Korg PS 3100 - Dynamic Low Pass Filter

The Low Pass Filter.
You have your standard controls ... Cut off frequency (Fc) & peak resonance (Q).
Remember that the PS 3100 has a independent filter for every key.
The Peak resonance (Q) knob adjusts the degree of emphasis at the cut off point, determined by the Fc knob

Below this is the KBD Filter balance & expand.
They allow you to adjust how the filter behaves along the span of the keyboard.

The KBD Filter Balance adjusts the Fc of each of the key's independent filters over a rising or falling slope corresponding to the scale of the keyboard.

The EXPAND knob adjusts the degree to which the Fc is varied by the envelope signal formed by the EM section (to the right).

Below the filter section is a modulation section.
You need to push the orange switch to "on" for this section to be active.

There are controls for the MG1, GEG & external CV modulation signals.
The MG 1  & GEG knobs adjust the depth of modulation.

 It's very similar to the mod section of the Signal Generators.