Change (or transient) detector
Any constant signal at input results in zero at output, rising signal will give positive value and falling negative (main output). Input goes through envelope follower which is compared to its own delayed version and difference of two followers is sent to output.
Top knob controls input gain/attenuation. Middle knob adjusts lag time for followers, bottom knob sets time difference between them. Upper three lamps monitor positive output (or Rise), lower three lamps monitor negative output (or rectified Fall).
Out is main output where goes bipolar signal. Rise is output of "attack" phase (positive signal), Fall is output of "release" phase but rectified so it's also positive.