Ableton Live 8 has native sidechaining that only works with the native Compressor plugin or any other possible Max4Live devices. Steinberg's VST3 specifications include functionality for VST3 compatible plugins to allow external sidechain signals but Live does not yet support the VST3 protocol. In this tutorial I will explain how to set up a third party sidechain enabled VST plugin in Ableton Live and I will present two example scenarios.
You can view my other related sidechaining tutorials in the links provided:
Side Chain Compression in Ableton Live 8 - Part 1
Side Chain Compression in Ableton Live 8 - Part 2
How Does It Work
During my research I realized that not all non VST3 sidechain plugins will work with the method I am going to present to you. That's because the third party plugin needs to have external sidechain input implemented into its code beforehand.
The way it works is by sending the signal that is going to activate the sidechain process to the plugin's external sidechain input in Ableton Live. Doing that will cause the signal to mute, though. Thus, another audio channel needs to be created to receive the source signal and output it back to the Master channel. Let's have a look at a couple of examples:
Sidechain Compression with Fabfilter Pro-C
Create a signal that is going to modulate the sidechain input like a kick drum. I used a drum rack preset from the Ableton library and draw a basic 4/4 kick pattern. In another channel create the signal that will be compressed by the sidechain input. I used an Analog pad preset and draw a long E1 MIDI note.
The loop currently sounds like this:
Depended on your sound, an instance of the Fabfilter Pro C either in mono or stereo can be used. In my case I dragged an instance of the Fabfilter Pro-C (Mono SC) as an insert to the pad channel.
Click the EXPERT button in the Pro-C interface and select Ext under the SIDE CHAIN heading to activate the sidechain input. On the kick channel, select the pad track from the Audio To dropdown. The Pro-C Sidechain input gets automatically selected on the Output channel underneath.
Dial in your preferred settings of Threshold, Ratio, Attack, Release, etc that you would like your compressor to have. Try playing back your loop and see how the sidechain compression gets activated when the kick MIDI event occurs. You can view the Pro-C graph that clearly displays the compression being applied. By clicking the Audition button in the SIDE CHAIN area of the compressor, you can audition the source signal that causes the compression. While you are in Audition mode you can use the filter controllers just over the Audition button to select a particular frequency range from the source signal that you would like to act as the source instead of the original one. Finally the sidechain GAIN knob can be used to further adjust the sidechain input level.
The loop would sound like this now:
The kick signal is muted at the moment since it's being sent to the compressor's sidechain input. To get the signal back, we need to create an extra audio channel, select the track where the kick is located and set the Monitor to In at the In/Out section of the audio channel. If we had more samples loaded and playing in our rack, we would potentially have had to select the particular kick sample from the Input Channel dropdown.
The final loop:
Sidechain compression with Cytomic's "The Glue"
Similarly, using Cytomic's The Glue compressor plugin we can create the same effect. Just drop it as an insert effect on the pad channel and reselect that channel from the kick channel's Audio To output type. The Sidechain input gets automatically selected on the Output channel underneath.
Select appropriate settings for you compression and hit the Ext In button at the bottom of The Glue's plugin interface. Like our previous example, you can use the HP knob to filter out any frequencies you don't want from your source signal. If you had the audio channel we created in our first example deleted, create another one, select the track where the kick is located and set the Monitor to In at the In/Out section of the audio channel.
As you can see sidechain functionality can easily be achieved with third party software in Ableton Live. There are exceptions where these plugins might not work, though, since they might need a host that supports the VST3 specification.