire
frequency range into a voltage that ranges from 0 to the reference
voltage with some kind of bizarre mapping.

Too bad.  It *is* simple.  Gene, did you say you actually built a
converter like this?

Tom.