I have been playing with side-chaining lately and came across ducking. Setting this up is pretty simple (the last 30 seconds of this video is pretty short and sweet). Basically he put a PC4K S-TYPE compressor in the main track's ProChannel, enabled Side-Chain, then simply put a send on a vocal track and inserted a send to Built In Channel FX.
 
That part is fine, but now I am confused about the compressor settings. Specifically, I am assuming (perhaps incorrectly) that the knobs on the compressor are affecting the main track's signal, so not sure where the "threshold" is that triggers the compressor? I am not sure where to look for a better understanding of the mechanics here, so any insight would be most appreciated.
 

Threshold acts on the input signal to the compressor's detector, and since you're side-chaining this is the side chain input signal. In's a situation where you are using a side-chained compressor to dim background music in response to commentary, you would put the compressor on the music track, with the dialog as the side chain input.
 
Next you adjust the threshold to the point where the compressor begins to dim (compress) the music when dialog is spoken. In this situation, you'll usually use a rather long release 500ms or more. Experiment if 4 or 10 dbs of compression gives the better effect.
 
 

I don't claim to be a ducking expert, but I do use it now and again. And I just pulled a project out where I used it to play with the knobs.
 
Someone can correct me if I am wrong, but I think the only difference between normal operation of a compressor and side-chaining is that the compressor is simply sensing a different signal instead of the track that is on. If thought of like that, then all the controls work just as they did before, including threshold. They are just sensing a different input signal.
 
Anyone.. can you confirm my statement as true or folly?
 
 
Rick
 

flameout
I don't claim to be a ducking expert, but I do use it now and again. And I just pulled a project out where I used it to play with the knobs.
 
Someone can correct me if I am wrong, but I think the only difference between normal operation of a compressor and side-chaining is that the compressor is simply sensing a different signal instead of the track that is on. If thought of like that, then all the controls work just as they did before, including threshold. They are just sensing a different input signal.
 
Anyone.. can you confirm my statement as true or folly?
 
 
Rick
 

Precisely.

Keep in mind you can use gain on the input side channel to control how loud that signal is to trigger the compression at your selected threshold where you want the compression to start on your main channel. Since it is only a trigger it doesn't matter if it clips.

Thank you! I have been messing around with it a bit more and found another couple nuances that would be nice to have in documentation (or maybe is, and I cannot find).
 
Using the set up from the OP, the song track is the Dry mix, and the voiceover is the Wet mix. With side chaining enabled, it appears the threshold is triggered by the voiceover (wet mix), but acting on the song (dry mix). The two knobs that were messing me up were Make Up (compression compensation), and Dry/Wet. It was not until I took those to extremes that I could understand what the "guts" were doing. The 4:1 ratio, 500ms release were prefect for what I was doing, thanks SuperG.
 
The Make Up was what was doing me in at first, since it was defeating the point of ducking in the first place on me (and auto makes it worse!!). The Dry/Wet comes in very handy to tailor the mix, and for "strict ducking" it best on settings > 70%.
 
The application I was using this in was using R-Mix to punch most of the vocals from a Billy Joel song to test out a new microphone, and although that is "pretty good" it is not perfect. Using "mild" ducking on that original track effectively removed the vocal artifacts the R-Mix missed and melded the performance much better.

I have been forcing myself to dig deeper into things I have never really used before for this course I am taking and chose this topic for Week 4, using Alloy 2 (which was also used in the class presentation). Here is the assignment I submitted for anyone interested (it is more a text walkthrough of how it is done). Alloy 2 is a good choice, since you can visually monitor the sidechain signal.
 
"Hi everyone,
For my Week 4 assignment for the Coursera Introduction to Music Production course I chose "musical application of compressors," but also chose a concept which introduces two new things from the lessons - ducking and sidechaining. In particular, I used these to demonstrate how to re-record a vocal on an already completed piece of work and minimize the impact of the original vocal (similar to karaoke).
 
As I reflected on how best to present this, I wanted to be sure that the concept was clear. Because of this, I chose a text-based format which would allow the reviewer not to be confused with my work inside my DAW (SONAR X3); but included necessary screen captures to illustrate important steps required to achieve this useful effect.
The file can also be found here.
 
Best regards,
Michael"

Check out this video (and others from Joel Cameron) for all kinds of side-chaining techniques to isolate drums. These should give you lots of insight into using side-chaining in new ways.

https://www.youtube.com/watch?v=AFIzwtAhSRw
 
These examples are shown in Pro Tools, but I easily adapted them to Sonar.