If you try to insert Avid’s DownMixer plug-in on a stereo master fader you’ll find that you can’t because it is missing from the plug-ins list. Why is that?

There is a useful tip we’ve featured before where you can use an Air Stereo Width plug-in to check mono compatibility. To do this just insert the Air Stereo Width plug-in on your master, set the width to 0% and you will have mono, bypass the plugin to route straight through for stereo and when you want to check mono un-bypass. Simple!

It occurred to me that as there as a DownMix plug-in in Pro Tools. Why not use that instead? Even though the Air Stereo Width trick works, there is something a little backwards about it for me. To check how well this might work I went to instantiate the Avid DownMixer Plug-in on the stereo Master Fader of a session and I couldn’t find it.

After searching, checking the plug-in name on Google (it’s not something I ever use normally) and manually checking both the multichannel and multi-mono plugins lists under “Avid” I thought I was losing my mind. I’d seen it in my plugins list minutes earlier, that was what had prompted me to think about using it as a mono check.

It was when I looked again on a stereo Aux track and found it straight away that I realised it was missing from the list on the Master Fader. I’d never noticed this before. Why would that be?

In the two images above you can see the effect, a DownMixer plug-in has on the mixer architecture. In the case of the StereoWidth plugin, the output with the width set to zero is still stereo, but both channels are carrying the same information so the signal is 2 channel stereo but summed to mono. In the case of the DownMixer, it is a single mono channel. The channel architecture has changed. The other notable difference between the two plug-ins is that there is no bypass available in the DownMixer.

Once this important difference, between a plug-in which sums two channels together into two mono channels and a downmixer, which creates a single channel of mono from a stereo input it becomes obvious why the DownMixer isn’t available on a Master Fader. Master Faders are summing points within the Pro Tools Mixer architecture. They already exist, creating a Master Fader track just displays the summing point. The change DownMixer Plug-in makes can’t be performed on a Master Fader so Pro Tools doesn’t display that plug-in in that list.