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Create sounds using analogue electronics (Part 4)

Posted: 10 Jan 2012     Print Version  Bookmark and Share

Keywords:voltage-controlled amplifier  oscillators 

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.

FM (LFO/VCO): FM uses the linear frequency CV input of the VCOs. It produces additional frequencies in the output signal.

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 synthesisers allows parameters like the reverberation time, flange speed and others to be controlled.

ControllersIn conventional instruments, the control of the sound production is often a mechanical linkage between the performer and the instrument. A saxophone player uses a number of levers to control the opening and closing of the holes that determine the effective length of the saxophone. Control over the timbre can be accomplished by how the lips grasp the mouthpiece and the reed as well as the use of the tongue. Further expression comes from the lungs with control over air pressure.

The interfacing between the performer and the synthesiser sound generation circuitry is accomplished by one or more controller devices. The main note-pitch controller is usually a modified organ-type keyboard, although sometimes weighted action piano-type keyboards are used. Changes in pitch are normally produced with a rotary control called a pitch-bend wheel, and a similar control is used to add in modulation effects such as vibrato or tremolo. Control over volume and timbre can be accomplished by using a foot pedal – as used in organs for volume.

Keyboard: The familiar music keyboard with its patterned combination of black and white keys is widely used as the main discrete pitch control for note selectionas well as initiating envelopes. Although normally connected together, the pitch selection and envelope triggering functions can be separated.

Pitch-bend: Continuous control over the pitch is achieved by using a 'pitch-bend' controller. These are normally rotating wheels or levers and usually change the pitch of the entire instrument over a specified range (often a semitone or a fifth). They produce a CV whose value is proportional to the angle of the control. Pitchbend controls normally have a spring arrangement, which always returns the control to the centre 'zero' position (no pitch change) when it is released. This central position is often also mechanically detented, so that it can be felt by the operator, since it will require force to move it away from the centre position.

Modulation: Modulation is controlled using rotary wheels or lever, where the CV is proportional to the angle of the control. Modulation controllers are not normally sprung so that they return to the centre position. Some instruments allow pressure on the keyboard to be used as a modulation controller.

There have been some attempts to combine the functions of pitch-bend and modulation into a single 'joystick' controller, but the most popular arrangement remains the two wheels: pitch-bend and modulation.

Foot controllers: Foot controllers are pedals which provide a CV which is proportional to the angle of the pedal. Although associated with volume control, they can be used as modulation controls or even as pitch-bend controls.

Foot switches: Foot switches are foot-operated switches, which normally have only two values (some multi-valued variants are produced, but these are rare). They are used to control parameters such as sustain and portamento.

- Martin Russ

Printed with permission from Focal Press, a division of Elsevier. Copyright 2009. "Sound Synthesis and Sampling" by Martin Russ.

To download the PDF version of this article, click here.

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