Velocity Sense and Sensability

[Magnus Danielson]

>>>>> "W" == WeAreAs1  <WeAreAs1 at aol.com> writes:

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

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

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

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

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

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

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

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

OK, to sum this up in the way thay I grasp it:

You want this knob to control the range for which the velocity may
govern the envelope amount sent to the filter (or whatevelow
control amounts will it just pass the envelope througth unmodified.
With full velocity you may at any time get the full envelope amplitude
just like you did with the control knob closed.

I think this little math expression would suffice to explain my
thougths:

	C = ((1 - x) + x * V) * E

Which really says that the amount of envelope to be passed on is

	(1 - x) + x * V

This is the amount the OTA has to be controled by.

If you let yourself to use a double ganged linear pot you can easilly
let one of the pots be the (1 - x) part by putting the reference
voltage in reverse order over the pot (+10V at closed knob, 0V as
fully open knob).
The velocity signal you put over the other pot (no velocity signal at
closed knob, full velocity signal at open knob).

The taps should be buffered and summed properly.

This sum is then sent of to the OTA to ajust the envelope signal.

A diffrent way to accomplish the same thing would be to use the first
equation and instead let the pot directly do the crossfade between the
full envelope signal (in reversed order) and a envelope*velocity
signal (in normal order).

To go even further on that trail you migth take the envelope and
envelope * velocity signals properly buffered onto each end of a
signel linear pot and you can now work it as the doubleganged ways.
The trick here is that the pot resistance is spread in R(x) and R(1-x)
chunks between two diffrent potentials and the middle tap will be the
output of a voltage divider... the total resistance of this divider is
always the same, so...

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

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

I hope I at least gave you some inspiration if not the solution
itself, please feedback to me if I where missing something out or if
it turned out to be succesfull.

However, I am not sure about the musical use of this thing, since it
will effectively modify the amount of envelope being fed, but then I
can't say I am a master at the keyboards (well, synth keyboards that is).

Cheers,
Magnus