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Welcome to Sound On Sound's indispensible, regularly updated, explanations of technical terms from the fields of Recording, Audio Production, Music Technology, MIDI, Music Software, Audio Plug-ins, Mac and PC Computing, Live Sound, Acoustics, Electronics and more...

If we do not explain a particular term below, please email and we will add it to our next update.

Last updated: 01/02/24

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Also known as a ‘tube’ in America. A thermionic device in which the current flowing between its anode and cathode terminals is controlled by the voltage applied to one or more control grid(s). Valves can be used as the active elements in amplifiers, and because the input impedance to the grid is extremely high they are ideal for use as an impedance converter in capacitor microphones. The modern solid-state equivalent is the Field Effect Transistor or FET.

Vari-Mu Compressor

An audio compressor that employs a valve (tube) as the variable audio attenuator. Mu is an engineering term for gain, so this is a variable-gain compressor. In essence, the side-chain signal continuously adjusts the bias o the valve to alter its gain appropriately. Vari-Mu compressors are fast and smooth, with low distortion.


Voltage Controlled Amplifier. An amplifier in which the gain (or attenuation) is controlled by an external DC voltage. VCA's are used in a wide range of audio and musical equipment, such as fader-automation systems in large format mixing consoles, audio compressors, and synthesizers.

VCA Compressor

See VCA. VCA compressors tend to be fast-acting (at least in comparison to opto-compressors), a wide dynamic range, and low distortion.

VCA Group

Found in large mixing consoles. The fader levels of a number of separate channels assigned to the VCA group can be adjusted together by the VCA Group fader but without mixing their signals together. Usually referred to as a DCA Group in a digital console.


Computer display screen (See also Monitor).


The rate at which a key is depressed. This may be used to control loudness (to simulate the response of instruments such as pianos) or other parameters on later synthesizers.


Pitch modulation using an LFO to modulate a VCO. (cf. Tremolo)


A signal processor that imposes a changing spectral filter on a sound based on the frequency characteristics of a second sound. By taking the spectral content of a human voice and imposing it on a musical instrument, talking instrument effects can be created.


The capacity of a synthesizer to play a single musical note. An instrument capable of playing 16 simultaneous notes is said to be a 16-voice instrument.

VU Meter

An audio meter designed to interpret signal levels in roughly the same way as the human ear, which responds more closely to the average levels of sounds rather than to the peak levels. (cf. PPM)