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Appendix Synthesizer Basics 423
Envelopes
What does the term envelope mean in this context? In the image, you see an
oscillogram of a percussive tone. It’s easy to see how the level rises immediately the
top of its range, and how it decays. If you drew a line surrounding the upper half of the
oscillogram, you could call it the envelope of the sound—a graphic displaying the level
as a function of time. It’s the job of the envelope generator to set the shape of the
envelope.
The screenshot shows a recording of an ES1 sound created with these ADSR (attack
time, decay time, sustain level, and release time) parameter settings: attack as short as
possible, medium value for decay, zero for sustain, medium value for release.
When you strike a key, the envelope travels from zero to its maximum level in the
attack time, falls from this maximum level to the sustain level in the decay time, and
maintains the sustain level as long as you hold the key. When the key is released, the
envelope falls from its sustain level to zero over the release time. The brass or string-like
envelope of the following sound—the envelope itself is not shown in this graphic—has
longer attack and release times, and a higher sustain level.
The envelope generator can also control the rise and fall of the cutoff frequency. You
can also use envelope generators to modulate other parameters. In this context,
modulation can be thought of as a remote control for a given parameter. There are
more sources that can serve as a modulation source: the pitch (note number), velocity
sensitivity or the modulation wheel, for example.