The original SEM modules included two attack/decay/sustain (ADS) envelope generators. These function much the same as more common attack/decay/sustain/release (ADSR) envelopes, the only difference is that the decay and release stages are combined into one control.
How They Work
When a voice sees a gate voltage from a note, the envelope outputs a dynamically changing voltage, according to the settings of its stages. The Attack stage defines how long it takes for the output voltage to rise from 0 to 5 volts. Once the Attack stage reaches 5V, it moves to the Decay phase. If the key is released, the Decay knob defines how it long takes for the voltage to fall back to zero. If the key is held, the Decay time defines how long it takes to fall to the Sustain level setting. The note then holds at the Sustain level until it is released, and fades to zero at the time set by Decay knob - the Decay knob effectively does "double-duty," acting as a decay and a release control.
Attack- Defines the length of time for voltage to rise from 0V to 5V when a key is played.
Decay- If the key is released, the Decay knob defines how it long takes for the voltage to fall back to zero. If the key is held, the Decay time defines how long it takes to fall to the Sustain level setting.
Sustain- Sets the voltage (i.e. the level) the envelope holds at following the Attack and Decay phases.
Vel (Velocity)- Defines how much the envelope affects amplitude via keyboard velocity. When set to zero, keyboard velocity has no effect on amplitude; all the way up results in maximum dynamic range.
Envelope 1 and 2 Routing
As indicated on the front panel, Env 1 affects the VCA, i.e., amplitude. It's hard-wired to the VCA, so no other routing is necessary. If VCO 1's three-way mod routing switch is set to the Env 1 position, it will also modulate VCO 1's pitch.
Env 2 is mainly intended to modulate VCF cutoff frequency, but because it's not hard-wired, VCO 2's three-way mod routing switch needs to be in the Env 2 position for cutoff mod to occur. In addition to filter cutoff frequency, Env 2 can also be used to modulate VCO 2's pitch when VCO 2's three-way mod routing switch is set to the Env 2 position.