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. Note 1: Signal compression is usually expressed in dB. Note 2: Instantaneous values of the input signal that are low, relative to the reference level, are increased, and those that are high are decreased. Note 3: Signal compression is usually accomplished by separate devices called "compressors. " It is used for many purposes, such as (a) improving signal-to-noise ratios prior to digitizing an analog signal for transmission over a digital carrier system, (b) preventing overload of succeeding elements of a system, or (c) matching the dynamic ranges of two devices. Note 4: Signal compression (in dB) may be a linear or nonlinear function of the signal level across the frequency band of interest and may be essentially instantaneous or have fixed or variable delay times. Note 5: Signal compression always introduces distortion, which is usually not objectionable, if the compression is limited to a few dB. Note 6: The original dynamic range of a compressed signal may be restored by a circuit called an "expander. " 2. In facsimile systems, a process in which the number of pels scanned on the original is larger than the number of encoded bits of picture information transmitted.