The dbx company grew out of the need to reduce tape machine noise.  It was founded by David Blackmer and the first products used a solid-state voltage control amplifier (VCA) coupled with a RMS based detection and control circuit.  The idea was essentially to compress the dynamic range of the incoming audio signal, then record the result on tape.  At this point, the audio would exist on the tape as well as the inherent tape noise.  However, since the audio signal was compressed before it was recorded, the volume of the signal on tape was always relatively high.  Upon playback, the dbx system would “expand” the entire replay signal (noise included now) with essentially the reverse of the compression curve.  Now when the signal was quiet, the noise was expanded downward.  During louder passages, the noise was for all practical purposes “drowned out” by the original audio signal.  The system did not require elaborate input and output calibration, making it easier to use than the popular Dolby system.  Unfortunately, I have always felt that the dbx noise reduction system created too many noticeable artifacts, such as “pumping” and unnatural attack transients.