Several videos show a side chaining button within Nectar.  I can't get this button to show up.  What I was hoping to do was to send an audio file (piano e.g.) to the Nectar harmonizer module to detect the chords.
In reality, I don't think Nectar can do this.  It seems that all you can do is to send MIDI notes, but I haven't even figured out how to do that.  The Nectar documentation says that we must set the properties for the Nectar VST to configure as synth.  I did that, but still don't see a way to send MIDI into Nectar.
 
It is more likely that I would rely on Nectar default harmonies, and make any modifications through automation lanes.  This definitely works, as Nectar exposes about 100 parameters that can be automated.  That brings up a second problem.  The list of automation values is way too long for my monitor.  There is a little triangle scroll button at the top of my screen, but nothing at the bottom.  If I move Sonar to my first monitor (smaller than the other monitors) the scroll triangles show up at the top and bottom, but not on my other two monitors.

Can you post a link to the video you are referring to?

cparmerlee
 It seems that all you can do is to send MIDI notes, but I haven't even figured out how to do that.  The Nectar documentation says that we must set the properties for the Nectar VST to configure as synth.  I did that, but still don't see a way to send MIDI into Nectar.

Once you have configured Nectar as a synth in the plugin manager, insert it on the audio track you wish to harmonise, then create a new midi track and Nectar should show up as an option in the midi tracks output drop down menu.

KingsMix
Can you post a link to the video you are referring to?

It was here https://youtu.be/TMfFuls5IKA?t=359
 
As I look at that, I think the author was using Logic, and that sidechain pull-down must be part of a Logic window frame.
 
But I still don't see how to do the side-chaining in SONAR.  I was hoping to side-chain an audio stream, but I think Nectar only accepts MIDI on the side-chain.

tenfoot
Once you have configured Nectar as a synth in the plugin manager, insert it on the audio track you wish to harmonise, then create a new midi track and Nectar should show up as an option in the midi tracks output drop down menu.

OK.  I'll try that.  Thanks. 
 
What I was hoping to do was to send the keyboard track into Nectar so that it could detect the key context on the fly.  I don't think Nectar can do anything like this:
 
a) It seems to have only statically selected keys, which can be changed through automation
b) it only accepts MIDI ion the side-chain and that is only for the purpose of providing the specific notes to be used in the harmonization.
 
I think for my purposes, it will be adequate to set the primary key, then use automation to dynamically turn the harmonization on and off, and to occasionally change the key root and/or scale if the Nectar harmonies are too strange.
 
It would be cool for Nectar to evolve to the point that it can recognize the actual chords and harmonize the vocals accordingly.  Other VSTs are able to detect chords.
 
I might play around with using Melodyne to save the keyboard part into MIDI notes and then feed that MIDI into Nectar just for fun.