Transient (Envelope) Generator

In Moog-speak this module is called a transient generator but is also known as an envelope generator.  What it does is fairly simple: when the "Gate" signal from the keyboard controller goes to a "1" (turns on - note is pressed), a varying voltage is produced at the output.  When the "Gate" signal goes back to "0" (turns off - note is released) the varying voltage drifts back to zero.

The following figure shows the relationship between the "Gate" signal and the output of the envelope generator:

Most envelope generators these days have four parameters that can be adjusted by controls on the operator panel.  Each of the controls corresponds to a particular element of the envelope produced.  The panel controls are most often labeled:

ATTACK        DECAY        SUSTAIN        RELEASE

The following figure depicts which elements of the envelope are affected by the operator controls:

The ATTACK control adjusts the amount of time it takes to ramp up from zero to +5V.  This is the "Attack Period".  The attack time can be varied from about one millisecond to perhaps twenty seconds.

The DECAY control adjusts the amount of time it takes to ramp down from +5V to the sustain voltage.  This is the "Decay Period".  The decay time can be varied to a degree similar to the attack time.

The SUSTAIN control adjusts the "Sustain Voltage" level depicted in the above figure.  The sustain voltage is a sort of average voltage level for the envelope.  The sustain period length is solely determined by how long the "Gate" signal is on.  Any note could be held for several seconds or indefinitely!  That is why I said the sustain level sets the "average" signal level for the envelope.

The RELEASE control adjusts the amount of time it takes to ramp down from the sustain voltage back to zero.  This is the "Release Period".  The release time can be varied to a degree similar to the attack and decay times.

Some envelope generators have a second output that mirrors the primary output, except the voltage polarity goes negative instead of positive.  Such outputs look like this:

Envelope generators are used to change the dynamics of the sound in some way.  Check out my other module discussions to see how envelope generators might be used!