Dynamics processors typically use the detected amplitude of their input signal to trigger gain reduction. This split-off signal is known as the
side-chain. The Compressor/Limiter and Expander/Gate plug-ins feature external key capabilities and filters for the side-chain.
With external key side-chain processing, you trigger dynamics processing using an external signal (such as a separate reference track or audio source) instead of the input signal. This external source is known as the
key input.
With side-chain filters, you can make dynamics processing more or less sensitive to certain frequencies. For example, you might configure the side-chain so that certain lower frequencies on a drum track trigger dynamics processing.