Many compressor plugins out there (including the ones from Sknote like SDC, Disto, STA-Limit, etc.) have them, and most of us have already seen or even used them: sidechain high pass filters. Especially when working with bass heavy material or (sub-)mixes, the sidechain high pass filter of a compressor can do wonders in terms of consistency and transparency.
But why is that so? Isn’t the purpose of a compressor to bring the audio material closer together; particularly when the plugin has been put on a group or master channel? How is that supposed to work if one actually excludes a part of the frequency spectrum from the compression circuit, and wouldn’t it rather sound inconsistent in the end?
To understand why a sidechain high pass filter can be such a valuable mixing tool, let’s first have a look at the frequency spectrum of a (random) master track:
As you can see, the loudest parts of the signal are pretty much all below 250Hz, which is nothing special actually – you are likely to find similar images on various (sub-)mixes, no matter what genre. However, this is precisely what can cause problems if you now put a compressor on the mix bus. Remember that a compressor starts to kick in as soon as the volume of the source material exceeds the threshold of the compressor.
If you now look at the frequency spectrum again, you can see that you would have to lower the threshold considerably before the compressor would even start to affect the mids or even highs at all. At that point, meanwhile, the bass frequencies would have been compressed a lot – the result would sound rather inconstant and inhomogeneous.
Enter: sidechain high pass filter. By activating it, the louder bass parts of the spectrum (i.e. the lows) will be excluded from the compression circuit. The threshold can now be adjusted more precisely so that the effect can work on all (of the other) frequencies with greater care.
In our example track this would look like this:
You see, it’s well worth activating the sidechain high pass filter on a compressor when working with sub or full mixes – just give it a try!