When using some plugins in your DAW, you may be prompted to choose between post FX, pre FX, and post fader. Let’s identify what is post FX, pre FX, and post fader with regards to determining the behavior of a plugin.
What is Post FX
Let’s start by answering what is post FX. Let’s say you’re using sidechain compression in Ableton (or any DAW for that matter), you’re compressing one track based on the behavior of another track.
One common application with sidechain compression is to compress your bass dependent upon the behavior of your kick. The kick and bass and particularly sub bass share the same frequencies as I discussed in my low end mixing tutorial.
Using sidechain compression or sidechain EQ, you can duck the bass out whenever that kick plays, giving dominion of the low end to the kick just for that intermittent fraction of a second when it needs it.
This keeps your bass relatively transparent while creating space in your low end.
To use sidechain processing, you need to select a source track to control the behavior of your track and then designate whether it’s post fx, pre fx, or post fader.
Each of these determines at what point in that track’s signal chain the plugin should take the audio/input level from.
To answer the initial question of what is post FX, selecting this setting means that the audio will be taken at the end of the signal chain but without considering the fader position.
In other words, it will take the signal that’s been EQ’d, compressed, whatever other effects you have in your signal chain for that track and use that audio regardless of where the fader is set.
This is what will determine the behavior of the sidechain processing that you’re doing.
As I gave as an example earlier, sidechaining bass to kick is a common application to get the bass out of the way for the kick. In this case we’d drop a compressor on our bass and set the input/audio from source as that kick track:
When this compressor is set to “Post FX”, it’s compressing our bass by the behavior of the kick’s signal at the end of all of its processing. So again, whatever state or volume the kick is at the end of any processing on the kick drum track, that’s the level informing the compressor on that bass.
What is Pre FX
Pre FX on the other hand takes that signal at the start of your chain before any processing has been done.
In other words, this is basically the raw recorded audio itself which is determining the behavior of whatever sidechain processing you’re doing.
So in keeping with our sidechained bass to kick compression example, our bass compressor would ignore any processing the kick has on it and instead us the raw recorded or sampled kick drum to influence it.
What is Post Fader
Lastly, post fader (as in pre fader and post fader) does the same thing as post FX but also takes into account where the volume fader on the kick track is set.
In other words, if that fader is dropped all the way down, the compressor on the bass will be doing nothing because there’s no input level to affect its threshold.
Turn the fader up on the kick, and there will be more signal going into that compressor on the bass.
Pre FX vs Post FX Vs Post Fader – Which is Best
There’s no right or wrong answer when it comes to deciding which setting is best.
The important thing to mention is that you’re aware of which setting you use so that you can set your threshold on the sidechain compressor accordingly.
I say this because if you have the sidechain compressor set to post fader and set the compressor up the way you want, if you forget this then change the volume of the kick via the fader, the threshold on the sidechain compression on the bass will need to be adjusted.
For that reason, I personally prefer post FX as it’s the most set it and forget it friendly while using that processed and more practical version of your kick as the guide.
Of course with post FX you need to be mindful if you add any subsequent processing or adjust any existing processing on your kick track which might wildly change its output at the end of the signal chain, but it’s still my personal preference.
So now you know what is post FX, pre FX, post fader, and the differences between them all and can side your sidechain processing accordingly!