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Side Chain Processing In Pro Tools
This week at the corner we will look at a mixing technique known as side chaining. After breaking down some basic concepts, I will walk you through the process of setting up a keyed gate in Pro Tools, a great skill to add to your bag of mix tricks. While the simplicity of the Pro Tools mixer actually makes this task fairly easy, side-chains are generally considered an advanced mixing technique so you may want to brush up on the basics of compressors and gates before you proceed.
What is a side chain?
Typically a feature found in dynamics processors (compressors, limiters, gates and expanders), side-chain inputs allow the processor to be triggered by a separate signal as opposed to the direct signal it is set up to process. When you place a compressor on a track, that compressor controls the dynamic range of the signal based on the interaction of that signal with the threshold. As the signal breaches the threshold compression occurs, with the amount of gain reduction defined by the ratio and speed determined by attack/release settings. By using a compressor’s side-chain input, the signal interacting with the threshold (controlling the amount of gain reduction) is actually separate from the signal being processed.
Basically, a side-chain allows us to control the dynamics of one signal using the dynamics of another. Why would anyone want this you ask? By controlling a dynamics processor with an external signal or “key” we can achieve some pretty cool things.
For example, to control the dynamics of a bass guitar you would generally use a compressor. But let’s say the bass is a bit loose and we want to control its dynamics using the dynamics of the kick drum instead. To accomplish this, you could send some of the kick drums signal over to the bass compressor’s side-chain. When the kick drum strikes, the bass guitar’s compressor reacts as if it where inserted on the kick drum track. This example is often called compression keying, since the bass compressor is being controlled or “keyed” by the kick drum signal. For even more control, you might use a “keyed” gate to control the bass against the kick. Confused yet? Don’t worry. Once we walk through an actual set-up in Pro Tools it will make more sense.
Using Side Chains In Pro Tools
Most dynamics plug-ins in Pro Tools support side-chain inputs. The side-chain or key input on these plug-ins can be found at the top left hand corner of the plug-in window. Typically one would send the control signal over a free mono bus and set the key input (side-chain input) on the destination plug-in to receive from the same bus.
Plug-ins that support side-chains often have switches that activate the key input, refer to the specific plug-in’s manual to learn where this control is located.
Example: Beefing Up A Kick Drum With A Keyed Gate
Setting up a keyed gate is a classic example of side-chain processing. In this scenario we are looking to double the kick drum with a low frequency sine wave (using signal generator). Our goal is to trigger the sine wave at the exact same time as the kick drum. To do this we will use the DigiRack Expander/Gate Dyn 3 plug-in after our signal generator. By sending the gate some of the signal from the kick drum track we can trigger it to open and close in time, allowing the continuous sine tone to pass through only when the kick drum strikes.
First, create a new mono aux track next to your kick drum and insert the plug-ins Signal Generator (Other>Signal Generator) and Expander/Gate Dyn 3 (Dynamics>Expander/Gate Dyn 3). If you don’t have a kick drum track to play with, you could create an instrument track and use Xpand to generate the kick.
Set the signal generator plug-in to generate a 40hz sine wave tone. If you are using headphones or speakers incapable of producing sub-bass frequencies try something higher (like 80hz) to practice the exercise.
Hint: try using a frequency that is in the same root key as your track. For example if my song were in the key of A, I would use a 55hz sine tone. While you are setting up the gate, you may get better results using a higher frequency to tune the attack and release times and then lowering the frequency as sub-frequencies can be hard to hear when trying to choose precise settings.
Now we can set up the side chain. On the kick drum track, create a send to a free mono bus (in this case I have chosen Bus 1, you could use any bus just as long as you aren’t using that bus for any other routing). After creating the send, set it to unity gain and check the pre-fader button. To make a send pre-fader, simply click on the “P” within the send window. This allows you to set and forget the key level without worrying about the level of the kick drum tracks main fader.
Remember Pro Tools key inputs are monophonic
After feeding the control signal into a bus, bring up the expander/gate plug-in and configure its side chain input to receive from the same bus (in this case bus 1).
To activate the side chain, click on the external “key” icon (the key icon should light up in blue when the side chain input is active). This tells the gate to react to the key input (our kick drum) instead of the sine wave.
In the gate plug-in set the ratio control to 100:1 and the range to -80dB, this will create a true gate. Now trim the threshold parameter on the gate to react to the incoming kick signal. The gate should open only when the kick drum strikes, staying closed at all other times. Be careful when setting the threshold to avoid gate chatter (where the sine wave stutters its way through the gate, instead of passing smoothly), use a higher threshold setting if you experience this. In this case a threshold of -10dB worked for me, but you may need to adjust based on the level of the incoming kick signal.
Hint: use the gates side-chain “listen” function (the small speaker icon next to the “key” icon) to fine-tune your threshold.
So the kick sounding pretty thick, all that is left for us to do is adjust the gate’s attack, release and hold times to achieve the desired effect. Use fast attack and release times to add a little extra punch, or try using longer hold and release times for a cool 808-style effect. Adjust the balance of the kick and signal generator to taste.
Other Uses For Side-Chains
Dynamics processors with side chain inputs can be used for a variety of creative mix tricks and techniques. Try setting up a homemade de-esser or frequency specific compressor/gate by feeding a filtered version of the signal into the key input. Many compressors’ side-chains have built in filters designed just for this purpose. Use side-chains to automatically “duck” the volume of music under dialog by keying the music tracks compressor with the dialog. Once you get the hang of side-chains in Pro Tools I think you will find they open up a whole new world of creative possibilities in your mix.
Pro Tools Tidbits:
- Line 6 announced Pro Tools RTAS support for their GearBox amp simulation plug-in.
If you have a Pro Tools related question that you would like me to take a shot at answering or have any feedback, please send it to brian@audioMIDI.com. I will do my best to select the questions that seem to stump the most people.
As always, stay tuned to the audioMIDI.com eNews for the latest reviews and new product info.
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