Community member Trond Nedberg sent in a question, which we touched on in Podcast 307 and promised to explore in a separate article. Trond asked about how the linking in Pro Tools actually works when using multi-mono instances of plug-ins especially with regard to plug-ins with side-chains as it appears that Avid is using parameter linking rather than side-chain linking. Over to you Trond...
I've noticed many engineers tend to use compressors or limiters in multi-mono on surround sources. Either because they don't have any dedicated surround compressors or, most likely, because they really like a certain compressor and take it for granted that a multi-mono instance will do the job. Because after all they're linked, right?
To some of you, this may be elementary school curriculum. But I had an awakening when I was working on making an R128 adaption of a feature film and noticed that the reverb was acting strangely. I suspected it had something to do with the linking in multi-mono by the way the sound image behaved and so I began researching. I had little luck searching the internet, and the Pro Tools manual gave me 250+ on any search term I tried, so I gave up on that and decided to do some testing on my own.
I created a test-session in Pro Tools, which contains an audio track with a test tone that switches back and forth between the left and the right channel. The top track has two multi-mono compressors inserted with the controls linked. This is the way Pro Tools works by default. On the lower track, I've also created a mono bus (summed) that feeds the sidechain of the two lower compressors.
Multi-Mono Plug-in Parameter Linked - Watch What Happens To The Gain Reduction...
If you put a stereo compressor on a track it should do the same amount of gain-reduction on both channels, as you can see from the images above from the test-session, the left and right gain reduction meters don't move identically, which demonstrates that the gain reduction is not linked, even though the controls are.
When AVID say linking with Multi-mono plug-ins, they are referring to the linking of the parameters. When you adjust the threshold on one channel, the threshold on all channels will follow by the same amount, parameter linking. However what the plug-in might be doing under the hood is not affected by the linking, be it gain reduction, saturation or something else.
So what's the deal? Is this bad or good? Let's look at an example: Say we have a stereo car pass-by where the car drives from right to left across the screen. The sound pans with it. What would happen if the gain reduction is not linked, is that first the right side gets pulled down by the compressor, leaving both sides equally loud. When the car reaches the left side of the screen the left channel gets pulled down effectively eliminating the level-difference between the speakers, counteracting the sound designer's intentions. If the compressors are truly linked, both channels would be compressed equally, leaving the level-differences between the speakers as intended, and just lowering the overall volume by the same amount.
Multi-Mono Plug-in Parameter Linked And Side-chain Fed With Mono Summed Signal - Watch What Happens To The Gain Reduction Now...
So how can we link the gain reduction? To link two mono compressors properly, both channels must be summed into one mono signal. This mono-signal must again be feed to the side-chain of both the left and the right compressors. Only then will the right side of the compressor compress by the same amount as the left even if only the left channel reaches the threshold and by doing so retaining the stereo image intact. While most compressor plug-ins are available in both mono and true stereo, this is not the case in the world of surround, and that is why there are specialised surround compressors.
By making a sidechain send, as shown in the test-session project, any compressor can now be used in surround as well, and be properly linked but you need to create a mono sum of all the channels on a bus to feed into the compressor's side-chain. I hope this will be useful to some of the Pro Tools Expert community.
Thank you Trond for digging into this, it makes complete sense now that you have explained what is going on. This is really a significant weak point in multi-mono plug-ins that use a side-chain because although the multi-mono plug-ins are parameter linked (when you adjust a control on one channel they all follow) the same cannot be said for the side-chain, there is effectively no side-chain linking in multi-mono plug-ins in Pro Tools.
To make sure this wasn't a bug with just the Avid stock compressor, Mike checked it with a number of other dynamics plug-ins including the Avid Pro Compressor and the Nugen Audio ISL2. This would seem to confirm the issue is a function of the design in multi-mono linking in Pro Tools and all users of multi-mono plug-ins, especially those that have a side-chain like compressors, limiters, gates, expanders etc, should be aware of this issue and think carefully before using a plug-in multi-mono mode because there isn't a multi-channel version available.