A stereo dynamics compressor with the controls a mixing compressor needs: peak or RMS detection, a soft knee, hold, automatic release, a ceiling on how much it may reduce, and a detector you can filter without touching the audio you hear.
The idea
Threshold, Ratio and Knee describe where the compressor is heading: how much quieter a signal above the threshold should become. Attack, Release and Hold decide how quickly it gets there and back. That is why a drum hit and a sustained chord react differently at identical settings.
The detector filters are the part worth learning. A high-pass on the detector stops a kick's low end from pulling the whole mix down, and a low-pass stops the compressor reacting to cymbals and sibilance. Neither filter changes the sound itself, only what the compressor listens to, and at the ends of their ranges they are switched off entirely.




