Introduction to Sound Synthesis Routing modulation
Modulating the modulation
The amount, rate or shape of one modulation can itself be modulated by another source. An envelope that controls an LFO's depth makes vibrato fade in over each note; an envelope that controls its rate makes the vibrato speed up as the note is held.
In the modulation matrix each connection had a fixed amount. That amount is a setting like any other, and a synthesiser that exposes it to modulation lets one source control how strongly another acts. On a modular synthesiser the depth control is a VCA between the source and the destination: its control input sets how much of the source gets through, and any other signal can be patched there. On synthesisers with a matrix, the same thing appears as a slot whose destination is the amount of another slot, or as a dedicated LFO delay or fade setting.
The tool below plays held notes with vibrato. The depth of the vibrato can be fixed, faded in by an envelope on each note, or moved slowly by a second LFO, and its rate can be fixed or raised by the same envelope. The plot shows the pitch movement in blue and the depth setting that produced it as a grey outline.
Modulating a vibrato
An LFO moves the pitch of held notes. Its depth comes from a fixed setting, an envelope on each note, or a second, slower LFO, and its rate can be fixed or raised by the envelope.
Listen for With a fixed depth, every note has full vibrato from its first moment. With the envelope on depth, each note starts steady and the vibrato grows over about a second, as a singer or a violinist often plays a held note. With the slow LFO on depth, the vibrato comes and goes across several notes. With the envelope on rate as well, the vibrato also speeds up through each note.
Depth, rate and shape
The depth of a modulation sets how far it moves the destination, so modulating depth makes the effect grow and shrink, as in vibrato that fades in or a filter sweep that widens over a phrase. The rate sets how fast it moves, so modulating rate makes it speed up and slow down, which on an LFO sent to pitch gives a vibrato whose speed rises and falls. On an LFO with a continuously variable waveform, the shape can be modulated too, so that a sine changes gradually into a square.
Several layers of modulation
Each layer can be modulated again: an LFO controlling the depth of an LFO that controls the rate of a third. When each layer is slower than the one it controls, the result changes over longer and longer spans of time, which is a common way to make a patch change over minutes without anyone touching it. Each added layer is harder to hear apart from the ones beneath it. The next chapter covers macro controls, in which one control moves several settings at once.
Common questions
What does modulating the modulation mean?
It means using one modulation source to change the amount, rate or shape of another. An envelope that controls an LFO's depth makes vibrato fade in over each note, and an LFO that controls another LFO's rate makes a modulation whose speed rises and falls.
How do you make vibrato fade in on a synthesiser?
Send the LFO to pitch through an amplifier or modulation amount whose level is set by an envelope with a slow attack. At the start of each note the envelope is at zero and there is no vibrato; as it rises, the vibrato deepens. Many keyboard synthesisers have an LFO delay or fade setting that does the same thing.
How is modulation depth controlled on a modular synthesiser?
It is controlled with a VCA between the modulation source and its destination. The VCA's level sets how much of the source gets through, and because that level is itself a control input, any other source can set it.
Published Glossary