Logic Pro How to crossfade between two channels (not regions)

[stevefal]

Anyone know a way to automate a constant-power cross-fader between two channels? I've cobbled one together using transformers in the environment, but I'd like a simpler way that can be set up easily.

An example use, in sound design, is transitioning between two ambience tracks without incorrect level in the middle of the crossfade. A character moves from one location to another starting and stopping. When stopping, the crossfade pauses, and when moving, it continues.

I could do this by automating volume on both tracks, but within the crossfade, there's no telling whether the sum of the two channels will be the proper level. With a proper crossfader, I could just automate its position, and know that the sum level is always unity.

 

[Peter Ostry]

Use the automation curve tool. Drag upwards/downwards to make a single curve instead of an S-shape. Do this on both tracks. At the point where you want the crossing, both curves should produce –6 dB. You can see this in the automation value field when you move the SPL to this position. Adjust the curves until you get the correct value. I don't know an easier way and this type of crossfade works perfectly.

You can check this on two tracks with the Logic Test Oscillator plugin and white noise. Just put a level meter into your master bus and it should show a stable output level. Don't use sinus as test signal, it may partially cancel or amplifiy the signal.

 

[stevefal]

Thanks Peter.

Since I need starts and stops, I need a more dynamic solution. Since my tracks are part of a sub, I was able to do it with SmartControls. Here's how I did it.

1) Add a dummy audio plug-in to the Sub parent. This is required to activate SmartControls on the channel (I think). SmartControls for a Sub parent exposes child channel parameters for mapping. I used a Gain and disabled it.
2) Create two mappings to each of the sub channel Volume parameters.
3) Create unity sum scaling curves for each SmartControl mapping, like you did above.
4) Now automate the SmartControl at will. No matter what value you set, the two sub-channel volumes will sum to unity.

 

[Peter Ostry]

Better than drawing curves on two tracks.
Clever idea, thank you for sharing!