Side-chaining uses the signal level of another input or an equalized version of the original input to control the compression level of the original signal. For sidechains that key off of external inputs, when the external signal is stronger, the compressor acts more strongly to reduce output gain. This is used by disc jockeys to lower the music volume automatically when speaking; in this example, the DJ's microphone signal is converted to line level signal and routed to a stereo compressor's sidechain input. The music level is routed through the stereo compressor so that whenever the DJ speaks, the compressor reduces the volume of the music, a process called ducking. The sidechain of a compressor that has EQ controls can be used to reduce the volume of signals that have a strong spectral content within the frequency range of interest. Such a compressor can be used as a de-esser, reducing the level of annoying vocal sibilance in the range of 6-9 kHz. A frequency-specific compressor can be assembled from a standard compressor and an equalizer by feeding a 6-9 kHz-boosted copy of the original signal into the side-chain input of the compressor. 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, while still keeping the bass out of the way of the drum when the drum hits.
A stereo compressor without a sidechain can be used as a mono compressor with a sidechain. The key or sidechain signal is sent to the first (main) input of the stereo compressor while the signal that is to be compressed is routed into and out of the second channel of the compressor.