An electronic filter that modifies the frequency response (amplitude and phase versus frequency) of a system for a specific purpose. While filters in some applications perform the conceptually simple operations of rejecting specific bands of frequencies and passing other bands of frequencies, equalizers typically realize a more complicated frequency response in which the amplitude response varies continuously with frequency, amplifying some frequencies and attenuating others. An equalizer may have a response fixed in time or may be automatically and continuously adjusted. However, its frequency response is usually matched to some external physical medium, such as an acoustic path or communication channel, and thus inherently needs to be adjustable.