The Barton BMC053 module uses VCAs arranged in a particular way to achieve two outcomes, a voltage controlled panner and a four-quadrant multiplier. The signal flow is illustrated to the right. The triangles with circles on the end represent inverters. If you're unfamiliar with a fourquadrant multiplier, it's a specialized VCA where negative control voltage causes the output to invert. The quadrants are mapped out in the diagram below. To acheive 4QM, the audio input is sent to two VCAs. The MOD input controls the positive VCA and an inverted MOD controls the negative VCA. The negative VCA's output is then inverted and mixed with the positive output.

CONTROLS/INS/OUTS

1.Audio Input – This is the signal that will be attenuated/inverted by the 4QMP. It does not have to be an audio signal. There is an attenuator for this input.
2.Mod Input – This signal controls the attenuation/inversion of the 4QM output and controls whether the +O or -O is active. There is an attenuator for this input.
3.Offset Input – This signal will be summed with the mod input and inverted mod input in controlling the VCAs. It's effect on the 4QM output is negligible when working properly, but when using the module as a panner, it keeps a channel from turing all the way off when the MOD polarity changes.
4.AC Switches – These toggles bypass decoupling capacitors in the circuit controlling whether the MOD and AUDIO inputs are DC or AC coupled.
5.-O Output – This output is active when the MOD input goes negative, the more negative the greater the signal.
6.+O Output – This output is active when the MOD input goes positive, the more positive the greater the signal.
7.4QMO Output – This is the 4 Quadrant Multiplier Output.