Envelope modulation

On a subtractive synthesizer, envelope modulation is the amount that a modulation envelope effects a certain parameter (filter cutoff, oscillator pitch etc.)

Most synthesizers include an ADSR envelope, meaning:

* Attack
* Decay
* Sustain
* Release

Here is an example volume envelope from a software synthesizer called the TS404:

For instance, a note is played. The attack value determines how long it will take for the sound to reach its peak after the key has been struck. The decay value determines the length of time it takes for the sound to decrease to the sustain level. The sustain parameter determines the sustain level at which the modulated parameter remains throughout a key press. Finally, the release parameter determines how long the sound will keep going before fading out after the key has been released.

Envelopes can control many parameters besides volume, such as a filter, for example.

Also, there is an "envelope modulation" parameter on many synthesizers with filter envelopes. Often it is abbreviated as "env mod", particularly on the TB 303. The purpose of this parameter is to determine how harshly the envelope affects the filter. If env mod is turned all the way down, the filter will produce a flat sound with no envelope. When the parameter gets turned up the envelope becomes more noticeable, expanding the minimum and maximum range of the filter.