Introduction to Sound Synthesis Routing modulation
The modulation matrix
A modulation matrix connects modulation sources to destinations, each connection with its own amount. It does the job of patch cables on a synthesiser with fixed internal connections, and usually lets any of its sources be sent to any of its destinations.
The classic voice has its modulation wired in: one envelope to the filter, another to the amplifier, an LFO to pitch. A modulation matrix replaces that fixed wiring with a table. Down one side are the sources, such as LFOs, envelopes and random generators; across the top are the destinations, such as pitch, cutoff, level and pan; and each cell holds an amount, which can be positive, negative or zero. On a modular synthesiser the same thing is done with cables and attenuators, one per connection. Some synthesisers show the matrix as a grid like the one below, and others as a list of slots, each with a source, a destination and an amount.
The tool below is a drone with four sources and four destinations. Drag across a cell to set its amount: to the right of the centre line is positive, to the left is negative, and a double-click returns the cell to zero. The dots beside each source show its present value. The bottom row plots, for each destination over the last few seconds, what each connected source adds as a thin line in that source's colour, and the sum of those contributions as a thick blue line. The shaded band is the range the sum can reach with the amounts as they are set.
A modulation matrix
Four modulation sources can be connected to four destinations in any combination. Each cell sets an amount from −100% to +100%, and several sources can share a destination.
Listen for LFO 1 on pitch gives vibrato, and LFO 2 on cutoff gives a slow sweep. Add the envelope to cutoff as well and each beat starts with a bright attack on top of the sweep. Random on pan moves each beat to a new position. Set the envelope to −100% on level and the drone ducks on every beat instead of swelling.
Sources and destinations
A source is any part of the synthesiser that produces a control signal: LFOs, envelopes, random generators, and the player's own controls, such as key velocity, the modulation wheel and aftertouch. A destination is any setting the synthesiser exposes to modulation, such as pitch, cutoff, resonance, level, pan, pulse width, wavetable position or an effect setting. A matrix with more slots allows more connections at once, and on any matrix a connection is made or changed by editing an amount, so no cables need to be moved.
Adding and inverting
When two sources reach the same destination, their contributions add, so the destination moves by the sum of the two at every moment. An envelope on cutoff gives each note a bright start, an LFO on cutoff gives a slow sweep, and with both connected the bright start of each note is added to the position of the sweep at that moment. A negative amount inverts the source, so an envelope that would open the filter closes it instead, and an envelope that would raise the level on each note lowers it. Many matrices also let the amount of one slot be controlled by another source, which is the subject of modulating the modulation.
Common questions
What is a modulation matrix?
A modulation matrix is a table on a synthesiser that connects modulation sources to destinations. Each connection, often called a slot, has a source, a destination and an amount, and the amount can usually be negative to invert the source. It does the job of patch cables on a synthesiser with fixed internal connections.
What happens when two sources modulate the same destination?
Their contributions add. An LFO and an envelope both sent to cutoff move it by the sum of the two at every moment, so the envelope's rise and fall on each note is added to the LFO's slower sweep.
What does a negative modulation amount do?
It inverts the source before it reaches the destination. An envelope with a negative amount on cutoff closes the filter at the start of each note instead of opening it, and an LFO with a negative amount moves its destination in the opposite direction to one with a positive amount.
Published Glossary