Compressors generally use the detected amplitude of their input signal as a control source. However, you can also use other signals, such as a separate reference track or an external audio signal as a control source. This is known as side-chain processing.Side-chain processing lets you control Impact compression using an independent audio signal (typically, another Pro Tools track). In this way you can compress the audio of one track using the dynamics of a different audio track.The reference track or external audio source used for triggering side-chain processing is referred to as the Key Input.