Side-chaining
When side-chaining, the compressor uses the volume level of an input signal to determine how strongly the compressor will reduce the gain on its output signal. This is used by disc jockeys for ducking – lowering the music volume automatically when speaking. The DJ's microphone signal is routed to the compressor's sidechain so that whenever the DJ speaks the compressor reduces the volume of the music. A sidechain with equalization controls can be used to reduce the volume of signals that have a strong spectral content within a certain frequency range: it can act as a de-esser, reducing the level of vocal sibilance in the range of 6–9 kHz. A de-esser helps reduce high frequencies that tend to overdrive preemphasized media (such as phonograph records and FM radio). Another use of the side-chain in music production serves to maintain a loud bass track without the bass drum causing undue peaks that result in loss of overall headroom.
A linked stereo compressor without a sidechain input can be used as a mono compressor with a sidechain input. The "key", "trigger" or sidechain signal is sent to the left input of the stereo compressor while the signal that is to be compressed is routed through the right channel.
This technique is not to be confused with Parallel compression, which was referred to as "side chain" compression in a 1977 Studio Sound article by Mike Bevelle.
Read more about this topic: Dynamic Range Compression