Alternative to MIDI-CV revisited

[Don Tillman]

   Date: Thu, 9 May 1996 11:25:03 -0700
   From: Tom May <ftom at netcom.com>

   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.  

Gack, I've been found out!  True enough, it won't work.  This brings
up two questions: Why not? and How close does it come?

It doesn't work because what's really needed is a log discharge curve
and not an exponential discharge curve.  (Ahhh!)

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

Another part of the curve comes pretty close for linear F-to-V, and
I'm in fact using this feature for a Mellotron servo motor control
board I'm building.  The prototype works very well, but then again a
Mellotron motor doesn't have a huge adjustment range.

I'm sure that with a little cleverness one of us could come up a
simple variation on this scheme that performs accurate logF to voltage
conversion.

  -- Don