I've been working on a vector synth but realised I've just been using a crossfade separately on the X and Y axis so it's not really working like a vector mixer at all. I just can't figure out how to do this properly.
Ideally the closer the X or Y value is to any of the extremes it should reduce the value of everything else.
Are there any existing modules that will help?
In a modular synth, this can be done with three crossfader modules. One connect to A and B, and the other to C and D inputs. The third connects to the outputs of the other two. The same CV (the x axis on a joystick for example) is applied to the first two crossfaders and the second CV (the Y axis) is applied to the third crossfader. The final output is taken from the third crossfader.
To do quadraphonic panning, you just do it in reverse but use panners and not crossfaders.