Pro Tools Tutorial: Digging Deep Into The Sidechain

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Use a sidechain filter to boost your frequencies, remove the low-end, and improve your track no end. Mike Hillier gets the lowdown…

Sidechain compression is a classic trick, and one we’ve discussed frequently in MusicTech. It usually involves using a kick drum to trigger the compressor on another track, such as a synth pad, to create a pumping sound.

This can be a great trick and has been used to great effect on many dance hits. But it is only one of a myriad of techniques that can be employed using the sidechain to create unique and interesting compression effects.

Many compressors, even those without an external sidechain, feature a sidechain filter. This takes a separate copy of the audio signal, filters the low-end and uses the filtered signal to trigger the compression. The compressor, however, continues to act on the dry, unfiltered signal. This is particularly useful for preventing a track from ducking in response to excess low-frequency energy. But with an external filter we can get far more creative in our use of filtering, opting to use our own favourite EQs, or even other effects in the sidechain.

The creative benefits of this are huge. By using a fully featured EQ instead of the built-in filters we can shape the incoming signal in any way we want, not simply by removing low-end, but by boosting frequencies that we do want the compressor to respond to.

Understanding how the compressor is going to respond as you do this can get quite complicated, and remembering that the compressor is still acting on the full, un-touched signal is key. Removing low-end results in the compressor ducking the signal in response to mid- and high-frequency material, producing a stronger, more present low-end.

While boosting the low-end in the sidechain would result in the compressor ducking the signal more in response to the low-end, producing a bass weak mix.

The same is true across the spectrum: as you boost the signal going into the sidechain, the compressor will push the mix down more in response to this frequency, which will have the effect of lowering that frequency in the mix.

In fact this is how early de-essers worked, in that they would filter out all of the signal apart from the sibilance in the compressor sidechain, therefore ducking the whole signal whenever the sibilance pushed the compressor into gain reduction.

This is similar then to multi-band compression, except that with multi-band compression only that frequency band is reduced, whereas by reducing the full spectrum in response to a specific range, using the sidechain can result in a much more natural sound.

But why stop with EQ? What would happen should you add dynamics processors, such as another compressor, across the sidechain? This would reduce the dynamic range of the sidechain signal, causing it to cross the threshold of the primary compressor less often, reducing the amount of compression that occurs, but also smoothing off the level of gain reduction, and lengthening the attack and release.

This can be useful if you want to compress a signal without reducing the transients. Add a limiter to the sidechain to chop the transients off before they hit the primary compressor sidechain detection circuit. The compressor will now only respond to the signal after it’s been limited, ignoring the transients and letting them through un-compressed.

Focus On External Sidechains
Many plug-ins and hardware units have external sidechains, not only compressors. Once you’ve got your head around the concepts described here you could easily apply them to other types of effects, such as gates or expanders.

How about triggering a gate on a kick drum only in response to the very low-end, in order to prevent accidental mis-triggers by the snare? Or filtering the low-end off from the snare’s sidechain, to prevent the kick from triggering it?

Step By Step – Adding a Sidechain Filter

  • 1: Add an instance of Dynamics III to the Drums channel. Like many dynamic plug-ins this has a built-in sidechain filter section. We can use this to remove low- or high-end from the sidechain signal.
  • 2: The built-in sidechain filters are fairly minimal, so let’s use an external sidechain. Start by duplicating the Drums Aux channel on to a new track, though this time without the Dynamics III insert. We’ve chosen to name it Drums SC.
  • 3: We’re just using this new Aux. as a sidechain for the compressor, so we need to set the output of this new channel to a spare buss. Remember, we don’t want this channel being routed to our mix, so don’t route this new buss to its own Aux. channel.
  • 4:  In Dynamics III on our Drums channel we now need to assign the external sidechain. Select the buss we just created from the drop-down menu that’s next to the key icon in the top-left of the plug-in window.
  • 5: Now any changes we make to the sidechain signal will alter the way the compressor responds. We can still use high- and low-pass filtering, but now we can use any EQ we choose. Here we’re using the free Brainworx Cleansweep V2 to filter the signal, which has smoother slopes than the filters built into Dynamics II
  • 6: Setting the filters to act as a band-pass filter we can use our sidechain to focus on the sibilance, turning our compressor into a simple de-esser.
  • 7: We can be far more adventurous with our EQ now too, as we are no longer limited to just high- and low-pass filtering. Here we use the Softube Tilt plug-in to change the balance of the sidechain signal, similar to shelving EQs.
  • 8: Combining shelves and filters together we can make the compression respond quite differently to a standard compressor, and often in very interesting ways. Here we’ve used a high-pass filter at 60Hz to stop the compressor from responding to excessive low-end, and boosted the top-end with a shelf, to push the compressor to respond more to the cymbals. This results in a very mid-forward mix.
  • 9: EQ is only one of the tricks you can employ on the sidechain. In this example we’ve added a limiter to the sidechain. This removes any stray peaks and smooths out the compression behaviour. This technique can be especially useful on a stereo buss.
  • 10: Alternatively, if you want to really clamp down on the peaks in your mix, you could place an expander across the sidechain, forcing the compressor to respond more than usual to the peaks in your mix.
  • 11:  We can even use the external sidechain trick to force our compressor to ignore the mono signal and respond only to the sides. Here we’re using Brainworx bx_solo to quickly solo the sides in our sidechain. There is no advantage here however of soloing the mids, or changing the stereo width, as the sidechain input is mono, and therefore mids-only anyway.
  • 12: If you want to listen to the sidechain as you process it, you can always engage the “sidechain listen mode” on Dynamics III. Just be sure to turn this off once you’ve finished to once again hear the actual effect of the compressor on your signal.


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