Making sounds with analogue electronics - Part 4: Voltage-controlled amplifiers

Modulation is another type of modifier. Any parameter that can be voltage controlled is a potential means of modulation. Although VCAs are available from the front panels of many analogue synthesizers, they are also used inside to allow CVs to act as modulators – anywhere where a CV is used to change the amplitude or level of a signal or CV.

Some of the many possible ways that sources can be modified using modulation are as follows:

• LFO (LFO/envelope/keyboard): LFO modulation changes the rate or frequency of the LFO. This can be used to produce vibrato or tremolo whose rate is not fixed.
• VCO mod (LFO/envelope/keyboard): LFO modulation of a VCO produces vibrato. Envelope modulation produces pitch sweeps. Keyboard modulation changes the scaling of the VCO: it can change the keyboard so that an octave on the keyboard represents any pitch interval to the VCO.
• Filter mod (LFO/envelope/keyboard): LFO modulation of a filter produces cyclic timbre changes. Envelope modulation produces dynamic timbral changes during the course of a single note. Keyboard modulation controls how the filter 'tracks' the note on the keyboard.
• PWM (LFO/envelope/keyboard): PWM changes the timbre of the source waveform.
• AM (LFO/VCO): AM with low frequencies produces tremolo. At higher frequencies it adds extra frequencies to the audio signal (see Section 3.4).
• FM (LFO/VCO): FM uses the linear frequency CV input of the VCOs. It produces additional frequencies in the output signal (see Section 3.4 and Chapter 5).
• Cross-modulation (VCO): Cross-modulation connects the outputs of two VCOs to their opposite's frequency CV input and so each frequency modulates the other. This produces complex FM-like timbres, but it can be difficult to control and keep in tune.
• Pan (LFO/VCO/envelope/keyboard): LFO modulation of the stereo pan position produces 'auto-pan', where the audio signal moves cyclically from one side of the stereo image to the other. VCO modulation can spread individual harmonics across the stereo image. Envelope modulation moves the image with the note envelope. Keyboard modulation places notes in the stereo image dependent on their position on the keyboard.
• Other sources: Many other sources and modifiers can be modulated. The effects section of many analogue synthesizers allows parameters like the reverberation time, flange speed and others to be controlled.