Chapter Four: Synthesis
5. Sub-audio rate modulation
Modulation is the application of AC control voltage from a VCO, LFO (Low Frequency Oscillator) or noise source to other synthesis parameters, such as frequency, filter c.o.f., filter Q amount, amplitude, or pulse width. The modulating module providing the oscillating control voltage is known as the modulator. A VCO in the signal path whose frequency is being modulated is known as the carrier. A module will be deviated above and below initial state by the undulating voltage of the modulating wave if the modulating source is bipolar, meaning it rises an equal amount above 0 volts as it falls below 0 volts (so for example, a sine wave that peaks at both +5v and -5v wouldbe considered bipolar). In the case of a carrier oscillator or filter, the initial state is called the center frequency and is set by the offset control of the module. Often a modulating signal is summed with another c.v. source, such as a keyboard to provide, for example, vibrato at different pitches. VCO’s may be modulators OR carriers depending on how they function in the signal/control paths.
Three parameters are key to modulation: rate, depth and wave shape. Regarding wave shape, if a sine wave was applied to the frequency control input of an oscillator, a smooth up and down vibrato would ensue. However, if a square wave was applied, a trill would ensue.
Sub-audio rate modulation = the frequency of the modulator is tuned below 20 Hz, often accomplished by using a Low Frequency Oscillator (LFO) to save audio rate oscillators for other functions.
Audio-rate modulation occurs when the control oscillator (or noise source) is set above ~20 Hz. This can often create additional frequencies, sometimes called sidebands. An effective synthesis technique may be to sweep the frequency of a modulating oscillator from a sub-audio rate frequency to an audio-rate frequency, causing something like a trill or vibrato to blossom into a panoply of additional frequencies. In this instance, a regular VCO would be used as the modulator, since most LFO's don't go very far into audio-rate frequency range.
Types of sub-audio rate modulation:
- frequency modulation (FM) = vibrato. An LFO is applied to a VCA in the signal path causing a fluctuation of frequency
- amplitude modulation (AM) = tremolo. An LFO is attached to an amplifier in the signal path, causing fluctuations of loudness
- pulse width modulation = timbre modulation. An LFO is attached to the pulse width input of a VCO that is set to output a pulse wave.
- filter c.o.f. modulation = timbre modulation. An LFO is attached to the cutoff frequency control causing the filter to "sweep" as the cutoff frequency moves. Increasing filter resonance (Q) enhances this effect.
- filter Q amount modulation = timbre modulation. An LFO attached to a filter's Q (resonance) control modulates the amount of enhanced resonance at the cutoff frequency (see filters:page2)
Parameters, such as frequency or pulse width of a modulator can be controlled by a second modulator. Such a case is called double modulation. For example, if you would like a trill to speed up and slow down, you would route the second modulator into the first modulator’s frequency control input.
Patches (click on synths for enlarged images in a new window):
Waveform of the modulator controls shape of deviation. For example, a triangle wave will give a smooth gliss up and down, a pulse wave will give a trill, a sawtooth wave will give a “whoop” up or down.
Depth of modulation is normally controlled by the attenuator at the c.v. control input of the module being modulated. The more of the modulating signal allowed into the module, the greater the deviation (or effect) of the parameter being modulated. On many modern synths, the depth of modulation is controlled by the mod wheel (duh!)—what sort of modulation is determined by the synth’s patch.
Rate: The frequency of the modulator determines the rate of modulation. Above 20 Hz for most forms of modulation, stranger things, such as additional frequencies begin to sound.
Exponential or Linear Control Input:
The control inputs of modules are often labeled exponential or linear. If applied to a VCO freq input, exp will deviate the signal up and down an equal musical interval around the center frequency (cƒ). If linear (which we use for audio-rate FM), will deviate an equal number of cps up and down around the center frequency, so a smaller musical interval above the cƒ than below.