Korg PS 3100 - Envelope Modifiers (EM)

These EMs establish the changes in volume that occur over time when a key is pressed.
It's basically a VCA with an attached envelope generator.

Looks simple enough. .... An attack, a Decay & sustain knob.

In addition to controlling the VCA, it also can be used to control the filters.... specifically their cut off frequency. This effect is controlled by the EXPAND knob in the filter section.

The RELEASE is interesting ... it's a switch with 3 settings:
Damped, half damped & release.

 Re the KBD HOLD switch:
When the switch is ON, all EM controls (with the exception of Attack) are disconnected.
To experiment with this feature:
1. Turn the attack knob to the range of 7 to10.
2. Turn on the HOLD switch
3. Press a number of keys , one after another leaving a small space of time between each.
     Wait ... about 5 secs if the attack knob is at 10... sound will suddenly appear.... in the order you played the keys . Set the attack knob to 7 if you wish to hear the sounds sooner.

To release the HOLD, turn attack to 3 or below & turn the hold switch to off.