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Glossary of PA Terms - V

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The glossary pages provide definitions for over 1900 PA-related terms. If you can't find the term you are looking for, or would like any of the existing definitions to be expanded, please email me − likewise of course if you find any errors in the links etc. Use of this information is conditional upon acceptance of the Disclaimer on the PAforMusic home page.

V * VA * Vacuum tube * Valve * Variable bit rate * VBR * VCA * VCA group * VCD * VCF * VCR * Velcro tie * Velocity of sound * Verb * VESA * VGA * VHF * Vibrato * Video * Vinyl * Voice * Voice coil * Voltage * Voltage-matched * Voltage controlled amplifier * Voltage controlled filter * Voltage gain * Volume * Volume control * Vox * VT * VU

The definitions for these terms are given on the assumption of their use in the context of PA systems; many of the terms have more general meanings when used in a wider context. Where more than one definition is given for a term, the definitions are numbered (1), (2) etc.

Some of the definitions themselves use terms (such as "signal") in a specific way − most of these are links (just the first time they are used, in each definition), so just click on them to see the meanings that are intended.

V
An abbreviation for volt. Note: Due to font conversions, on some web pages the Greek capital letter Omega, the symbol for ohm, may be displayed by your browser as a 'V'; it should look like a horseshoe shape with a flat base, which, if your browser displays it correctly, now follows: Ω.

VA
An abbreviation for 'volt-amperes', i.e. the RMS value of the voltage supplied multiplied by the RMS value of the current drawn (or available). So, to determine the amount of supply current drawn by mains-operated power-utilising equipment that is rated in VA, simply divide the VA figure by the mains voltage. Likewise, to determine the amount of supply current which may be supplied (or carried) by AC power generation or distribution equipment (such as a transformer) that is rated in VA, simply divide the VA figure by the supplied voltage. Also known as the 'apparent power'. See Power factor for further details.

Vacuum tube
See Valve.

Valve
Short for 'thermionic valve', a device generally consisting of several electrodes spaced apart inside an evacuated glass envelope. Connection to the electrodes is made by means of a 'base' consisting of several pins. The base plugs into a special socket called a 'valve holder'. There is sometimes an additional 'top cap' connection.

In a valve, a current flows by means of a movement of electrons from the cathode electrode, which is heated, to the anode electrode. This flow of electrons is controlled by the voltage applied to an intervening 'control grid' electrode, so enabling the amplification of a signal applied to the grid. This basic design is called a triode (meaning 'three electrodes'). More advanced designs (tetrode, pentode, etc.), containing additional grid electrodes, are usually used for high-power amplification.

The valve has a number of disadvantages (including fragility, poor efficiency and a relatively short life), so is now largely superceded by the transistor. However, it is still used in some guitar amplifiers because of the particular type of distortion that it produces when operated at (or close to) its maximum output level. Also used in some very expensive studio microphones and microphone pre-amplifiers, and in a few models of compressor. The American term for a valve is a 'tube' (short for 'vacuum tube'). Compare Solid state.

Variable bit rate
Describes a digital signal having a bit-rate that is not constant, as in the case of a particular variant of MP3 bit-stream. Often abbreviated to 'VBR'.

VBR
An abbreviation for 'variable bit rate'.

VCA
An abbreviation for 'voltage controlled amplifier' (the term is rarely used in full). A signal processing device whose gain (or loss) is determined not by a potentiometer acting directly on the audio signal, but by a DC 'control voltage'.

It has many applications, such as in dynamics processors, in enabling the level of a signal to be controlled remotely, or in enabling some processing (such as noise filtering) of the control signal to be carried out (as in an active fader). A further application is when the same control voltage is applied to several VCAs simultaneously − this allows the level of several different signals to be controlled together, even though the signals themselves are not mixed (so effectively forming a 'virtual group', usually called a 'VCA group'). When incorporated as part of a more complex integrated circuit (e.g. a compressor/expander chip) a VCA is sometimes termed a 'gain cell'. See also VCF.

VCA group
See Group.

VCD
An abbreviation for 'video compact disc'. A standard for the recording of digital video information on a CD, giving a reduced picture quality as compared to the DVD standard. See also AVCD.

VCF
An abbreviation for 'voltage controlled filter' (the term is rarely used in full). A signal processing device whose filtering characteristics are determined not by physical controls that are part of the filter circuit, but by a DC 'control voltage'. This allows the filter to be controlled remotely, and also allows the control of several filters at once. See also VCA.

VCR
An abbreviation for 'video cassette recorder'. See also VT.

Velcro tie
A short narrow strip of fabric material, equipped on one side with miniature 'hooks' and on the other side with miniature 'loops', that engage with each other when the strip is wrapped around something. Such strips are useful for securing coiled cables in transit or storage, to prevent them becoming tangled, as an alternative to the use of releasable cable ties or PVC tape.

Strips designed for this purpose usually incorporate a means of attaching them to the cable, to avoid the strip becoming lost while the cable is in use. For a list of the advantages of using Velcro ties (as compared to PVC tape), see Cable tie. The name Velcro is a registered trademark.

Velocity of sound
See Speed of sound.

Verb
A slang abbreviation for 'reverb'.

VESA
An abbreviation for the Video Electronics Standards Association, an organisation which defines standards for video source and display equipment, and for interfaces between such equipment. As well as electrical aspects, its standards include physical aspects such as display mounting methods. Their website is www.vesa.org.

VGA
An abbreviation for 'video graphics adaptor'. A standard interface for the connection of display equipment (such as monitors and projectors) to computers. The standard gives a resolution of 640 pixels horizontally and 480 pixels vertically. The aspect ratio is 4:3. Note that as VGA was the 'original' standard, its name has become something of a generic term and so may be used to refer to cables, connectors (usually a 15-pin high-density D-type), etc., even though they may be used to carry the signals of higher resolution analogue computer graphics standards such as SVGA, XGA, SXGA, WXGA or UXGA (but not DVI, which is a standard with digital capability).

View VGA (HD15) image

The most commonly encountered standards, their resolutions and aspect ratios are tabled below.

VHF
An abbreviation for 'very high frequency'. Refers to radio frequencies in the range 30 MHz to 300 MHz, used by some types of radio microphones, radio instrument systems and in-ear monitoring systems. Most professional radio systems now use UHF frequencies or operate in the 2.4 GHz frequency band. For more details, see the radio mic information on the Microphones page.

Vibrato
A deliberate repetitive variation in the pitch (frequency) of a musical note, as an effect to improve the interest of the sound. Most commonly encountered in the context of keyboard instruments. Often confused with tremolo (for example, the effect produced by the tremolo bar of an electric guitar is actually vibrato, not tremolo). See also Leslie.

Video
Visual programme material (picture information) represented in an electrical form for passing between items of equipment or for storage. Depending upon the interconnection format, used, video may be analogue or digital and may be carried using a single or multiple signals. Some common analogue formats are listed below in order of increasing quality − follow the links for further information.

• Composite video
• S-video
• Component video
• RGB

See also Frame (2), Field (2), SCART, BNC, DVI, HDTV, HDMI, Pixel, VGA, Scaler, Aspect ratio, VT, VCR, DVD, VCD, VESA, Sync, Raster, Interlace, Progressive scan, Blu-ray and DL.

Vinyl
A term adopted to describe the 'original' type of audio recording disc, having a continuous groove with physical deviations to represent the audio information in an analogue manner. The disc is played on a 'record deck' or 'turntable', in which the audio information is read from the groove by a stylus and cartridge, located at the end of a tone arm. The stylus rests in the groove and physically follows its deviations, the resultant vibrations being converted into Left and Right audio signals by the cartridge, to which the stylus is mechanically coupled.

The clarification became necessary to provide a distinction from more recent 'disc-type' recording formats such as the compact disc, mini-disc, DVD, etc. Although the very first continuously grooved discs were not made from vinyl, most of those that remain in use today are. See also RIAA.

Voice
Apart from the obvious meaning (human speech or song), a term referring to the particular type of sound (as opposed to its pitch) being created by a musical instrument, particularly a programmable synthesiser or an organ. See also MIDI and Timbre.

Voice coil
The coil of wire, inside a driver or a dynamic microphone, which carries the electric current and is mechanically coupled to the cone or diaphragm.

In the case of a driver, the voice coil generates a magnetic field which interacts with the field of a stationary permanent magnet, and the resulting mechanical force causes motion of the cone that corresponds to the direction and magnitude of the current, so producing sound waves.

In the case of a dynamic microphone, sound waves impacting on the diaphragm cause motion of the voice coil within the field of a stationary permanent magnet, and this causes a voltage corresponding to the incident sound pressure to be generated across the coil. See also Ferrofluid.

Voltage
The 'pressure' of electricity. Measured in volts or millivolts (a thousandth of a volt). No current can flow without a voltage to drive it around the electrical circuit. Strictly, a voltage can only be said to exist between two points (usually two separate conductors), rather than 'at' a single point, so another term for voltage is 'potential difference'. However, when a particular voltage is said to exist at a single point, it can usually be assumed that the voltage relative to earth is what is meant.

A voltage can be either DC or AC, or even a mixture of the two. In the case of AC (such as an audio signal), the instantaneous value of voltage is continually rising and falling, first in one direction and then with the opposite polarity. Therefore, a special means is needed to quantify the effective value of AC voltages, over a complete cycle of change. This is the RMS value. Furthermore, in the case of an audio signal, there are usually constant fluctuations in the effective value of voltage, as the level of the represented sound changes. Several measurement schemes have been developed to accommodate this complexity, the main ones being PPM and VU.

In general, the higher the voltage, the greater the capability (when misused) for causing injury or death by electric shock, so the greater the caution required; any voltage above 50 volts should be considered hazardous. For further safety information see the Safety page. Audio signal voltages are usually less than 2 volts RMS − except for the signals which directly drive the speakers, which may be as much as 100 volts RMS. Mains voltage is 230 volts RMS in the UK and Europe (400 volts RMS on 3 phase supplies), and 110 volts RMS in the U.S.A.

For those with a scientific interest, the number of volts represents the amount of energy (in joules) that is (or would be) transferred per coulomb of electrical charge moving through that potential difference. See also Electromotive force and Potential.

Voltage-matched
Describes an interconnection in which the output impedance of a signal source and the input impedance of the equipment that it connects to are co-ordinated so that the output voltage of the source equipment is not significantly reduced as a result of making the interconnection. It is of course also necessary for the source voltage (signal level) to be compatible with the interconnected equipment (see Matching). See also Impedance. Compare Impedance-matched.

Voltage controlled amplifier
See VCA.

Voltage controlled filter
See VCF.

Voltage gain
See Gain.

Volume
An informal term for loudness.

Volume control
A term used only with domestic Hi-Fi equipment etc., to refer to a control which is provided to enable adjustment of the loudness of the sound produced. It is too general a term to be of any use in the context of PA systems and the like, because in such a system loudness is not a function of a single control. The loudness of a particular amplified sound source is affected by many controls in the audio chain (e.g. channel gain control, channel fader, group fader, main fader, crossover level adjustments, amplifier gain, etc.), and additionally the loudness at a given listening position is affected by other factors − especially by its distance from the speakers and by the room acoustics. See also Inverse square law.

Vox
Short for 'voice' or 'vocals'. See also Lead (2) and Backing.

VT
An abbreviation for 'video tape'. Often refers not to the tape itself but to a recording on it, or to the machine it is to be played on, as in the producer's phrase "Run VT", meaning to start the playback of a particular recording.

VU
An abbreviation for 'volume units'. More properly known as a "standard volume indicator", this is a type of level meter found on mixers used in broadcast and recording studios, whose response is specially tailored to indicate the average (rather than peak) level of the audio signal and so give an indication of perceived loudness. The required behaviour is detailed in IEC standard 60268-17, which specifies a time of 300 ms to reach a 99% indication (about 1 dB down). The decay characteristic is similar. As this response is unable to register short-term peaks and transients, it is common for a 'Peak' LED to be included in the meter to indicate the presence of high-level peaks in the signal.

Although the meter has a scale marked 'VU', its reading is actually a value in decibels relative to an agreed '0 VU' level. The standard scale runs from −20 to +3 VU, with the portion above 0 VU being marked in red. For broadcast equipment, a continuous tone at +4 dBu (equivalent to 1.23 volts RMS) is usually arranged to register as 0 VU (though calibration standards vary). However, because of the dynamics of a typical programme signal, a programme reading of 0 VU would be roughly equivalent to the normal peak programme broadcast level of +8 dBu (equivalent to 1.95 volts RMS) as registered by a PPM. See also SOL.

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This page last updated 14-Nov-2009.