Part 2 of our sidechaining in Cubase / Nuendo tutorial.
If you have not read the previous article please click here
In the previous tutorial we covered one way of routing to allow us to rhythmically gate a static pad part. This worked fine for our purposes but there are other applications of sidechaining where this method is not suitable.
Compression is another process that can be triggered by a key input and this is common in both music production and broadcast environments. In a broadcast environment compression can be used to automatically bring down the level of background music when a presenter/DJ speaks - this process is often called ducking.
In music production, especially in some forms of dance music, compression can be used to bring down the level of one signal whilst another is apparent - the most common application of this is the bass guitar/synth being momentarily reduced in volume every time the kick drum is struck.
If you followed the previous article you may be wondering why we would need to change the routing when it worked fine. Well, in our previous example our key input (our hi-hat track) was created specifically for the purposes of triggering the gate and wasn't intended to be heard on it's own - we only wanted to hear the gated synth. However, when working with sidechained-compression it is common to want to hear both signals.
We are going to demonstrate this with a synth bass line and a 909 style kick drum. As both of these instruments are operating in the low frequency area, using sidechain compression is common to make 'room' for the kick drum.
First lets have a listen to the bass line and kick drum together before sidechain compression has been added:
Synth Bass and Kick Drum - No Sidechain Compression
What is noticeable in this example is that although both kick drum and bass are audible, it's all a bit messy and there's no real punch to the kick drum as the bass is overpowering the low frequencies. What we want to do is compress the bass synth, but only trigger the compression when the kick drum sounds.
Initially, as in the previous tutorial, we need to setup a stereo group channel track (I've labeled mine 'Sidechain Comp') and then place 'Waves C1 comp-sc' as an insert on this group track. Remember to use the stereo version of this plugin!
Ok, we now need to look at how we are going to route our signals to the compressor. Firstly we want to route the output of the bass synth to the group making sure that the bass is panned fully right.
Unlike yesterday we don't want to route the key-input (kick drum) to the compressor via the channels output as this will effectively mute the kick drum. 
Instead we are going to route the signal via an aux send. If we open up the kick drums channel we can see the aux sends on the right hand side so turn on the send, select our group track ('Sidechain Comp' in my case) and move the send level until it is at 0.00dB as seen on the right. If you remember the previous tutorial you will realize that we need to send this to the left hand side of the group channel. To do this we need to access the pan controls for the aux sends - to do this you click on the 'Send Display Modes' button and select 'Routing' as seen on the right. This will bring up a new set of controls for the aux send as seen in the next screenshot. From here you can change the panning of the send - for the purposes of this tutorial we need to set the pan control so that it is fully left.
Now we should have both of our signals running into the stereo group - we now need to set the compressor up to accept the sidechain input. As with the previous tutorial we need to change the 'Key Input' mode from 'Stereo' to 'L->R'. We also need to change the pan controls on the group by right clicking on the pan control, choosing 'Stereo Dual Panner' and then panning the right hand channel to the centre.
The routing is now complete so we can open up the compressor again and start playing with the controls. For the purposes of demonstration i have set the ratio at 3:1, the threshold to give about 6dB of gain reduction and an instant attack. Let's listen to a couple of different release times:
50ms Release - You can hopefully hear the reduction in the level of the bass synth which manifests itself as a clearer, punchier kick drum
150ms Release - In this example the synth is lowered in volume and takes longer to return to normal - this gives an unnatural 'pumping' sound which many dance producers utilize in their productions.
