Sidechain
What is a sidechain input?
A sidechain is an input on a compressor that allows you to use a separate equalizer for frequency-dependent signal processing. This is most commonly used for de-essing vocals (removing loud sibilants). To accomplish this, you connect an equalizer to the sidechain input on the compressor. Then boost the levels on the equalizer at the frequencies you want tied to the equalization. Note that this boost on the equalizer does not affect the signal that passes through the compressor. It merely triggers the compressor when loud levels are encountered on the selected frequencies. For de-essing, a boost of about 6-12 dB on the range between 7 kHz and 13 kHz on the sidechain equalizer works well.