Side-chain filter does what exactly?

Question

I'm sure this must be covered somewhere, but I can't find specifics in the manual or previous posts here.
Which signal does the side-chain filter in LP8's compressor affect--the side chain signal (i.e., compression only occurs when certain frequencies are present in the sidechain source signal) or the audio that's being compressed (i.e., only compresses certain frequencies in the track in which the compressor is inserted)?
If it's the former, how does one compress only specific frequencies using a side-chained signal? I'm asking partly because I want to use a kick drum to compress a somewhat bright bass, but I'd like to try compressing only the lower frequencies of the bass, down where the kick drum's frequencies overlap.
Hope I've explained this all clearly.

Answer 1

Randall--thanks so much for all your info. Like Dalek, I initially thought the side-chain filter does what I needed to do, but I guess not. The solution is a bit complicated, but that's not your fault; you didn't write the software, and at least you thought of a clever way to do selective-frequency ducking in Logic. I haven't had time to try it yet, unfortunately, but hope to tomorrow. I've taken the liberty of pasting below your post that you linked to on the Logic Pro Help board (hope this is OK), mostly so that anyone else with this same problem can find the answer all in one thread.
Randall's post:
Pursuant to my previous tip on improving noise gate performance:
http://logicprohelp.com/viewtopic.php?t=17553
. . . a couple of people asked me if I'd write up how to do selective frequency bandwidth ducking to improve a signals presence in a busy mix. So I thought Christmas day would be a good time to write this up and create an example as I'm deliberately not doing any "real" work today!
So here's the typical problem. A bunch of noisy guitars driving along and you want to inject an acoustic guitar or vocal lead in there once in a while. But you can't get the track hot enough or eq'd enough to overcome the noisy guitars. Ducking the guitars with either automation or a sidechained compressor might be too noticable, so this trick allows you to just duck the specific frequency range that is masking your lead track. Here's how to do it:
Note: the attached LP8 example just uses some stock Apple Loops that everyone should have, (I hope).
1. This is a mix technique and we'll be assigning plugs to auxes, so make sure PDC is set to "All" in your preferences.
2. Assign your noisy guitar tracks to a bus. We'll use bus 10 as in my example.
3. Assign your lead track to your main outputs. Create a Send on this track to bus 11 and set its level to unity, (ctrl-click the send).. This will also create an aux in your mixer. Delete this aux.
4. Create 2 new aux channels. Assign both their inputs to Bus 10, and both their outputs to your main outs.
5. Don't put any plugs on the first aux. On the second aux, (from top to bottom) place a Linear Phase EQ, a Noise Gate, then a Gain plug.
6. Set the EQ up with a Lopass and a Hipass only, with steep 48dB/oct slopes. Leave the Q at .71.
7. Set the Noise Gate as follows.
Threshold & Reduction: -100dB
Attack, Hold & Release: 0mS
Hysteresis: -20dB
Lookahead: 20.000mS
Sidechain: Bus 11
8. Open the Gain plug and enable Phase Invert(s). Close the Gain plug.
Here's what's happening in my example. When the acoustic guitar plays, the gate opens up on the second aux track that has the flipped phase noisy guitar signal. With the EQ bypassed and the aux faders at unity, you'l hear none of the noisy guitars as they will cancel out. In my example, I want to hear more of the body of the acoustic guitar which is about 225 to 1300 Hz so that's what I've set the EQ to pass. So now everything below and above that band won't be canceled. If I drop the fader down on the second aux, then the signal isn't cancelled as much.
When the acoustic plays, the body of the guitar sound comes through the mix a bit more as I'm attenuating the noisy guitars just in that band and just while the acoustic is playing. So your guitars can keep driving along!
You have a lot of control as you can tweak the reversed aux's fader for the amount of ducking and tweak the lo and hi pass freq parameters on the EQ to tune the band that gets ducked.