[sdiy] XOR as 'digital' ring modulator

David G. Dixon dixon at interchange.ubc.ca
Thu Nov 11 06:18:15 CET 2010

> As a hopefully interesting side note, there are 2 ways to obtain the
> "ring
> modulator" function. The popular one for the synth crowd is a  linear
> multiplier
> like the AD633. The other way is by polarity reversal.  This is how true
> "ring
> modulators" work: the carrier signal goes  through an amplifier that has a
> selectable gain of + or - 1.  The  modulation signal's sign determines the
> gain
> polarity: when the mod  signal is positive the gain is +1, when negative,
> -1.

But for true ring-modulation the amplifier's gain is not just selectable
between -1 and +1, but sweeps smoothly between those two limits.  My
"Multiplying Modulator" does this with two linearized VCAs by controlling
one with a modulating signal and the other with its inverse.  One VCA
processes the signal, and the other processes its inverse.

By panning the signal from one VCA to the other, you can sweep from positive
two-quadrant multiplication to balanced four-quadrant multiplication to
negative two-quadrant multiplication.  You can get a wide range of
"ring-mod" flavours at the various "unbalanced" settings.  I also have
switches to rectify both signal and modulator up or down, which also adds a
lot of character.

One really nice use of this device in four-quadrant mode is to feed a slow
LFO signal to the modulator, and an envelope to the signal, then send the
output to the FM of a filter.  In that sense, it is like a
voltage-controlled bipolar attenuator, which is another way to think about

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