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Compression
  • A process in which the dynamic range of a signal is reduced by controlling it as a function of the inverse relationship of its instantaneous value relative to a specified reference level.
  • Note 1: Input levels that are low relative to the reference level thus realize a relative increase, and levels that are high relative to the reference level thus realize a relative decrease. Note 2: Compression is usually accomplished by separate devices called compressors and is used for many purposes, such as: improving signal-to-noise ratios, preventing overload of succeeding elements of a system, or matching the dynamic ranges of two devices. Note 3: The amount of compression (expressed in decibels) 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 4: Compression always introduces distortion, which is usually not objectionable, provided the compression is limited to a few decibels
  • 2. In facsimile systems, a process wherein the number of pels scanned on the original is larger than the number of encoded bits of picture information transmitted.
  • 3. See also compander, compression ratio, compressor, expander, expansion, level, redundancy.