In addition to the normal signal input, a compressor has a 'side-chain' input.
In normal use, the amount of compression is related to the level of the input signal. The side chain allows the signal passing through the unit to be controlled by the level of another separate signal.
De-essing is an important compression technique using the side chain. Many singers have high level sibilants – 'sss' sounds – which detract from the quality of their performance. Equalizing the signal will reduce the sibilants, but also make the overall vocal sound dull. The sibilants can be selectively removed by compressing only when there is an excessive level of high frequencies.
The microphone channel is routed to a group with the compressor patched into the group insert points. The microphone channel is also paralleled into another channel via the line input.
The signal in the second channel is equalized so that high frequencies in the sibilant range are boosted. This channel is fed via an auxiliary output to the compressor side chain input.
Now, the compressor will react whenever there is a sibilant, reducing the gain for the duration of the sibilant and cleaning up the vocal sound.
It is also possible to use the side chain to fix a bass that 'booms' at a certain frequency, or other situations where a band of frequencies is occasionally obtrusive.
The side-chain input can also be used on a radio station phone-in. The caller's voice goes into the normal input of the compressor, and the presenter's goes into the side chain.
So whenever the presenter speaks, the caller's voice is pushed down in level!
That's not fair surely…?