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What is Sidechain Processing? Mixing Techniques De-Mystified

In this article, Headliner takes a look at the sometimes confusing world of sidechain processing and attempts to give you an overview of, as well as some of the more common uses for this incredibly useful technique.

What is Sidechain Processing in Music Production?

In pure layman’s terms: ‘Sidechain processing is the manipulation of one audio track by means of another.’

Let’s refer to the audio track we want to process as the ‘Original’ audio, and the second track we use to process the original audio as the sidechain. You may also have come across the terms ‘Key’ or ‘Trigger’ - these also refer to the sidechain audio track. What’s useful to know is that the sidechain could be a copy of the original audio that has been manipulated in some way, or a completely different audio track altogether.

However, to fully appreciate the concept of sidechain processing, it’s necessary to delve into a little bit of studio history and scrutinise a couple of iconic vintage analogue audio devices.

Drawmer DS201 Dual Noise Gate

In the early ‘80s, the Drawmer DS201 rack unit became the ‘industry standard’ throughout the world. Whether for studio or live, it became the go-to gate for drum isolation as well as noise reduction, ducking and other sound design duties. The unit offered a Key (sidechain) source selection switch from either the Internal Audio (Original) or a separate ‘Key Input’ external audio source via a balanced jack input. While this was not a new feature, the ability to apply high and low pass filtering to the sidechain audio path from the DS201’s front panel was.

How does this improve the function and accuracy of the noise gate, you may ask. Say for example you’re recording a jazz drummer and there’s a lot of intricate ghost strikes to the snare, as well as busy and often quite loud hi-hat work. To avoid crosstalk that is relatively high in level, as well as spurious triggering of the gate by unwanted components in respective microphones, this is done by selecting Key Source and using the high-pass filter on the Hi-Hat channel to remove low end audio information produced by the snare and likewise, the low-pass filter on the snare channel to reduce the high end audio information from the hi-hats.

This ability to sidechain process the gate’s trigger information adds a frequency dependent as well as level to the gate’s audio trigger, where level alone may keep the gate open more often than you’d like during loud sections – or worse still, during quieter passages not open at all – causing takes to be unusable. That is obviously an extreme outcome, and you’d negate that by range reduction, but I hope you can start to see how sidechain processing can quickly become your friend.

In terms of vintage gear, the term ‘sidechain processing’ refers to the manipulation of a duplicate of the original that is manipulated in some way, and then utilised as a means of triggering the processing of the original signal.

You may now be getting a feel for sidechain manipulation of the internal (original) signal from a problem solving perspective, but what about an external key input? The classic example of this became a mainstay of many radio stations during the ‘80s. With the DS201 switched to Link and Duck and inserted across the music subgroup on the studio mixer, key input channel ‘one’ could be fed from an AUX on the DJ’s Mic channel. This would in effect Duck the level of the music by a pre-determined amount set with the range control, every time the DJ spoke. I’m aware that this is also a technique that can be employed on compressors with sidechain processing and key input capabilities, but compressing music that harshly changes the nature of the sound rather than simply turning it down.

These days it’s almost a rarity to find a production or mix that doesn’t feature some form of sidechain processing.

Another important role played by sidechain processing is in the reshaping of existing sounds. An example of this can be heard on the Shaman’s Ebeneezer Goode. In this instance, a MIDI track was created in Steinberg’s Pro 24 for Atari, the sequence then used to generate a series of short drum hits. These were then fed into the DS201, inserted across two guitar channels via the key input to produce a pulsing guitar sound to what was otherwise continuous guitar feedback. They were by no means the first to use this technique, in fact it was commonplace and one of numerous sidechain processing tricks to be employed by EDM and other genres.

In the example above, simply switching between Gate and Duck would change the guitar stabs from beat to offbeat. Just another example of a problem-solving tool being employed as a creative tool!

I briefly touched on sidechain compression, but this technique became common during the ‘80s for keeping kick drums prominent and not crowding them with bass lines from bass guitars or synths alike. The compressor is inserted across the bass instrument and the kick drum is used as the sidechain trigger to compress the instrument during the kick strike. This has the effect of maintaining the prominence of the kick while the reduction of the bass instrument is rarely noticeable.

Again, as EDM came to prominence in the late ‘80s and early ‘90s, sidechain compression became more widely used across a range of instruments to create those classic pumping-style dance groves. There are so many examples of this in action, but check out Daft Punk’s 1997 album Homework, for example after example. Daft Punk also provides excellent examples of using sidechain compression across the entire mix. Check out Discovery as well for these!

Of course, the good news is you’re pretty much guaranteed your DAW comes complete with a whole heap of very usable plugins that, where necessary, have sidechain selection available. This usually takes the form of a drop-down menu in which there’s a list of available channels and input sources from which to pick up your sidechain audio. These days it’s almost a rarity to find a production or mix that doesn’t feature some form of sidechain processing. Examples to listen out for and try in your own work are: using the lead vocal or vocal group to trigger compression or reduction of guitars, keys or other instruments that occupy the same frequency range as the lyrics you want to put across. Similarly, this technique could be used to reduce the accompaniment behind a solo or key melody. Imagine the amount of time involved in trying automation and balancing and re-EQing without the magic of sidechain processing.

I hope this brief explanation and examples has given you food for thought and you’re itching to go try some of these techniques out for yourself. I have only scratched the surface of what sidechain processing can do and when it can be used; let’s just say it is limited only by your imagination!