A means of reducing distortion in a system by introducing networks that compensate for the particular type of distortion over the required frequency band. For example, if a loudspeaker system has a poor response to bass frequencies, bass boost (amplification of the bass frequencies with respect to the treble frequencies) may be introduced into the system to compensate.

In communication systems, the frequency response of a communication channel is usually not known to sufficient accuracy or is not sufficiently stable to permit a single optimum receiver to be designed. A filter, known as an equalizer, is commonly employed to match the receiver to the channel thereby optimizing receiver sensitivity. An adaptive equalizer is an equalizer the characteristics of which are dynamically changed to match variations in the channel.