We’ve looked at the subject of side-chaining quite a few times in the past but there is one area of the subject that hasn’t been touched upon. Frequency dependent side-chaining is a process which allows us to home in on a specific frequency range for processing.

In this tutorial I’ll show you how to set up side-chain filters using Fabfilter’s ‘Pro-C’. Don’t worry if you don’t have this I’ll suggest some alternatives as we go. You should be able to create almost identical effects using Reason, Logic or Cubase.

Step 1 - The Source And Destination

The first thing to think about here are the specific sounds you want to use. There is the ‘source’ which will be used as the trigger for our compression effect and the ‘destination’ being the audio we actually want to process.

Of course depending on the situation these two sounds could vary drastically but in this case I am using a drum loop to apply some syncopated gain reduction to a synth line.

Download audio file (1.mp3)

The two untreated sounds we’ll be using here

As you can hear our drum loop contains sounds of a few different frequencies. If we were to apply traditional side chaining / ducking the synth line would be reduced in level every time a transient event occurred. Using frequency dependent side chaining we can home in one element in our drum loop and use it to apply the reduction. Here’s how…

Download audio file (1b.mp3)

Treated with traditional side chaining the synth loop is compressed continually

The synth line is side chained using the whole loop as a trigger

Step 2 - Some Basic Routing

Ok, so first up things need to be set up correctly. Luckily the routing here is pretty much identical to a standard side chain compressor. Things will vary a little depending on which DAW you are using but in Logic it’s just a case of telling the compressor which audio stream you would like to use a side chain input.

The Pro-C is routed for side chaining

Ableton Live, Cubase, Reason and Pro Tools all now include side chaining as a native function and the process is equally straight forward in all of them.

Go ahead and place the compressor you are using on the channel you want to process (in this case the synth) and select the source as the side chain input. (the drum loop). This is as complex as it gets and you should be ready to go in no time.

Step 3 - Finding The Frequency

With this basic routing in place we are ready to start tweaking the compressors settings to ensure we have targeted only the chosen frequency range for our side chain effect. For this to work you have to double check that the compressor you are using has a side chain filter. Logic’s built in compressor does as does Reason 6’s channel strip dynamics.

Reason 6 is capable of side chain filtering

… As is Logic’s stock compressor

No matter which compressor you can use there will be an option to activate the side chain filter. In Logic’s compressor it’s in the drop down section of the plug-in, in Reason it’s in the filter section of the channel strip… Again every DAW and plug-in differs but the theory remains the same regardless.

The Pro-C has an ‘expert’ section that slides open and reveals the side chain filter. Once engaged high and low pass filters can be applied to the incoming side chain signal. To hear what you are doing here the effect can be auditioned in isolation.

Pro-C’s Expert section reveals the side chain filter

Here I simply applied a low pass filter until all that could be heard was the kick drum in the loop. This means that the side chain effect will only occur when this low frequency signal is present. A great trick.

A simple low cut is applied to home in on the kick.

Download audio file (3.mp3)

The low pass filter is isolated to help identify the correct area

Step 4 - Tweaking The Ducking Effect

From this point on it’s simply a case of adjusting the threshold and ration of the compressor to attain the desired amount of gain reduction. This is also identical to using a standard side-chain compressor.

It’s really important at this stage to get the attack and release times set correctly. I like to use a very fast attack time for any side chaining application. This is so the effect is as near to instantaneous as possible. Saying that a very small amount of attack (a few ms) can help to eradicate clicks and pops.

With these simple settings in place you are done. You should find that you can use this in the same way as any other ducking effect with the added bonus of being able to target a very specific area of your sound.

The final effect in action

Download audio file (4.mp3)

The two sounds together with the final effect in place

Step 5 - Some Ideas And Other Uses

So as we’ve seen this technique is awesome for clamping down on a kick drum in a loop but what else could it be used for. Well essentially you could apply to any signal. I’ve used it to create clutch groups in the past, ducking a closed high hat pattern with an open high hat signal and also to clean up busy percussion loops.

Another obvious and more traditional use for this type of processing is de-essing. By feeding a duplicate of your vocal to the compressor as the source an intense high pass filter can be employed to eradicate any sibilance in the signal.

Ultimately the way you use this is up to you but I find that it allows much more freedom than standard static side chain compression.

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