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Many Cubase users are unaware that they can perform sidechaining in the VST environment. This tutorial will look at a simple way of achieving this with a gate.
Although this facility is not as well implemented as it is in other platforms such as Digidesign's Pro Tools, it is still possible to feed a sidechain input to a gate or compressor that supports the function. At present there are only a handful of VST plugins that support sidechain inputs - for the purposes of this tutorial will use Waves C1 compressor/gate.
Lets first look at using a rhythmic sound such as hi-hats to trigger a gate - this is commonly used to add some interest to static pad and guitar lines.
Firstly I opened up an instance of Embracer and played in a C chord for 2 bars - not very inspiring I'm sure you'll agree;
2-Bar Pad
I've opened up LM-7, the drum VSTi bundled with cubase and created a 2-bar hi-hat pattern that i will use as the sidechain input;
2-Bar Hi-Hat Pattern
The next thing we need to do is create a stereo 'Group' track which can be done by going to the Project Menu->Add Track->Group Channel->Stereo. 
For the purposes of this tutorial, name the group channel 'Sidechain'. Open up the group channel and add the Waves C1 comp-gate as an insert, remembering to choose the stereo version of the plug, not the mono version.
Now we have to get down and dirty with the routing!
Firstly we need to route our hi-hats to the group, so in the mixer find the LM-7 VSTi channel and change the output routing to 'Sidechain' as seen here, and then move the pan control on the LM-7 channel fully left. Whilst we are in the mixer, find the Embracer channel and route that to 'Sidechain' also, but pan this channel fully right.
Now you need to bring up the C1plug-in. The C1 offers sidechain facilities by treating one side of the signal (either left or right) as the sidechain and the other as the signal we want to gate. As we have panned the hats to the left we want the left to act as the sidechain input. In the C1 plugin window find the dropdown option labeled 'Key Mode' - it is probably set to stereo at present but we need to change this to 'L->R' as pictured below. This treats the left hand side as the sidechain input.
Pressing play now will result in silence as we need to set the gate parameters so that the threshold is exceeded by the hi-hats. In this instance I used the settings as shown in the picture below. However, depending on the exact sound you are after, you may need to use different settings.
We're almost there although the more eagle eyed amongst you (or those of you who have just pressed play) will have noticed that due to our routing, our gated pad sound is only coming out of the right channel - we want some degree of control over the placement of this so we need to change the panning of the right channel independently. The default 'balance panner' will 
not allow this degree of control so we need to change the pan type. This can be achieved by right-clicking on the balance control and selecting 'Stereo Dual Panner' - we now have 2 pan controls for the 'sidechain' group rather than the previous single balance control. We can now choose the pan control for the right channel and drag it to our desired position - in this 
case I have panned it dead centrally as can be seen in the screenshot.
With any luck you should now have a rhythmically gated pad sound which you can change the rhythm of by changing the MIDI drum pattern on the LM-7s MIDI track. This is what my pad sounds like:
2-Bar Gated Pad
The next tutorial will look at using similiar principles to perform 'ducking' or 'pumping' with compression - it'll also cover alternative methods of routing the signal to the sidechain. Until then, happy gating!
