Making an equal power crossfader with bump scanner units

Trying to make an equal power crossfader using two mixer units, each containing a bipolar VCA, controlled by a bump scanner unit. The control is a Mutable Instrument Shades, outputting a 0-8V offset. I’ve tried a lot of settings on the bump scanner, but am having difficulty getting a cross fade across the 8V offset. The idea is at 0V, VCA 1 will be completely open, and as the voltage increases to 8V, VCA 1 closes down. Conversely, VCA 2 starts fully closed at 0V, and opens as the voltage increases to 8V.

I’ve tested a lot of settings in the bump scanner, but the ones that seem to make the most sense are:

Bump Scanner 1
Width = 0.8
Center = 0
Fade = 0.8 or 1.6?

Bump Scanner 2
Width = 0.8
Center = 0.8
Fade = 0.8 or 1.6?

I think my problem is the behavior of the fade parameter. It looks like it ramps the voltage by half the set value on both sides of the center value. If that’s the case, you’d expect a fade setting of 1.6 would ramp the voltage over 0.8 in one direction, but that doesn’t seem to be the case. This may all be my lack of understanding of how the bump scanner works, but I’ve run through a lot of parameters trying to get this to work. Any suggestions appreciated. Thx.

Yeah, it’s not super easy to understand, is it. :slight_smile:

The attached chain works for an equal power (linear) crossfade based an incoming signal from Frames (0 to 10V). For Shades, go into each mixer, then into the subchain of each Linear VCA following the Sine Oscillator. Use the existing VCA and Offset that are in that subchain to condition your signal so that 0-8V input gets translated to -5 to 0. Do this in the other mixer too. Then I think this chain should work for you and hopefully help you understand it better.

crossfade.chain (16.2 KB)

Edit: Basically, I think you just need to change the gain on the VCAs in the bottom subhains to 0.625 and I think it will work as expected, but I don’t actually have an 0-8V knob to confirm that.

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I really need to get stuck into building my “2 deck” monome / ER-301 DJ performance mixer …

Thanks Joe! I’m looking over your chain now, but it’s late here and my brain is mush–haha. Will see if I can sort it all out tomorrow.

I don’t think you need to use the Bump Scanner for just a 0-8V linear cross-fade.

VCA #1

  • modulation gain = 1.25
  • modulation bias = 0

VCA #2

  • modulation gain = -1.25
  • modulation bias = 1

This is not equal power but neither is the Bump Scanner method.

FYI, there is an “equal power” cross-fade object (CrossFade and StereoCrossFade) in the Middle Layer which is used for the wet/dry controls on any units that have it. I put “equal power” in quotes because it uses part of a parabola rather than a cosine curve:


Hey there, since I did not get the time to actually get into the Middle layer yet, I was wondering if someone more experienced e.g. @Joe might find the time and be so kind to make an equal power crossfader unit from the above mentioned crossfade object in the Middle Layer. I think that would be super helpful to some people.

I’ll add it to my list to look into.


@Joe just curious if you ever got the time to explore this, a equal power crossfader would be amazing! Sorry to exhume an old thread. :zombie:‍♂


I didn’t. But I’m thinking it might be time to dust off the Lua skills here soon.


That would be amazing, thanks for all of your contributions! Love the videos.