[sdiy] VC bipolar pot

[David G. Dixon]

Tonight, on the way home from work, I was daydreaming about how cool it
would be if I could put the bipolar CV knobs on my 2164 Korgasmatron Dual
VCF (dual MS-20 clone) under voltage control.

Thinking about this a bit, I realized that, given the standard -1 to +1
adjustable gain opamp setup, one would need two voltage controlled resistors
to emulate the pot on both sides of the wiper, with their control voltages
arranged such that their combined resistances added up to a constant.  This
can be achieved by feeding a (suitably biased) CV signal and its inverse
into a pair of OTAs or linearized VCAs.

Then I remembered:  This was exactly the original impetus behind my
Multiplying Modulator design (13700 and 2164 versions), and is nothing more
than a four-quadrant multiplier.

So, in case you were wondering: a four-quadrant multiplier (or
"ring-modulator") can be used as a voltage-controlled bipolar pot, provided
that its inputs are DC coupled.