Introduction to Sound Synthesis Modulation
Envelopes as modulation
An envelope can be sent to the same settings as an LFO. It moves the setting once for each note, starting when the note starts, so that a note can open bright and then darken, or start with a bend in its pitch.
In the chapter on the VCA, an envelope controlled the level of each note. An envelope produces a control signal like any other, so it can control a filter's cutoff, an oscillator's pitch or any setting that accepts modulation. Many synthesisers give the filter its own envelope for this reason, separate from the one controlling level, and many let either envelope be sent to further destinations through a modulation matrix.
The tool below has two envelopes. The amplifier's envelope is fixed so that each note sounds at a steady level while the gate is held. The second envelope has its own controls and can be sent to the filter's cutoff or the oscillator's pitch. Hold the gate for single notes, or start the phrase to hear a repeating pattern.
A filter envelope
A sawtooth plays through a lowpass filter and an amplifier. A second envelope, with its own attack, decay, sustain and release, moves the filter's cutoff or the oscillator's pitch by a set amount for each note.
Listen for With a short decay and a low sustain sent to cutoff, each note starts bright and darkens within a fraction of a second, while its level stays the same. A long attack makes each note open up slowly. A negative amount lowers the cutoff at the start of each note. Sent to pitch, a short decay gives each note a quick bend into its pitch.
Amount
An envelope runs from zero to its peak and back, and the amount control sets how far that moves the destination. On a filter, the cutoff control sets the cutoff while the envelope is at zero, and the amount sets how far above that the envelope's peak raises it. A low cutoff with a large amount gives notes that start bright and end dark. A high cutoff with the same amount lets most of the harmonics through the whole time, so the envelope changes the sound much less. In the tool above, full amount is four octaves of cutoff or twelve semitones of pitch.
Many synthesisers allow the amount to go negative, which inverts the envelope. The cutoff then falls when a note starts and returns as the envelope decays, which gives notes that start dull and brighten. A negative amount needs the cutoff set high enough for the envelope to lower it audibly.
Two envelopes, two timescales
The amplifier's envelope and the filter's envelope are separate so that the loudness and the brightness of a note can change at different speeds. When a string is plucked, its high harmonics die away faster than its fundamental, so the note gets darker as it gets quieter. On a synthesiser that is a filter envelope with a short decay and a low sustain under an amplifier envelope with a long decay or a high sustain. Swapping the two gives a note that fades quickly while staying bright.
Pitch
Sent to pitch, an envelope bends each note. With a small positive amount, a short decay and the sustain at zero, the note starts slightly sharp and falls to the pitch you played; a negative amount makes it start flat and rise to the pitch instead. Any sustain above zero leaves the note held above or below the pitch you played for as long as the gate is high. A large amount with a fast decay drops the pitch steeply at the start of every note, which is how many synthesised drums are made, and is the subject of envelope as pitch.
Common questions
What is a filter envelope?
A filter envelope is an envelope sent to a filter's cutoff, so that the cutoff rises and falls with each note. With a fast attack and a short decay to a low sustain, each note starts bright and quickly darkens, which is a common sound for synthesiser basses and plucked sounds.
What does envelope amount do?
It sets how far the envelope moves the setting it is sent to. On a filter, the amount is how far above the cutoff knob's setting the envelope's peak opens the filter. A negative amount inverts the envelope, so the cutoff drops at the start of each note and rises back.
Why do synthesisers have two envelopes?
Two envelopes let level and tone change on different timescales. The amplifier's envelope sets how the note's volume rises and falls, and the filter's envelope sets how its brightness changes. A note that stays loud while it darkens needs the two to differ.
What is the difference between an envelope and an LFO as a modulation source?
An LFO repeats at its own rate whether or not a note is playing, unless it is set to restart with each note. An envelope runs once for each note, starting when the gate goes high, so the movement it produces starts again with every note and lasts as long as its stages and the gate allow.
Published Glossary