Introduction to Sound Synthesis Signals
Control signals and audio signals
A control signal and an audio signal are the same kind of signal running at different speeds. Whether a signal is heard or used to move a setting depends on where it is connected.
The chapter on what a signal is described a signal by its amplitude, frequency and waveshape. None of those three properties determines whether the signal is meant to be heard. A sine wave at 220 Hz sent to a speaker is a tone. The same sine at 2 Hz sent to the pitch of an oscillator moves that pitch up and down twice a second, which is vibrato. Both are signals of the same kind, and the tool below sends one signal to either place so you can compare.
The same signal as audio and as control
One signal, with its rate set anywhere from 0.3 Hz to 600 Hz, is either sent straight to the speakers or used to move the pitch of a second oscillator.
Listen for Sent to the speakers, the signal is silent or a series of clicks at low rates and a tone above about 20 Hz. Sent to the pitch of the second oscillator, a slow signal moves that pitch up and down where you can follow it, and as the rate passes about 20 Hz the movement turns into a change of timbre.
Speed
Below about 20 Hz people hear changes in a signal as movement: a pitch rising and falling, a filter opening and closing, a level swelling. Above about 20 Hz the same changes fuse into a tone, and when that tone moves a setting, the setting changes too fast to follow and the result is heard as a different timbre. The two ranges have different names: signals that are listened to are called audio, and signals that move settings are called control signals, or control voltages (CV) on analogue hardware.
The LFO, or low-frequency oscillator, is an oscillator built to run slowly, usually from a few minutes per cycle up to a few tens of hertz, and it is normally used as a control signal. The boundary between the two kinds of oscillator is loose, because many LFOs reach into the audio range and many audio oscillators can be slowed down to LFO rates. What makes an oscillator an LFO in a given patch is that its output is sent to a setting rather than to the speakers.
On a modular synthesiser
In a modular synthesiser every signal is a voltage on a patch cable, so in principle any output can go to any input. An envelope can be listened to, and an oscillator can open a filter. In practice, the result depends on how each input is coupled. Audio inputs are often AC-coupled, which means a capacitor in the input blocks steady voltages and weakens slow ones, so an LFO sent to an AC-coupled input arrives much smaller or not at all. Control inputs are almost always DC-coupled and pass signals at any rate from a steady voltage up to audio rate, which is why cross-patching usually goes into a CV input.
On other synthesisers
A synthesiser with fixed internal connections does the same job through a modulation matrix: a table where you choose a source, such as an LFO or an envelope, a destination, such as filter cutoff, and an amount. In software the signals are streams of numbers rather than voltages, and a MIDI controller sends messages rather than a continuous signal. Some digital synthesisers calculate their control signals at a lower rate than their audio to save processing, so their LFOs stop well short of the audio range. Where a synthesiser does let a fast signal reach a control, the result is the subject of audio-rate modulation.
Common questions
What is the difference between a control signal and an audio signal?
In a modular synthesiser and in most software, they are the same kind of signal. A signal below about 20 Hz is usually used to move a setting, and one above it is usually listened to. The difference lies in how fast the signal changes and where it is connected.
What is CV?
CV stands for control voltage: a voltage used to set something rather than to be heard, such as an oscillator's pitch or a filter's cutoff. In software and in synthesisers controlled over MIDI, the same job is done by a number or a MIDI message rather than a voltage.
Can I plug audio into a CV input on a modular synthesiser?
Usually, yes. A CV input will follow an audio-rate signal as well as a slow one, and doing so changes the sound's timbre rather than moving the setting audibly up and down. The chapter on audio-rate modulation covers what you hear.
Why does my LFO do nothing when I plug it into an audio input?
Many audio inputs are AC-coupled: a capacitor in the input blocks any steady voltage, and with it most of a slow signal. A 1 Hz LFO through an AC-coupled input arrives much smaller than it left, or not at all. DC-coupled inputs pass slow signals unchanged.
What is a modulation matrix?
A modulation matrix is a table on a synthesiser with fixed internal connections that lets you choose a source, a destination and an amount, for example the LFO to filter cutoff at 30%. It does the job that patch cables do on a modular synthesiser.
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