Sidechain Compression in Logic

This is the second instalment in an informal series of Logic quick-reads, following on from my previous article on sampling drums in EXS24. Today we’re covering the topic of sidechain compression.

Compression is a huge topic in on itself and it really deserves its own space on this site, and while understanding compression in its own right can help you better understand today’s topic, that doesn’t prevent you from using sidechain compression in your own work. Let’s start off with some basics first…

What is Compression?

Broadly speaking, compression gain regulation – put simply, it’s a glorified volume control. Traditional compression, on say, vocals, analyses the incoming signal and when it passes a certain threshold it attenuates it (meaning it makes it quieter).

Here’s the excellent LOOK MUM NO COMPUTER demonstrating how an optical compressor works:

Of course there’s more to it than just that. A typical compressor would have controls for:

I want to avoid doing a huge compression 101, as no doubt I’ll get to it later and there’s already a good amount of literature out there on the subject. Special mentions go to Patches.zone’s animated guide to compression. It’s Ableton based but don’t let that scare you off as it’s very nicely presented.

What is Sidechain Compression?

Sidechain compression is the process of using an external sound source to trigger a compressor, typically a kick drum. It’s a sound you’ve heard a billion times and it’s littered all over EDM, Techno, Disco, Progressive and French House etc.

Here’s one half of Daft Punk (plus DJ falcon) with their 2002 hit So Much Love to Give. I’ve embedded from just before 2.09 where the compression kicks in (using the bargain Alesis 3630 compressor). You can hear a few bars of the untreated sample before the 909 bass drum comes in and ducks the whole mix.

Sidechaining in Logic

So how do we get sidechain compression working in Logic X? Until recently, compression and the analysis of sidechain triggers in Logic was a lot more fiddly, including the need to route MIDI instruments to busses and what-not. Luckily since Logic 10.3.2 it’s a little easier now.

Anyway, let’s get down to it. I’ve recorded eight bars of Island And Holiday’s 1984 b-side Living into Logic. Here it is:

We’ll start off demonstrating how to sidechain to an audio kick. I’ve imported a kick drum on to another audio track and sequenced it on each beat of the bar (imaginative I know). Here’s the sample and the kick with no sidechain compression.

We’ll be using Logic’s own compressor for this exercise, but most compressors have a sidechain analysis feature. Here’s what the default preset looks like:

First off we need to route the kick into the compressor’s sidechain analysis circuit. On the top right of the plug-in window is a dropdown menu titled Sidechain.

Click here and route the relevant audio track into the compressor.

This is where it’s helpful to have all of your tracks correctly named 👀

You should now see the VU meter in the middle of the plug-in reacting every time the kick hits and you should hear an audible ducking sound. You could leave it there, but lets tweak the parameters a little.

I mentioned earlier the ratio and threshold determine the amount of compression – you’ll need to set these to taste. Having a ridiculously low threshold and high ratio will give you an artificial pumping sound akin to a more EDM/Guetta/Prydz-type sound. Here the threshold is at -25 dB and the ratio is 20:1:

I’ve rested on something more conservative to let the original sample breathe a little. From there on in, the rest is sort of down to you.

Attack and Release Top Tips

For sidechain compressing using drums as a trigger it’s recommended to have a short attack time – drums have sharp transients that you want the compressor to allow to attenuate the source material; having a lengthy attack time would reduce the impact of this type of compression.

However if you were sidechaining some guitars with a vocal part, then setting a more subtle attack time might be the right approach. I’ve set my attack at as close to 0 ms as possible, some compressors might introduce a clicky artefact if you set the attack too short, so be careful and listen out for that, backing off a little if it becomes a problem.

The release is how quickly the compressor will return to normality after the trigger has passed the threshold. Shorter release times give a snappier sidechain and longer release times are better suited to a lazy feel.

Some producers like to calculate their release times dependant on their BPM (I am not one of them – I prefer to use my ears) but if you’re interested in this approach you can follow this link to read more about it.

If that was tough to follow, try switching from the compressor’s meter display mode to the graph and adjust the attack and release values with a heavy compression. This visual aid might help marry the concept with what you’re hearing.

You might also notice that there’s an “auto” option for the release. As I mentioned above I tend to prefer adjusting the release times manually as it allows for more control, but be my guest to experiment.

Kneed for Speed

Similarly to the attack and release times, the knee will affect how the compression sounds. It’s a little harder to explain but rotating the control from anti-clockwise (hard knee) to clockwise (soft knee) will hopefully clear it up.

I generally prefer the sound of the softer knee compression as to me it sounds more natural where the hard knee sounds a bit brutal, but it’s rarely as black and white as that.

There’s an entire article dedicated to it on Home Studio Corner, so I’d suggest reading that if you’re still struggling to hear what it’s doing.

Lastly on the compressor’s features is makeup gain, or sometimes called auto-gain. In an ordinary mixing scenario you might enable the makeup gain to compensate for volume lost during compression, as compression is at its heart a glorified automatic attenuator.

With sidechain compression I can’t see a good reason to have it switched on so set the dial to 0dB and have the auto gain set to “off”. Having it on seems counter intuitive and some compressors (Live’s native compressor for example) will automatically disable the feature when sidechain mode is engaged.

Software Instruments and Backwards Compatability

This is great for an audio kick drum, but what if we’re triggering our kick from a drum machine such as Ultrabeat? This is relatively straight forward as contemporary installs of Logic will allow the compressor to see software MIDI instruments.

However this was not always the case. If you’re running an older version of Logic you’ll need to follow the proceeding steps.

I’ll be using Ultrabeat for this example, but most drum machines or samplers will work in a similar manor.

Firstly, create a multiple-output instance of Ultrabeat. You can do this when creating the instrument or you can change a stereo track to a multiple-out version retrospectively.

This will allow you to route different voices from Ultrabeat to different outputs (EXS24, Battey and most other drum samplers use this exact same method). In Ultrabeat, click where it says Sub 1 and route route your kick to an appropriate output.

Output 3-4 is the next the next available but if you’re thinking ahead you could use one of the mono outs, starting at 17.

In the mixer, you’ll see a plus symbol just above the solo and mute buttons (1). Clicking this will automatically create an auxiliary track with the next available Ultrabeat outputs as the aux input (2). You can re-assign the input to any available Ultrabeat output if you want (3).

Great – so we’ve separated our kick from any snares, claps, hi-hats and other percussion. This is necessary otherwise each element of the drum kit would trigger our sidechain compressor.

Next we’re going to route the kick to a return track. This used to be a necessary step for software instruments but isn’t anymore, however it still has its advantages.

Create a send by clicking and holding down on the send slot on the newly created auxiliary track and choose the next available send.

Now, send the kick to that return out at unity gain (0 dB) by clicking on the send and turning it clockwise.

N.B In the compressor ensure you have the sidechain dropdown set to the correct bus.

Next is the clever part – click and hold down on the send and change it form Post Pan (the default) to Pre Fader. Pre Fader means the kick will be send out at unity gain before any changes you make to the kick’s channel strip.

Pre Fader sends are commonly used in live situations to get different monitor mixes for the musicians on stage.

This means you can have the kick lower in the mix while still getting a nice healthy sidechain trigger.

You might notice that your kick is now twice as loud – not ideal. This is a really common mistake and is caused because the newly created auxiliary is playing the kick at the same time, thus doubling the volume.

In order to treat is just as a trigger we need to remove its output. Click and hold where it says Stereo Output and change it to No Output.

The very last thing to do is to make the channel strip solo-safe. This is useful because it means the channel wont mute even when other tracks are solo’d, meaning we can work on the compression on our track without needing the kick to be playing at the same time.

To do this ctrl + click on the Solo button and a red slash through the button will indicate it’s working.

Sidechain Filter

This is the last topic on sidechain compression today – it’s that of using the inbuilt filter in the sidechain analysis circuit.

I never knew what this was for and when someone showed me it was a bit of a no brainer why it’s so useful.

To demonstrate why this is useful, let’s first think about a kick drum: there are two things to notice about this kick I’ve made. The amplitude (volume) decreases but also so does the frequency.

As you can see, the distance between the cycles gets greater as the kick quietness – this is the frequency dropping. So the beginning is the sharp transient and the end is where the subbier frequencies are.

We can use an in-built filter in the compressor to high-pass out these frequencies, giving us a much snappier sidechain trigger.

Let’s hear the difference: here’s a relatively heavy sidechain compression. Notice the original track is almost completely ducked.

To combat this, click the Sidechain toggle to the right of the compressor.

The detection options only really apply to stereo tracks (max/sum) and Peak and RMS can be switched to taste. RMS is average volume where as peak will look out for the highest point of the signal.

[This in an incredible video on the difference between peak and RMS metering. Ed.]

Turning our attention to the filter, set the Mode to HP (high-pass – filtering out frequencies below the cutoff) and adjust the Frequency to taste. I’ve settled on ~120 Hz, but each kick is different.

Leave the Q (resonance) where it is for now.

You can click the Listen button to hear your filtered kick. The aim is to isolate the pokey transient at the beginning of the kick to get a quicker sidechain signal.

Keep an eye on the gain reduction VU meter and ensure your release is quick enough to hear the results. If you want a lazier sidechain you can still pile on the release but this allows for much more control.

Here’s the same compression settings as before but now with our filtered sidechain signal. As you can hear it’s letting more of the track through as the compressor is closing more quickly.