Introduction to Sound Synthesis Shaping control signals
Switches
A switch passes one of several signals to its output, or sends one signal to one of several outputs. A sequential switch moves to its next position on each trigger, so a patch can change which source or destination is connected while it plays.
In the examples so far, each source has stayed connected to the same destination for as long as the patch played. A switch changes which source is connected to which destination while the patch plays. A many-to-one switch has several inputs and one output and passes one input at a time, so the voice it feeds receives whichever source is selected. A one-to-many switch has one input and several outputs and sends the signal to one output at a time. In electronics the two are called a multiplexer and a demultiplexer. On a modular synthesiser a sequential switch is a module with a clock or trigger input that moves it on one position each time. Synthesisers with fixed connections offer related controls, such as a source selector in a modulation matrix or a pattern change on a sequencer.
The tool below is a four-to-one switch in front of a synthesiser voice. Each input is a different pitch source, and the switch can be advanced by hand or by a clock.
A sequential switch
Four pitch sources feed a four-to-one switch whose output sets the pitch of one voice. The switch advances by hand, on every bar or on every beat.
Listen for On every bar the switch moves on one input, so the line changes from the rising pattern to the falling one, then to the random notes and then to the held note. On every beat the voice plays two notes from each source in turn, which produces a new line made of fragments of all four. With Manual, press Advance to choose when the line changes.
Sequential and voltage-controlled
A sequential switch moves one position for each trigger. Clocked once a bar, it changes a part every bar, and clocked from a clock divider set to eight or sixteen bars, it can move a piece from one section to the next. A voltage-controlled switch instead sets its position from the level of a control signal, so a slow ramp from an LFO sweeps through the positions in order and a random signal jumps between them. Many switches have a reset input that sends them back to the first position, so that they line up with a sequencer started at the same time.
Switching audio and control
A simple switch moves from one signal to another in an instant. When the signal is pitch or another control signal, the jump changes the setting at that moment, and if it coincides with the start of a note it is heard as the next note. When the signal is audio, the jump is a step in the waveform, which is heard as a click, so switches meant for audio often crossfade over a few milliseconds. In this tool the switch passes pitch, a control signal, and the voice's own envelope sets the level at the start of each note, so the switching is not heard as a click.
A switch as a sequencer
A sequential switch with a fixed voltage at each input is a step sequencer: every trigger moves it on, and the voltage at the new position becomes the next note. Many early analogue sequencers were built this way, a row of knobs feeding a switch that a clock stepped through. A one-to-many switch, stepped by the same clock, sends each note of a single sequence to a different destination in turn, which is the basis of hocketing.
Common questions
What is a sequential switch?
A sequential switch passes one of several inputs to its output, or one input to one of several outputs, and moves to the next position each time it receives a trigger. On a modular synthesiser it is used to change which source or destination is connected while a patch plays.
What is the difference between a sequential switch and a voltage-controlled switch?
A sequential switch moves one position for each trigger it receives. A voltage-controlled switch sets its position from the level of a control signal, so a rising control signal sweeps through the positions in order and a random one jumps between them.
Why does switching audio click?
Switching from one audio signal to another in an instant jumps the waveform from one level to another, and the jump is heard as a click. Switches meant for audio often crossfade over a few milliseconds to avoid it. Control signals that change slowly, such as a held pitch, can be switched cleanly.
Can a switch be used as a sequencer?
Yes. A sequential switch with a fixed voltage at each input is a step sequencer: each trigger moves it to the next input, and the voltage there becomes the next note. Many early analogue sequencers were built this way.
Published Glossary