Side-chaining is one of the subjects that I get asked about the most from students and fellow producers alike. Using side-chaining as an effective signal flow technique in the studio is very handy, and has a ton of uses besides ducking your bass with your kick.
The EVOC 20 is a vocoder built into Logic that takes an audio signal from one channel strip and uses it to drive the vocoder's synthesis engine.. The most common way that the signal is routed in Logic is to have the output of a channel strip feeding the vocoder directly. Any audio channel strip that is set up in your arrange window automatically shows up as a potential source in the EVOC 20, making for some easy vocoding. I prefer to use busses instead.
The idea of using busses to feed an EVOC 20 is to allow more than one source to feed the EVOC 20. In this case, we have a vocal and a drum beat feeding the same EVOC 20.
Create a software instrument channel strip, and pop an EVOC 20 on it. Import a drum loop from the Apple Loops library to the arrange window, creating a new track for it. Once a loop is in the arrange window and rockin', record yourself saying something on a track with a new audio channel strip in the arrange window. You should now have two channel strips in the arrange window besides the EVOC 20. Give yourself about 8 bars.
Now open the mixer and select the two audio channel strips. Go to the '˜Sends' section and choose Bus 1:
For additional control, click and hold on the sends you just created, and choose Pre-Fader. Now both audio channels are feeding the Bus, and we will have access to that bus in the EVOC 20.
What we want to do now is go over to the Aux channel that was automatically created when we assigned Bus 1, click and hold on the output slot (I/O section), and select No Output. This may seem confusing, but the bus will still feed the EVOC 20, even though the Aux channel isn't playing through the output. This way we don't hear the audio signals from the auxiliary channel strip.
In the EVOC 20, go to the Side-chain menu in the upper right of the instrument, and choose '˜Bus 1'. Turn the sends up on the audio channel strips to 0 dB. We're almost there...
The last step is to choose a patch from the library of the EVOC 20 that is using the side-chain input for vocoding. Try Vintage Vocoder > Clear Voice Vocoder.
Press play, select the instrument track, and hold down a chord on your MIDI controller. Yes, we still hear the audio, but we can also hear the audio's effect on the vocoder. Remember that step where we made the audio channel strips pre-fader? Pull down the volume on the audio channel strips to Adjust their volume to taste. If you want to hear the EVOC 20 only, simply turn the audio channel strips all the way down in volume. The send section will still feed the EVOC 20.
Vocoding in this way can add new dimensions to the vocoded signals. Adding drum loops to a vocoder can make for some interesting rhythmic stuff!