Velocity Sense and Sensability

[WeAreAs1 at aol.com]

Hello DIY list,

I've been trying to come up with an elegant solution to a tricky little
design problem, and I'd like to know if anybody has any ideas on the subject.
 Here's the problem:

I use a Roland MPU-101 to do MIDI-to-CV stuff.  It has a Velocity CV output
for each of its four CV voices.  This output is 0 to 5 volts, I believe
(maybe it's 0 to 10 volts - I can't remember right now..).  I would like to
build a circuit into some of my synths that would allow me to attenuate the
synth's Filter Envelope with the MPU-101's Velocity CV.  Of course, this is
easily accomplished by running the envelope output through a standard OTA
circuit (3080, 3280, 13600, 13700, etc).  

Here's the tricky part:  I would like there to be one knob on the synth that
acts as a "Velocity Sensitivity" knob, and I want this knob to behave like
the velocity sensitivity knob on (for instance) the Roland MKS-80 Super
Jupiter, or like the Velocity Sense parameter on a DX7 FM operator.  This new
knob will be in addition to the synth's already existing Filter Envelope
Modulation Amount knob.  (for the sake of this discussion, let us assume for
now that I'll be installing this into a Minimoog, even though the principles
remain the same for most analog synths)

Why is this problematic?  If I simply run the Velocity CV through an
attenuating pot (labled "Velocity Sense") and then send it to the control
input of an OTA (the one with the envelope running through it), the following
would result:  As I turn the "Velocity Sense" knob DOWN, there will be less
and less voltage getting to the OTA, and the filter envelope amount will also
go down.  With the knob fully off, the OTA would be at full attenuation, and
the apparent Filter Envelope Amount would be at ZERO.  Hence, the synth would
act at all times as if you were playing with very low velocities (even though
the Minimoog's Filter Envelope Amount knob may be turned all the way UP).  In
my opinion, this is NOT how a "Velocity Sensitivity" knob should behave.

Here is how I want it to behave:
-----------------------------
* With the Velocity Sense knob fully open, the envelope will be allowed to
reach its peak voltage with a velocity of 127, and would be fully attenuated
at a velocity of 0.

* With the Velocity Sense knob fully closed, the envelope will be pased
through the OTA and allowed to reach its full amplitude, at all times,
without regard to the current incoming velocity CV.  (it would basically act
as if there were no velocity-controlled OTA, i.e - the Filter Envelope Amount
would be the same at all velocities)

* At medium settings of the Velocity Sense knob, the envelope output would be
*partially* attenuated, and could be offset back to a higher level by higher
incoming Velocity CV levels.  However, if you played notes with very low
velocities, the envelope amount would not be attenuated to Zero, as it would
if the Vel. Sense knob was all the way open.  As you further opened the Vel.
Sense knob, the OTA would require greater and greater Velocity voltages in
order to be fully opened up.

Now, maybe there's some real obvious solution to this problem, but so far it
hasn't occurred to me.  I've drawn up several different circuits which can
accomplish the desired effect, but none of them are very elegant or easy to
build.  I even built one into a Minimoog at one time, and it worked
reasonably well, but it required an oddball dual-ganged pot and several
offset trimmers.  Frankly, it was more trouble than its musical value was
worth, and I don't look forward to building more of this particular design.  

I could post a schemo of my old design, but I would much rather see how the
rest of you would approach this problem, first.  I hope to come up with
something new that's simple and effective.  A low parts count would be nice,
but not nearly as important as musically functionality. (However, I would
like to avoid using dual-ganged pots, if at all possible..)

If anybody has any ideas on this, I'd love to hear from you.  I'd be happy to
attempt to further clarify this concept, if I haven't yet explained it
clearly enough.

Michael Bacich 
WeAreAs1 at aol.com