(Fr. commande par tension; Ger. Spannungssteuerung; It. controllo di tensione).
In electronic music, a means by which the functions of certain electronic devices may be controlled by the application of external voltages. A change in the voltage applied results in a proportional change in the behaviour of the signal-producing or processing device that is being controlled. Theoretically any function of such a device that can be controlled manually (for instance, by turning a knob) can be made to respond to applied voltages. In practice the most usual applications are to be found in the voltage-controlled oscillator (VCO) and the voltage-controlled amplifier (VCA), in which respectively frequency and amplitude may be controlled by an external voltage. The range of voltages used in musical applications of voltage control is normally quite small: in the case of the voltage-controlled oscillator it is typical to find a ratio of one volt per octave. The lack of a global standard for voltage ratios, however, meant that it was often impossible to create such links between devices from different manufacturers.
One of the first pioneers of electronic music to appreciate the importance of the principle of voltage control was the Canadian inventor Hugh Le Caine. His ‘electronic sackbut’, completed in 1948, incorporated a VCO; in 1955 he designed a multichannel tape recorder whose speed could be varied by control voltages operated from a three-octave keyboard or a ribbon controller. In the USA Harald Bode made limited applications of voltage control in a modular ‘signal processor’ or sound synthesizer constructed in 1959–60, which in turn influenced Robert A. Moog when he began experimenting in 1964 with what was to become, at the end of that year, the first commercially manufactured analogue synthesizer. In 1962 Donald Buchla constructed independently several voltage-controlled modules, from which the Buchla synthesizer was developed.
The principle of voltage control is central to the operation of analogue (especially modular) synthesizers, many of whose component devices are voltage-controllable. Thus a device may be used to control one or more others, which in turn may control further devices. To take a simple example, a low-frequency oscillator (that is, one producing frequencies below the audible spectrum) may be used as the source of a varying voltage to control another oscillator that operates within the audio spectrum; the frequency of the latter oscillator tracks the waveshape of the former, producing an audible variation in pitch (or, if applied to its amplitude, as a vibrato). This kind of automated frequency control led to the development of the sequencer, a device designed to deliver sequences of control voltages. Any piece of equipment capable of delivering a variable voltage within an appropriate range could be used as the controller of voltage-controlled devices, leading to a proliferation of control equipment employing many different principles of interaction between the user and the electronic circuitry.
Keyboards and ribbon controllers were available with the first analogue synthesizers. A synthesizer keyboard is generally of conventional design; to each key is assigned a voltage from a pre-set range of voltages divided into equal fractional steps, which can often be increased or decreased by the user, so that any desired interval steps (including microtones) can be delivered to a voltage-controlled oscillator. Some keyboards can deliver a second control voltage by means of a circuit that tracks the speed of the key as it is depressed. Typically this is sent to a voltage-controlled amplifier, so that a faster stroke gives a louder sound. However, the decision as to what is controlled by what is normally the choice of the user: the second control voltage could as easily control a second oscillator or a voltage-controlled filter. In other keyboards, pressure on the keybed or lateral movement of the key provides further sources of control voltages.
The glide strip or ribbon controller consists of a contact strip (usually of metal) placed above a strip of electrically resistive material. A single contact, made by pressing the finger down on the ribbon, causes a certain voltage to be sent to the receiving device, and the signal continues until the contact is broken by lifting the finger. By sliding the finger along the strip, continuous transitions between control voltages may be effected, resulting in glissandos if the output is sent to a voltage-controlled oscillator. Later control devices included the joystick, which simultaneously controls two voltage outputs, touch-plates, which deliver voltages proportional to the degree of capacitance between the performer and the plate, and photoelectric cells, the action of which is dependent upon the amount of light falling on them. External signals, such as those created by an acoustic musical instrument, can be converted into control voltages by means of an envelope follower or a frequency-to-voltage converter. A random voltage source, externally triggered, delivers unpredictable voltages from a source of white or pink noise. More unusual sources of control voltages have been tapped, for example the earth’s magnetic field (by Charles Dodge), and the alpha waves of the brain (by David Rosenboom and others), gathered by means of electrodes attached to the scalp.
In the early 1980s the American composer Morton Subotnick developed a system of control voltages in which amplitude-modulated oscillator tones were pre-recorded on tape (normally three to each track). During performance or recording, the voltage-control information was extracted by band-pass filters and envelope followers and relayed to the appropriate sound modules. In this way, several control voltages could be applied concurrently. This system, called ‘ghost electronics’ because only the effects, and not the sounds, of the pre-recorded tracks are heard, was used both for tape compositions (for instance the Butterfly series) and live electronic pieces. Voltage control provided composers and performers with many means of controlling their electro-acoustic sound material. The greater precision of control permitted by digital technology led to the development of a wider diversity of controller interfaces for composition and performance. Moog’s Big Briar company has mainly concentrated on this area, which is also featured in some non-commercial digital synthesizers. Voltage control has been superseded by MIDI (Musical Instrument Digital Interface), introduced in 1983, a far more wide-ranging digital system that can be applied to all MIDI-equipped devices, regardless of manufacturer.
H. Bode: ‘A New Tool for the Exploration of Unknown Electronic Music Instrument Performances’, Journal of the Audio Engineering Society, ix/4 (1961), 264–6
H. Le Caine: ‘A Tape Recorder for Use in Electronic Music Studios and Related Equipment’, JMT, vii/1 (1963), 83–97
R.A. Moog: ‘Voltage-Controlled Electronic Music Modules’, Journal of the Audio Engineering Society, xiii/3 (1965), 200–06
S. Tempelaars: ‘Voltage Control in the Utrecht University Studio’, Electronic Music Reports, no.1 (1969), 68–77
D. Johnson: ‘A Voltage-Controlled Sequencer’, Electronic Music Reports, no.4 (1971), 119–28
A. Strange: Electronic Music: Systems, Techniques and Controls (Dubuque, IA, 1972, 2/1983)
J. Chadabe: ‘The Voltage-Controlled Synthesizer’, The Development and Practice of Electronic Music, ed. J.H. Appleton and R.C. Perera (Englewood Cliffs, NJ, 1975), 138–88
H.S. Howe: Electronic Music Synthesis: Concepts, Facilities, Techniques (New York, 1975)
F.L. McCarty: ‘Electronic Music Systems: Structure, Control, Product’, PNM, xiii/2 (1975), 98–125
C.A.G.M. Tempelaars: Sound Signal Processing (Utrecht, 1977)
RICHARD ORTON/HUGH DAVIES