Typically an audio processor or plug-in, like a compressor-limiter, is controlled by the volume of the channel that it’s on. However, there are times when a signal processor will act on one channel, but be controlled by another. This is called a “side chain”.One very common application of a side chain is called a “ducker”, where typically a dialog track is used to control the gain reduction of the music. Then, whenever the dialog is present, the music automatically “ducks” under, so that the voice can be heard more clearly. To make this happen, a bus sends a portion of the dialog to control the gain reduction of a compressor/limiter that controls the side chain can also be used with an expander/gate. In this case, one instrument’s amplitude envelope can be superimposed onto another. For example the kick drum could be used to control the volume of the bass guitar, so that the bass guitar is only heard whenever the bass drum is played.