Many people may already be rolling their eyes at the sight of this headline to imply that they already know this. Yet, in my travels across the internets, it’s apparent that it is actually a much misunderstood area. But before we start, we need to clarify that in this article, we are not referring to the process of using one musical element to trigger the compression of another, such as the kick drum triggering the compression on the bass.
In some ways, that is a more complex process than what we are discussing here. But because many DAWs do the tricky stuff in the background for the user, it appears to be a little ‘out of mind, out of sight’ for some, and therefore easy.
In this article we are dealing with the effects of the sidechain (or detector circuit) over the SAME original sound that is feeding the device. We will use a compressor as the dynamics gadget of choice for this article, as it is the most ubiquitous.
Many compressors have a control for this but its label can vary from unit to unit. Just poking around my collection of plugins, I’ve found: Filter, SC Filter, SCF, SC HPF, Sidechain, SC Freq, Detection Filter, SC Hi Pass … you get the idea. You can see that this is a prolific control that probably appears on the majority of modern compressor designs.
It doesn’t appear on many plugins that emulate classic old designs because the original units generally didn’t feature this control. Some progressive plugin manufacturers have added it to their vintage emulations anyway, to give them more flexibility.
So enough preamble – what is it?
I’ll start by telling you what it isn’t, because this is the biggest area of misunderstanding with this topic. It is NOT a control that can (for example) prevent the deep bass from being compressed. It IS a control that tells the compressor what to REACT to. So it can, in this example, prevent the deep bass from TRIGGERING the compression.
Some people assume these two are the same thing or achieve the same end, but that is not necessarily true, though the end result may be similar.
So let’s listen to some real examples to illustrate this.
In the free video tutorial above, we have a very deep kick on the down beats and a very thin and bright snare on the up beats. Initially we compress them both without altering the compressor’s detector circuit. So the compressor is “hearing” the full range and making its compression decisions based on that.
Now let’s engage the detector or sidechain circuit to high-pass the signal that the compressor “listens” to, so effectively it won’t hear that deep kick. Because it can’t hear it, it won’t react to it. So only in this example can we support the argument that the detector is preventing the deep bass from being compressed. But in the real world, deep bass is almost never on its own like this.
Let’s look at example two where the kick is on all four beats but the snare is only on two and four. Now it is plain to hear that on beats two and four, when the snare triggers the compressor, the deep bass also compresses. Why? Because it’s a single-band compressor. Something told it to compress so it compressed! It might be ignoring the kick as a trigger but when the higher frequency snare trigger comes, everybody cops it - including the kick.
Therefore, this refutes the often shared myth that the detection filter prevents the (in this case) low bass from being compressed. It doesn’t – it just prevents the low bass from CAUSING the compression. But it is still a recipient of it.
The effect people often describe is that which is achieved by multi-band compression (or dynamic EQ but we won’t go there today!). In these cases, the compressor is split into two or more different paths that act individually. So yes, in this case we could have the deep bass/kick neither trigger the compression nor be pushed down when the high snare triggers the compression. But you cannot get that effect with a single-band compressor.