In telecommunication, the term signal compression has the following meanings: In analog (usually audio) systems, reduction of the dynamic range of a signal by controlling it as a function of the inverse relationship of its instantaneous value relative to a specified reference level. Signal compression is usually expressed in dB. Instantaneous values of the input signal that are low, relative to the reference level, are increased, and those that are high are decreased. Signal compression always introduces distortion, which is usually not objectionable, if the compression is limited to a few dB.