Chapter Four: Synthesis
7. Patches: 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 offset 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 +5 volts and -5 volts would be considered bipolar).
If the modulating wave was a positive unipolar one (a cycle that goes from 0 to +5 volts, for example), it will only be deviated above the initial offset setting. 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 act as 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 means the frequency of the modulating oscillator (or noise source) is tuned below 20 Hz, often accomplished by using a Low Frequency Oscillator (LFO) to save audio rate oscillators for other functions. LFO's may come with a sync option, which when selected causes the waveform to being again at a certain phase with each key on.
Audio-rate modulation, covered on the following pages, 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 and their effect (click on the modulation types below to play a short example):
Sub-audio Frequency Modulation (FM) produces vibrato. |
|
|---|---|
| Modulating Waveform |
Increasing Depth then Increasing Rate |
| Sine | |
| Down Saw | |
| Up Saw | |
| Square | |
Sub-audio Amplitude Modulation (AM) produces tremolo. An LFO is attached to a second amplifier earlier in the signal path, causing fluctuations of loudness. Normally the modulating wave is unipolar, meaning it does not provide negative control voltage values (using a two-quadrant amplifier setting if available), and the amplifier offset is set high enough so the sound is never completely cutoff (unless a pulse is desired). |
|
|---|---|
| Modulating Waveform |
Increasing Depth then Increasing Rate |
| Sine | |
| Down Saw | |
| Up Saw | |
| Square | |
Sub-audio Filter Cutoff Frequency (c.o.f.) Modulation also produces 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. The cutoff frequency offset sets the starting point of the c.o.f., and the c.v. attenuator control on the module determines how far the c.o.f. (or center frequency for bandpass and notch) will shift in frequency. |
|
|---|---|
| Modulating Waveform |
Increasing Depth then Increasing Rate |
| Triangle | |
| Down Saw | |
| Up Saw | |
| Square | |
Sub-audio Pulse Width Modulation (PWM) produces timbre modulation. An LFO is attached to the pulse width input of a VCO that is set to output a pulse wave. As the pulse width changes, the number and strength of harmonics change, and when rapid enough, it may even being to sound like a change of frequency. If the pulse width becomes narrow enough on either phase of the modulation, the sound produced may stop entirely. If this is undesirable, simply attenuating the modulating wave while assuring the pulse width offset is centered at a square wave duty cycle will solve this issue. |
|
|---|---|
| Modulating Waveform |
Increasing Depth then Increasing Rate |
| Triangle | |
Filter Q Modulation also produces timbre modulation. An LFO attached to a filter's Q (resonance) control modulates the amount of enhanced resonance at the cutoff frequency. See Filters | Page 2. The audio example is both modulating the carrier frequency with an LFO triangle while at the same time modulating the amount of Q with a square wave. |
|
|---|---|
| Modulating Waveform | Increasing Depth then Increasing Rate |
| Square | |
Double Modulation: Some parameters of a modulating oscillator, such as the frequency or pulse width, can be controlled by a second modulator. Such a case is called double modulation. For example, if you would like a trill created by a square wave modulator to speed up and slow down, you would route a second modulator into the first modulator’s frequency control input and perhaps use a triangle waveform for the second modulator. That is exactly what the audio example below is doing. One needs to carefully adjust the relative speed of BOTH modulators to get the desired results–usually the second modulator's frequency will be lower than the first one, but that is not absolutely necessary if more audibly random results are desired. In addition, controlling the depth of the second modulator's action will determine how much the first modulator speeds up in this example. |
|
|---|---|
| Modulating Waveform | Increasing and Decreasing Modulation Frequency |
| Triangle/ Square |
|
View the next two pages for the patches for frequency and amplitude modulation.
