(1) An early type of single-ended noise reduction system that uses one or two filters whose rolloff frequencies are controlled by the level of the signal. As the high-frequency signal level falls during soft passages, the high-frequency response is reduced; when the signal level is high, the full bandwidth is restored.
(2) A circuit used in aural exciters where a side-chain signal is combined with some dry signal in such a way that the original signal is modified both additively and subtractively to create the impression of an increase in both bass and brightness and the mid-range appears more focused. This type of spectral shaping is designed to be closely related to the way the human hearing system changes at different listening levels.