In this article Steve DeMott explains his approach to Bus Compression. Why is it important and what does it achieve?

Compression is a big deal in audio production. That’s probably why there are so many articles & videos about them. I admit that I love compression & compressors. Used properly they can be magic to a mix. With that said, this is not going to be an article that will explain the basics of what compression is, and how it works. That’s been covered ad nauseum. We’re going to get right into the nitty gritty of compression & discuss the intricacies of how it works. So, while this article could be about compression in mastering, it is really an article about mastering the art of 2-bus compression.

You hear a lot about the “glue” that compression adds to a mix, but what does that really mean? Is that cyanoacrylate super glue? Or just a good old fashioned white polyvinyl glue? We all know that compressors reduce dynamic range. We know about ratio, threshold, attack and release times, and how those interplay to effect compression. But what does that mean in practical terms?

I was recently having this discussion with a new assistant. We were going over a mix template and I was explaining what I was trying to achieve with the bus compression. Not the technical of what the compression is doing, but what is the net result to how we perceive the sound, post compression? I ended up explaining it this way: “what I’m really doing is trying to get all the sounds to cozy up to each other. I want to get them to interact a little more & react more as a whole.” She nodded, understanding what I meant.

So here is the big takeaway on the “glue” thing: when you add compression to a full mix or submix you are making all those sounds react to, and with, each other more than they were before. But if you have too much of it, you get a lifeless mix. Too little and you aren’t getting the most out of the compressor.

The Enemy In Stereo

A few weeks before the conversation with my assistant, I was talking with a friend of mine, and we were discussing mixes and mix techniques. Specifically, how do you make a mix “bigger” or “wider”, or whatever keyword you want to use. In the midst of the conversation I said to him “a good mix is unbalanced”. He gave me an incredulous look, so I explained. If every time you had something happen on the left side of your mix you had an equal “thing” happen on the right you’d have a very balanced mix that was also quite boring and never gave the impression of extra width. Consequently, if you pull the listener from one side to the other, the entire mix feels wider, bigger, but that is, by definition, unbalanced.

The problem is that a stereo compressor plugin tends to react equally on both sides of the stereo spread when it’s triggered by only one side. This reduces width and excitement. It also makes the compression more obvious. Both of these are not what I want.

So let’s look at 3 options to “unlink” our stereo compressors and get them to help add more width, space & excitement to our 2-bus, whether that’s a submix or on the master 2-bus.

Multi Mono

In Pro Tools, and I would assume most other DAWs, you can instantiate a plugin on a stereo bus as “multi mono”. This basically places 2 mono instances on the bus, one for the left side and one for the right. They are “linked” in that setting one side also sets those parameters on the other side, but they will react separately. I will mention that there is the option to set each side differently than the other, but that’s not what I want to talk about here. This setting allows any stereo plugin to react to each side differently.

Mid-Side

A third option for stereo compression is to use a compressor that has a Mid-Side or M/S setting. Some plugin companies make this a pretty standard feature, while others don’t bother with it, so you may not have any compressor plugins that have this option. It’s quite popular in mastering gear because it gives you a kind of control that even an unlinked stereo left/right compressor doesn’t.

What Mid-Side does is separate things that are happening on the sides of your mix from the things that are happening in the centre.

I remember my first experience with this idea was in the analogue world. At a studio I was working at in the late ‘90s we had a JoeMeek SC2 optical compressor. I remember loving that compressor on my master 2-bus because it just made the whole mix feel bigger. It wasn’t until years later someone told me that it was likely because the compressor converted the signal into mid and side, then compressed, and then converted it back to a standard left/right stereo to send back out. Whatever the reason, when that studio closed down I made sure to get my hands on that compressor.

Don’t Underestimate The Power… 

….of the low end!

When you compress a signal with a lot of low end information, the compressor is going to act/react to the low end first. It’s the physics of sound. The low end has a lot more energy than the high end at the same gain setting.

The way around this is to use a compressor that has a high pass filter side chain option. What this does is to roll off the low end of the compressors detection circuit, but not of the signal itself as you hear it, so that the compressor doesn’t overreact to the immense amounts of power in the low and sub frequencies.

I think of this feature as a kind of second threshold setting. The idea being that as you roll off the low end in the detection circuit you can control where the compressor kicks in and where to “focus” the compressor’s frequency response.

You can also create this feature as long as your compressor has a sidechain feature, but not the HPF sidechain feature. The problem is that it’s enough of a hassle, that it’s almost not worth it. So, if the compressor you’re using on your submixes or master doesn’t have a high pass filter side chain feature, get a different compressor. Seriously, it’s that important.

Talk Is Cheap

I once heard someone say “talking about sound is like dancing about architecture”. So, let’s listen. Here are a bunch of audio examples showing off what we talked about.

I did my best to balance these so that they were within about half a dB of each other to keep the comparisons as fair as possible. Some of the compressors had more exacting make-up gain than others, so I got as close as possible.

I also want to thank Greg Wolfe for his permission to use his song, Shine A Light On Freedom, for these examples.

Each bus group will go through the following examples:

Raw: no bus compression.

Multichannel: a FET style compressor as a standard insert that most people use.

Multi-mono: the same compressor as above, but inserted as a multi-mono plugin.

Mid-Side: The same analogue modeled FET as above set to M/S mode.

Multiband: a multiband compressor inserted in multi-mono

And the full mix will add 2 extra options

HPF: the same FET style as above, with a high pass filter sidechain set at 92Hz.

Mid-Side HPF:  The same analogue modeled FET as above set to M/S mode, with a high pass filter sidechain set at 92Hz.

Drums & Bass Bus

Instrument Bus

Full Mix (all submix/bus compression is bypassed):

What is your approach to bus compression? Do you link channels by default or do you use unlinked channels? Let us know in the comments below.