FILTERS on the Side-Chain

The FAIRCHILD 670 mkII is equipped with a Filter section on the Gain Control circuitry, (also called Side Chain signal.)
This allows to filter out the frequencies that you do not want to compress, thus giving you more Low End bellow 90 or 180 Hz.

Mid Range is more of an effect for De-essing applications, if you are after some special effects with top end compression...

AGC Filter