Output to bus changes volume and stereo width

Hello! First of all I should probably be clear that I’m using Ardour 2.8.12, because I’m still on an older version of Ubuntu and that’s the version in the repositories.

I’m mixing a song where I need to use a sidechain compressor, with the vocals controlling the compression of the guitars. For this, I have a bus with three inputs and two outputs. Input 1 gets its signal from the vocal track, and inputs 2 and 3 get their input from a stereo guitar bus. As a pre-fader plugin, I have an SC3 set to channel balance 1. On the bottom of the sidechain bus, there are two panning faders split to the extreme left and right.

This is no problem, it compresses like it should.

HOWEVER, what I’ve noticed is that – regardless of whether anything is sent to input 1 or not, or if the SC3 is active or not – routing the stereo guitar bus to inputs 2 and 3 on the sidechain bus changes how it sounds. The volume is increased by several decibels, and the stereo width appears to decrease. I would expect no difference at all. If I route the guitar bus to channel 1 and 2 of the sidechain bus (and ignore channel 3), there’s no difference from just outputting to the master.

What’s up with this? What am I missing?

This isn’t helpful for your problem, but the binary version of Ardour from ardour.org runs on ALL versions of Linux (on Intel-ish platforms), and can be parallel installed with distribution packages and even other versions of itself. There’s no excuse for using Ardour 2.x at this time.

Hopefully someone can help you. I think you’ve got SC3 wired up incorrectly, but I’ve never used it or looked at it. I believe that channel 3 is the sidechain, the other two are a stereo input pair.

I should have figured… is being lazy and scared of messing things up an excuse…? Let’s just say I’m stuck with 2.8.12 for this time and I’ll upgrade later.

It’s not the SC3, I get the same behaviour with no plugin at all. If I just do this, no processing at all:

[nothing] -> sidechain bus ch1
guitar bus ch1 -> sidechain bus ch2
guitar bus ch2 -> sidechain bus ch3
sidechain bus -> master

Guitar bus ch1/2 being L/R, and the panning on the sidechain bus (the two visible faders) being extreme L/R, I get an increase in volume of about 4 dB, and the stereo width seems to decrease compared to just routing the guitar bus to the master.

If I do:

guitar bus ch1 -> sidechain bus ch1
guitar bus ch2 -> sidechain bus ch2
[nothing] -> sidechain bus ch3
sidechain bus -> master

It sounds exactly the same as routing the guitar bus directly to the master.

I can’t offer any comments on Ardour 2.x behaviour. We haven’t worked on that code in more than 2 years. Ardour 3 is an entirely different beast.

In case anyone else has the same problem, I can report that I solved it by making two sidechain buses that got the left and right input respectively (both with the same trigger to channel 1), compressed them with the same settings and panned L/R.