MOTM

Hello all,

I have been experimenting with two-channel stereo panning and wanted to 
share my thoughts and get yours.  My 1980's era homebrew Electronotes 
synth used a pair of EN linear VCAs, for which I provided a panning VC 
input.  That did some convenient internal voltage conversion.  All the 
VCAs I use are similar to the MOTM-190 in taking a control voltage in 
the range of zero to +5V, with zero volts equating to zero gain and +5V 
to unity gain.  (I have two 190 VCAs, a modified Blacet Quad VCA/Mixer, 
and a Quad Low Pass Gates.  The latter is yet to be built, but I will 
calibrate it for a five volt control range.)  Let's say we want to pan a 
signal from left to right in stereo.  We need two VCAs, both taking the 
same signal input, with one going to the left and the other the right 
channel output.  We want to smoothly pan from one side to the other and 
back.  Stands to reason that we need one control going from zero to five 
as the other goes from five to zero volts.  Now my first problem is how 
to get a five to zero volt signal, starting from a zero to five volt 
one.  I can't just use an inverter, because the result will be going 
zero to minus five.  I have to add an offset of five volts.  In my synth 
I have several ways to do this.

I modified my MOTM-800 EGs so that the minus output goes from +5 to 
zero.  (Hint:  It takes one resistor.)  So I could use an 800.  But the 
exponential wave shape may not always be what I want for panning.  
Another module I can use is one of my Blacet EG2070s.  These also have + 
and - outputs like the MOTM-800.  The Blacet by design goes from +5V to 
zero on the minus output.   The upper voltage is adjustable, because 
Blacet standard VCA controls are zero to +10V.  I adjust mine for the 
five volt range to match MOTM.  The Blacet EG is a linear device and can 
self-cycle, so I have a pretty good panning controller there.  But what 
if I need my Blacet EG for something else?   A third candidate is my 
super-custom MOTM-OMS-820, which I modified to add a zero to +5V self 
cycle mode.  Again, that is way overkill for this purpose.  *I just want 
an LFO that goes zero to +5V!*   Blacet's new LFO has zero to 10V and 
10V to zero outputs.  It is built for panning the Blacet VCAs.  But I 
don't have one of those LFOs!  And if I did, I would have to modify it 
for a five volt range.  Back to my 1980's synth.  The panning input took 
a zero to five volt signal and did the conversion internally.  I've seen 
many VCA panners with this feature.  I suspect the long-awaited MOTM 
panning VCA has it.

That's not the end of it.  For proper panning you want a smooth 
transition of the sound field.  It should appear to be moving gracefully 
left to right, without extreme level changes.  This requires the right 
sort of response in the VCA and the right control contour.  I've been 
playing with different combinations.  Using a sine wave as the control 
with an exponential VCA response seemed pretty good.  (That takes a 
MOTM-320R and two of my Mixer-Comparator modules to rig up.)  I'm over 
my head here and just dabbling.  Surely some of you folks know a lot 
more about this.  I haven't researched my Electronotes archive yet on 
this one (I tend to drop those heavy binders on my toes!).

So what do you folks use for VC panning?

Cheers,

Richard Brewster