There is no such thing as too much compression. There's only Good Compression and Bad Compression. Two Stage Compression is a technique where you get good, transparent compression by compressing the sum of a compressed and uncompressed signal. You can do this through a bus routing method or with specific compressors, and it can be done in the analog and digital domains. The routing is a little complex in the abstract, so you may want to be in front of a console or DAW while reading this. Once the idea clicks, it's easy.
Hearing compression is a skill that one develops over time In issue #37 Michael Brauer explained that it took him three years to be able to hear the sound of compression, and he's known as one of the compression masters. The first step to understanding the relevance of the Two Stage Compression technique is recognizing Bad Compression, which is much easier if you know what to listen for. See the sidebar for some ear training tips on recognizing Bad Compression.
Two Stage Compression can be used any time you'd normally use a compressor and you can use any type of compressors. I'll use an drum subgroup for an example of how to set it up. Our hypothetical track will be a rock song with Nirvana-style dynamics — quiet verses and massively loud choruses.
Start by setting all of the drum channel faders so that they don't go to the stereo bus. Instead, assign them to two pairs of busses. We'll call the first stereo pair Bus A, and the second stereo pair Bus B. Bus A will send to the input of Compressor 1, can be single stereo compressor, or a pair of mono compressors used in stereo. The output of Compressor 1 should return to a pair of faders or stereo aux return. This return will not be sent to the stereo bus, but instead be sent to Bus B along with the uncompressed drum faders which we assigned initially. Bus B sends to Compressor 2 (again, one stereo or two monos) and its output returns to another pair of faders or stereo aux return. This return is the only part of the drum signal that gets sent to the stereo bus. None of the original channels, nor Bus A/Compressor 1 will be heard in the mix without being processed by Compressor 2 on Bus B. That's all there is to it. So, what is the point of this technique?
We've now set up Compressor 2 to compress the sum of our uncompressed drum tracks and a parallel compressed signal from Compressor 1. Understanding what parallel compression actually does is the key to understanding why there should be two stages of compression on the drums.
I usually consider parallel compression a "hack". Mainly it's in response to specific engineers I've seen use it mask a poorly set drum bus compressor. They smash the parallel signal and blend it the uncompressed sources. They like the sound of the fast release boosting the quieter parts of the signal, making the drum's decay and room ambience get louder very quickly and become closer in volume to the initial body of the sound. This can be exciting and explosive sounding, but it can be fatiguing and it often makes the cymbals flutter-pump. Their volume fluctuates very quickly with the compressors fast release making them sound extremely washy. In my experience, these same engineers like to use a fast attack, which removes all of the punch. The result is an unmusical, but loud, parallel compressed signal. They can get away with this by relying on the uncompressed signal to mask the Bad Compression and that's why I see this particular approach as a hack. All this does is make the soft parts, and average volume, louder without limiting the maximum volume of the uncompressed signal. This is the aspect of parallel compression that Two Stage Compression relies on. A little math explains how that works (see the sidebar). The first step after setting up your routing is to set Compressor 1 on Bus A. Solo its return, and set the attack and release as if it's the only signal you're going to monitor the mix. Don't just smash the signal, set it so that it sounds great on the quiet parts of the song. Don't worry if this compressor compresses too much when your drums get louder when you hit the chorus. Once you're happy with the setting for the quiet sections, take that out of solo and blend it with the uncompressed drums. This way these two signals will take turns masking each other. Compressor 1 will have a narrower dynamic range and allowing you to increase the volume with make up gain. That will cause it to mask the uncompressed signal during the quiet sections and we now have great sound for our verses. I'm assuming we chose a slow attack for some punch and a release that's slow enough to keep the drums tight, but fast enough to open up for the next hit to be fully volume and punchy. Compressor 1 has raised the volume of the drums and altered their envelope through its attack and release. However, there is now a small problem.
When the big chorus hits, the compressed signal will hit its ceiling within the compressor. Now the balance will change in favor of the uncompressed signal which can get louder and will now mask our envelope created by Compressor 1. The big loud section will not be as punchy and tight. Compressor 2 on Bus B solves this problem. We're sending it the sum of the uncompressed drum channels and the return from Compressor 1. We can now set Compressor 2 for sum of the two drum signals during the loud chorus we will have the proper envelope and volume, and any amount of dynamic contrast from the verse without Bad Compression artifacts.
This routing gives us two stages of compression, with Compressor 1 controlling the quiet verses and Compressor 2 controlling the loud choruses. For me, this type of control and resulting lack of compression artifacts is huge.
There's another way to look at Two Stage Compression. It can be used to reduce the dynamic range sent to a "primary" compressor. For instance, I really like the sound of my Pendulum 6386 compressor in its Fast mode on my guitar subgroup, and like to use it as my main guitar compressor. However, I don't like the results when the guitars push it to compress more than a dB or two.
If you want great sounding compression I think the ideal dynamic range to send to any compressor is around 3 dB. That's not too common in a musical performance. 6 dB is still manageable, but when you get beyond a 10 dB difference between the loudest and softest part of a signal, you're getting in to the range where you're going to start to hear compression artifacts no matter how great the compressor is.
I tried to get a couple of quotes from compressor designers as to what they think the maximum dynamic range is that you can send to a compressor and still expect it to sound good. Several pointed out that you can have far more than 10 dB of gain reduction and have a great sound. That's true, however gain reduction is not always a reflection of the dynamic range being sent to the compressor's input. You could send a steady state signal with a dynamic range of zero to a compressor and set it for 50 dB of gain reduction and not get compression artifacts. It's very easy to send a compressor a wider dynamic range than it can handle well.
Two Stage Compression allows me to use the 6386 as the primary compressor on the guitar bus without having more than a dB or two of gain reduction. Just add in a parallel compression stage before it and that will narrow the dynamic range hitting the input of the 6386. Since parallel compression brings up the low and average levels, you can send a dynamic range to the second compressor that's easier for it to handle because it's been narrowed.
