Sidechain Dynamics

Ah, sidechains.

So much potential for creative fun. So much potential for adding groove. For keeping instruments out of each others way. For totally rampant overuse throughout the last decade at least.

With that last point I'm referring to sidechain compression, which has been used to create heavy pumping effects in countless songs over the past while. It's use has become so prevalent it simply seems normal to me now.

I've used sidechain compression for a while now. I like the way it opens up space for my kicks and enhances groove. For the most part, I don't go too wild with it, though from time to time I'll get into some heavy pumping pad action.

Anyway, today's post is about sidechain processing - compressors, gates and expanders. Between these three processors, and a variety of input signals, lies a whole bunch of creative fun.

I'll just touch briefly on what sidechaining means and how it is typically set up.

Normally, when we run a sound through a dynamics processor, it reacts to the sound running through it - a compressor will turn the level down when the sound goes above the threshold, a gate opens when the sound goes above the threshold. With sidechaining, the processor doesn't care what's going on with the sound it's working on. It reacts to a separate signal, coming in via the sidechain input. The input signal (or "trigger") is typically set up on a separate channel in our sequencer, and is routed into the sidechain input. Depending on how we're using it, we may turn the trigger's level all the way down and send the signal to the sidechain input pre-fader. This means that it doesn't matter how much we move the trigger's fader, including turning it all the way down - the signal will be sent to the sidechain input at a constant level.

I'll go through some examples of how sidechaining can be used:

Sidechain compressing a bass to a kick: Low frequencies can quickly get out of control when both a kick and a low bass play at the same time. Sending the kick into a compressor acting on the bass means the bass will clear out of the way every time the kick plays. Problem solved! For the input trigger, there are a couple of options. We could use the actual kick we're using in the song, and send it post-fader. This means that only as much compression as required will occur - if the kick fades in or out at some point, the compression will gradually increase/decrease as required. However, sometimes we want to apply a steady, consistent pump to our bass, regardless of what's happening with the kick we can hear. An option is to put a copy of the kick on a separate track, program a steady beat, turn the level down to 0 and send it to the sidechain compressor. This way the bass will pump consistently throughout the song. Yet another option is to use a different trigger - kick drums often have long boomy tails, which can compress more of the bass than is actually required. For this reason, I generally use a very short closed hihat sample as the trigger. This allows me to set the timing of the pump very precisely by using the compressor's hold or release parameters.

Sidechain compressing any other sound to a kick: See above. Pads, pumping heavily on the offbeat, are common. Leads can come down very briefly, using a low ratio - hardly noticeable, but helpful for clearing out room for the kick. Percussive loops can groove in time with the kick. Heck, we could go so far as to send every sound except the kick into a group channel and compress the entire group every time the kick hits. Mega pump!

Sidechain compressing guitars/pads to a lead or vocal: This is similar to what radio announcers do when they speak over the top of a song that's playing - they start speaking, the music instantly drops down in volume so we can hear them clearly, then the music rushes back up when they stop speaking. We won't generally apply the effect in anywhere near as extreme a way - it's quite unmusical. But the principle remains - we want the centerpiece of our song to come through clearly - we want to make out every word, hear every note of the melody. So we lightly compress a wall of guitars using a vocal input, or a big pad using a lead input. Done subtly, most listeners won't even notice that it's happening - we've simply found a way to go over 0 dBs in the digital realm, that's all. (please don't take that statement at face value!) For this application, we want to send the vocal/ lead in post-fader, so the other sounds drop back only as much as needed. We'll generally want to use a release slow enough that the sound comes back up naturally when the vocal/ lead stops, rather than rushing up and letting the listener know exactly what was going on - though obviously this is a creative decision for each of us!

Sidechain compressing an instrument's reverb/delay to that instrument: This effect turns the reverb down when the instrument is playing, then brings the reverb back up as the instrument fades away. There are a number of creative applications for this, ranging from very subtle to quite extreme. It can be applied to short room reverbs and large caverns. Likewise, it can be applied to delays. It could potentially be applied to other effects (distortion, chorus and so on), but the effect would be much shorter in nature - it would only apply to the end of each note (unless it's a distortion which feeds back on itself!)

Sidechain gating/expanding a pad with a rhythmic pattern/loop: Using a short, snappy gate, a pad can be chopped into an interesting lead pattern by running a drum loop (or any other rhythmic element) into the gate's sidechain input. If the pad features some nice evolving modulation, this will keep running throughout the chopped up sequence - quite different to actually playing the same sequence and having the modulation start from the same point each time a note is triggered. Rhythmic loops with a wide dynamic range can work better than highly compressed loops - a wide dynamic range means there is a more defined space between "note on" and "note off". Using compressed loops will require finer tuning of the input threshold. A more subtle version of the chopped gated effect can be created by using an expander with a low ratio - the sound will only drop marginally between transients, rather than completely cutting out. Using automation, this also allows us to smoothly transition from unbroken pad chords/notes to heavily chopped chords/notes and back again.

Automating or changing the input trigger: A lot of the time, the automation will need to be reasonably heavy-handed to make an impact on the way the dynamics processor reacts. The input trigger can be heavily EQ'd - if we are punching out a rhythmic pattern using a percussive loop into a gate, an EQ can change the percussive loop so the kick comes through much more prominently (or another area of the loop). A transient designer can be attached to the percussive loop, transitioning it from very short and snappy to more smooth and drawn-out.

There are many more creative possibilities. Whether it's punching parts of sounds out or punching them in, whether it's pushing something away from another sound or locking it to it, or even whether it's a very subtle application which transparently solves an issue, sidechaining is a very handy resource to have at our fingertips.

Have fun, keep making great music!
Fabian