The mysteries of the sidechain can be elusive as this question from an inSync reader would suggest:
“Could you explain some of the details of using a sidechained EQ to trigger either a gate or compressor? I understand that the gate or compressor becomes frequency dependent, but is that from BOOST or GAIN from the EQ?”
In his infinite wisdom the inSync editor looks down from on high and responds, “Yes.”
This is exactly right. When you properly patch an EQ into the sidechain of any dynamics processor you are making it frequency dependant to the extent that you cut or boost various frequencies on the EQ. The “detector circuit” in the device is what tells the “gain cell” (often a VCA) to raise or lower the volume of the signal going through it, which is what allows the device to behave as a compressor or gate, or whatever. When you make that detector more sensitive to certain frequencies by turning them up on the sidechained EQ you are in effect making the compressor (or whatever) more sensitive to those frequencies. While it may not be easy to understand, the results are simple. Boost 4k on an EQ in the detector and your compressor will become more sensitive to signals at 4k. Lowering the high frequencies in the detector of a gate on a snare drum will make it less sensitive to the hi-hat, which should keep it from opening erroneously. There are tons of other applications.
Another thing to remember is that no matter what you cut or boost on the EQ the dynamics device is going to always act on the whole signal. Don’t get confused and think that boosting 4k on the sidechain is going to “compress” 4k in the mix more. It only makes the whole device more sensitive to that frequency, what it does with that sensitivity still happens to the whole signal. There are exceptions. Things like multi-band compressors and gates do have the ability to apply themselves differently across the frequency band, but that’s another article.