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 plug-in 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.