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Glossary of PA Terms - G

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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.

Gaffer tape (Gaffa tape) * Gain * Gain before feedback * Gain cell * Gain reduction * Gain riding * Gain stage * Gain structure * Galvanic * Galvanic connection * Galvanic isolation * Gamma correction * Gate * Gated reverberation * Gauge * Gear * Gel * Gender * Gender bender * Gender changer * Generation * Generator set * Genny * GFCI * GFI * GHz * Gig * GND * Go down * Gobo * Gooseneck * GPO jack * Graphic * Graphic equaliser * Grazing effect * Ground * Ground-compensated * Ground fault circuit interrupter * Ground fault interrupter * Ground isolator * Ground lift * Ground loop * Ground sense * Group * Group bus * Group delay * Group fader * GRP * Guitar pickup * Guitar processor * Gun microphone * Guru * Gyrator

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.

Gaffer tape (Gaffa tape)
A strong and very sticky adhesive tape, usually cloth-based, particularly useful for securing cables to floors. Generally available in black, silver, and (less common) white. Use of the silver-coloured variety is not recommended on anything electrical, as it contains an electrically-conductive layer which can cause problems. However, when it is important for the location of the cables to be visible, and hazard tape is not available or appropriate, the silver type is sometimes used as it is more easily seen.

Gaffer tape must not be used for securing heavy cables or other items overhead, as it may unexpectedly tear or become un-stuck. Also it must never be used as insulating tape, as it is not designed for this purpose and even the black variety cannot be relied upon to have good insulating properties. Further, as most types are flammable, gaffer tape must not be used close to hot surfaces such as lanterns, or in the vicinity of any source of ignition, such as pyrotechnics. It gets its name from the gaffers (chief electricians) of film sets, where the use of this kind of tape first became common. 'Advance Gaffa' is a registered trade mark of Advance Tapes International Ltd. See also Duct tape.

Gain
The amount of amplification given to a signal between two specific points in a system or within an item of equipment, usually considered as a multiplying factor expressed logarithmically as a value in decibels. A gain of 0 dB (that is, no change in level) is referred to as 'unity gain'.

Most commonly in PA work the term 'gain' is used as an abbreviation of 'voltage gain' − that is, the factor by which the equipment (or internal part of the equipment) increases the signal voltage level. It should be noted that this figure gives no information about the maximum output voltage level available from the equipment, nor about its current-supplying capability or maximum power output.

In other contexts, however (particularly radio-frequency ones), the term is used to refer to power gain − that is, the factor by which the equipment (or internal part of the equipment) increases the signal power level. Again, this gives no information about the maximum power output level of the equipment.

In the case of a control marked "Gain", its function is usually to set the amount of initial amplification given to an input signal by a pre-amplifier within that item of equipment, to raise the signal to an appropriate level for subsequent processing. So, it allows the equipment to be adjusted to accommodate a range of different input levels. On some equipment (e.g. by Mackie), this control is labelled "Trim".

As making an adjustment to a gain control will change the output level of the equipment (assuming that the input level remains the same), it is important that the gain is correctly set before the adjustment of other controls that affect the output level (such as the channel fader or Aux Send controls of a mixer). The correct setting of mixer gain controls is best determined by use of the channel metering facilities in accordance with the manufacturer's guidance.

The way in which overall gain is distributed throughout the signal chain is a very important factor in achieving the optimum overall signal-to-noise ratio and headroom − for further information see Gain structure. See also Ride (1). Compare Loss.

Gain before feedback
The maximum amount of overall gain that is possible in a PA system without the occurrence of acoustic feedback. See also Potential acoustic gain.

Gain cell
See VCA.

Gain riding
See Ride (1).

Gain reduction
Under given conditions of control settings and input signal, the amount by which a compressor has decreased its gain as compared to the situation of no input signal present (or input signal level below the set threshold, if relevant).

Or, under given conditions of control settings, the amount by which a noise gate decreases its gain when the gate is fully closed (input signal level persistently below threshold), as compared to its gain when the gate is fully open (input signal level persistently above threshold).

In both cases, the gain reduction is usually specified as a (positive) value in decibels.

Gain stage
Within an item of equipment, a stage of the electronic circuitry that provides gain.

Gain structure
The way in which gain is distributed throughout a signal chain. It is important that the gain structure is correctly optimised in order to achieve maximum dynamic range from the system, and to achieve the most appropriate balance between signal-to-noise ratio and headroom.

In a PA system, a pre-requisite to a good gain structure is ensuring that the channel gain controls of the mixer are correctly set for each channel. Ideally the remainder of the signal chain through the mixer, and each item of outboard equipment, would provide unity gain, but as this is rarely achievable in practice a good guideline is to avoid very low or very high settings on faders or other level controls. Setting the gain structure may involve the use of a test tone at the standard operating level. See also Metering.

Galvanic
Relating to the flow of current. This term is derived from the name of the scientist Luigi Galvani, who performed some early experiments with electricity. See also the next two definitions.

Galvanic connection
A connection which provides an electrically conductive path between two points, and so makes possible a flow of current between them. Compare Galvanic isolation.

Galvanic isolation
The absence of a conducting path between two points, making impossible the flow of current between those points. There may however be a means provided to enable power or a signal to pass between such points, using a device such as a transformer or an opto-isolator. Such an arrangement may be useful for safety purposes, or in order to prevent connection between the signal earths of different items of equipment, and so avoid earth loops. Compare Galvanic connection.

Gamma correction
In video signal processing, a deliberate non-linearity applied to the luminance signal (or, for greater accuracy, to each RGB signal individually) in order to compensate for the non-linearity of the display device. The correction is initially applied in the camera.

Gate
Short for "Noise gate".

Gated reverberation (Gated reverb)
A reverberation effect unit which is designed to automatically cut off the reverberation effect when the input signal falls below a particular level.

Gauge
The thickness of an electrical conductor, especially of the conductors of a cable. The gauge is usually specified using one of the following three methods − in each case it is the gauge of each conductor in the cable that is referred to, not the total:

• The cross-sectional area (CSA), usually in mm² (square millimetres) − most common in Europe
• A gauge-numbering scheme such as AWG − most common in the USA
• The number of strands in each conductor and the diameter of each strand

Some common examples of the latter method are given below; the figure before the slash is the number of strands, and the figure after it is the strand diameter in mm.

• 30/0.25 = approx 1.5 mm² (or 15 AWG)
• 42/0.20 = approx 1.5 mm²
• 50/0.25 = approx 2.5 mm² (or 13 AWG)
• 79/0.20 = approx 2.5 mm²
• 50/0.30 = approx 4 mm² (or 11 AWG)
• 84/0.30 = approx 6 mm² (or 9 AWG)

The gauge of cable conductors is of special importance for interconnections between power amplifiers and speakers, because of the high currents that flow in these interconnections and the need to maintain a high damping factor. The round-trip series resistance of various metric gauges of speaker cable is given below; if the conductors get warm in use or are installed in a hot environment (e.g. in proximity to stage lighting), then the high temperature figures should be used. Note that resistance applies only at DC − at high audio frequencies the value of the cable's series impedance will be significantly higher than its resistance, because of inductive effects. For AWG sizes of cable, see AWG.

For PA speaker applications the minimum size usually used is 2.5 mm² (13 AWG), but the minimum acceptable size will depend on the power being supplied to the speakers, the combined impedance of the speakers being supplied by the cable, and the total length of the cable between the amplifier and the speakers. As a rough guide, to maintain a good quality sound the recommended minimum gauges are given in the table below, but be sure that the actual type of cable that you use has an adequate current rating), in your particular installation conditions. For very high power systems the use of powered speakers is recommended.

In this table, the cable currents indicated assume the speaker has unity power factor, and the minimum damping factors indicated (in brackets) are given at DC assuming cable conductors at 70 ºC and a value of 0.1 ohms for the total of the amplifier output impedance and the connector impedances. The mediocre damping factors indicated for a 4 ohm load can be substantially improved upon by selecting the next higher cable size, where practicable.

The extremely large cable sizes indicated for long cables and low impedance systems (10 mm² is larger than will fit most types of Speakon connectors) are an important factor in the desirability of siting amplifiers as close to their speakers as possible, and in selecting an acceptable system impedance − especially in powerful systems. It can also be seen that where an amplifier has multiple speaker output connections, it is always better to run separate cables from these rather than 'daisy-chaining' a single cable (of the same gauge) from speaker to speaker.

Note that for high power systems and for 100 volt line systems, the voltage rating of the cable is important as well as its gauge. For further information on connecting amplifiers to speakers see the System Assemblers page and the Amplifiers and Speakers page. See also AWG.

Gear
A slang term for equipment. See also Kit (2).

Gel
See Filter (2).

Gender
Specifies whether a connector is of a male or a female type. Sometimes referred to as 'sex'. See also Gender changer.

Gender bender
A slang term for a gender changer.

Gender changer
An adaptor which converts a male connector into a female one, or vice versa. Sometimes called a 'sex changer'.

Generation
The number of copying operations that recorded material has undergone since being originally recorded. For example, a 'third generation' recording is a copy of a copy of the original. See also First generation, DRM, SCMS, SDMI and HDCP.

Generator set
An item of equipment consisting of an electrical generator powered by an internal combustion engine. Generator sets are usually used to provide a mains voltage supply in locations where a supply from the normal power distribution network is not available − especially when power is required in large quantities or for long periods.

The smaller models (up to about 5 kVA) usually use a petrol engine, whilst larger ones are usually diesel-powered. For reasons of electrical safety, it is usually necessary to install an earth rod in order to provide a safety earth. CAUTION: Generator sets must only be used in well-ventilated areas outdoors, because of the hazardous exhaust fumes produced by the engine. Slang term: genny. See also Inverter.

Genny
A slang term for a generator set, usually a portable or mobile type.

GFCI
See RCD.

GFI
See RCD.

GHz
A unit of frequency. One GHz is 1000000000 Hz, or 1000 MHz.

Gig
A slang term for any live event (not necessarily a music event).

GND
An abbreviation for 'ground'. See Safety earth and Signal earth.

Go down
A slang term for 'to cease working (properly)', or 'to fail'. For example, "The FOH centre feed has just gone down", or "My laptop has been down for a week". See also Dead (3).

Gobo
Just in case you wonder whatever the lighting engineer is talking about, this is a metal plate in the shape of a particular image that is to be projected by a lantern. It is inserted into the lantern at the focal point of the light beam.

Gooseneck
An arrangement for supporting a microphone, generally used with lecterns and table-stands. It employs a semi-flexible tube which may be bent as required and then holds its position. This allows a microphone, fixed at the end of the tube, to be readily pointed in any direction, retaining that position without the use of clamps (avoiding the need for loosening and tightening at each adjustment). Often the mic cable is routed through the tube, which may require the use of a special mic clip with a hole for the cable to pass through. Also frequently used to support means of illumination for mixers, lighting control desks, etc. See also Littlite.

GPO jack
See Jack.

Graphic
Short for 'Graphic equaliser' (see the next definition).

Graphic equaliser
A type of equaliser, usually having several slider-type controls. Each of these controls determines the amount of cut (that is, reduction) or boost given to a particular range (or "band") of frequencies, so that the complete set of controls enables adjustment of the cut or boost given by the unit across the entire audio-frequency spectrum. The controls are positioned side by side, so that the physical position of their knobs looks like a graph of the resultant frequency response of the unit, (which is the reason for its name). The budget versions divide the audio frequency range into a small number of bands, whilst the professional units divide it into a larger number of bands, giving a finer degree of control. The following arrangements are the most popular:

• 10 bands (octave types), with bands centered on 30 Hz, 60 Hz, 120 Hz, 240 Hz, 500 Hz, 1 kHz, 2 kHz, 4 kHz, 8 kHz and 16 kHz.
• 15 bands (two-thirds octave types), with bands centered on 25 Hz, 40 Hz, 63 Hz, 100 Hz, 160 Hz, 250 Hz, 400 Hz, 630 Hz, 1 kHz, 1.6 kHz, 2.5 kHz, 4 kHz, 6.3 kHz, 10 kHz and 16 kHz.
• 31 bands (one-third octave types), with bands centered on 20 Hz, 25 Hz, 31.5 Hz, 40 Hz, 50 Hz, 63 Hz, 80 Hz, 100 Hz, 125 Hz, 160 Hz, 200 Hz, 250 Hz, 315 Hz, 400 Hz, 500 Hz, 630 Hz, 800 Hz, 1 kHz, 1.25 kHz, 1.60 kHz, 2 kHz, 2.5 kHz, 3.15 kHz, 4 kHz, 5 kHz, 6.3 kHz, 8 kHz, 10 kHz, 12.5 kHz, 16 kHz and 20 kHz.

N.B. Although a PA graphic equaliser just looks like a larger version of the one you often find on a domestic hi-fi system, and what it does to the signal is just the same, the graphic equaliser on a PA system is provided for an entirely different purpose to the one on a hi-fi system. Whilst the one on a hi-fi system is (in practice) there to enable the listener to adjust the sound to suit his or her preference, the purpose in a PA system is to allow the sound to be adjusted in order to compensate to some extent for deficiencies in the acoustics of the room, taking into account the speakers used and their placement. This means that for a permanently installed PA system, once the graphic is correctly set up it should never be adjusted again, unless the acoustics of the room are changed (e.g. by a change to furnishings or carpeting), or the type or position of the speakers is altered.

Note, however, that a graphic equaliser cannot alter the room acoustics. A highly resonant space will always tend to ring at its resonant frequencies and cause colouration of the sound. The only solution to this is treatment of the room acoustics, generally by an increase in the quantity or effectiveness of absorbent material present. See also Parametric equaliser, Spectrum analyser and Q (1).

Grazing effect
Sound which travels from speakers that are on or near the ground to a listener at ground level, is reduced in level to some degree by virtue of its proximity to the ground − in addition to the reduction in level caused by dispersion over whatever distance it has to travel. This additional reduction is often not equal for all frequencies; usually the higher frequencies are affected most.

The extent of the additional reduction, which is called the grazing effect, and the frequencies of the sound which are most affected by it, is heavily dependent upon the nature of the ground surface across which the sound has to travel − a crowd of people would have much more effect than a bare wooden floor. The grazing effect can be reduced to some degree by positioning the speakers well above ground level; once this has been done, further advantage can be gained in an auditorium setting by banking the seating, so that the path of the sound to the people located at the back of the audience is well above the heads of most of the audience located in front of them. If the speakers are positioned at an insufficient height, however, banked seating will only make matters worse.

Ground
Same as 'earth'. (In the U.S.A., the term 'ground' is nearly always used, but it is becoming increasingly used in the UK).

Ground-compensated
Describes an output that, when used with a 3-conductor cable, is able to provide rejection of noise caused by voltage differences between the signal earths (or 'signal grounds') of the sending and destination equipment, even when feeding an unbalanced input. (Such an output is rarely referred to as 'earth-compensated', even in the UK.)

The output achieves this by using one of the conductors (called the 'ground sense' conductor) of the cable to sense the signal-earth voltage at the destination end of the interconnection. The output drive circuitry then superimposes that voltage on the signal voltage provided at the output, so that the signal input voltage 'seen' by the destination equipment, relative to that equipment's own signal earth, is (in theory) just the wanted signal voltage.

So, the ground-sense terminal of the output connector is actually an input connection. This connection is usually made through pin 3 of a 3-pole XLR, or the ring of a TRS jack. Pin 2 (or TRS tip) carries the signal ('hot' conductor) and pin 1 (or TRS sleeve) provides the signal earth connection (usually made via the screen of the cable).

The input impedance of the ground sense terminal is arranged to be essentially the same as the output impedance of the signal terminal, so that, when a ground-compensated output is interconnected with a balanced input, with the ground-sense conductor connected to the 'cold' terminal of the balanced input, the interconnection is able to operate much as a semi-balanced interconnection. (In such a case, as the ground-sense conductor is not connected to the signal earth of the destination equipment, no distant 'ground voltage' is sensed by the ground-compensated output and so no rejection is provided by the output; rejection of common-mode interference is provided by the balanced input, just as in the semi-balanced case. However, as any interference picked up on the ground-sense conductor will be added by the compensation circuit to the signal output on the signal conductor, the interference level on the two conductors will no longer be the same and the rejection provided by the balanced input will be impaired.)

Although the reduction in earth-loop induced noise obtained by interconnecting a ground-compensated output with an unbalanced input may be comparable with that provided by a true balanced interconnection, such an interconnection is inferior to the balanced case in that:

• There is no rejection of common-mode interference picked-up on the cable.
• It lacks the additional headroom provided by a balanced interconnection.
• It may induce a crosstalk signal into other nearby circuits that are not truly balanced.

A ground-compensated interconnection can only work with unbalanced destination equipment if the pin 3/ring (ground-sense) conductor is connected to the signal earth of the destination equipment. (This is in contrast to a normal balanced output, whose pin 3/ring ('cold') conductor should not be shorted to signal earth (as to do so would cause large currents to flow on that conductor due to the low output impedance of the line output; such currents could induce interference into other circuits − see Inductive coupling − and/or stress the line output components).

It is debatable as to whether or not the pin 1/sleeve (screen) conductor should be connected to the signal earth at both ends of the interconnection. Such a connection would (in the presence of an earth voltage difference) cause an earth current to flow in that conductor. Although the voltage difference should in theory be largely rejected by the ground-compensation arrangement, the current flow can cause effects such as a voltage drop between the sensing point (at the connector terminal at the destination end of the cable) and the true internal signal earth of the destination equipment, due to impedances in the signal earth connections (especially if dirty or worn jack connectors are in use). [In the case of poor quality cables, the current flow in the cable screen can also cause a small differential voltage to occur between the inner conductors, due to differences in the inductive coupling from the screen to the two inner conductors, but this applies equally to fully balanced interconnections.] However, disconnecting the signal earth (screen) at one end is inadvisable, as it turns the screen into a long aerial which can inject radio-frequency interference at the connected end, and leaves the inner conductors effectively less-well screened at the unconnected end (due to the screen impedance). Sometimes a compromise is adopted by connecting a capacitor (e.g. 0.01 µF) in series with the signal earth at one end; this allows RF to pass but effectively blocks mains-frequency earth currents. However, the usual preferred practice is to have the screen connected to signal earth at both ends.

Note that the ground-compensated scheme cannot work properly when the output is feeding multiple inputs (unless the signal grounds of those inputs are connected by an extremely low impedance − for example when they are inputs on the same item of equipment). Compare Balanced, Unbalanced, Semi-balanced, Quasi-floating and Pseudo-balanced. (A table comparing the most common types of balanced interconnections is provided under the 'Balanced' entry.)

Ground fault circuit interrupter
See RCD.

Ground fault interrupter
See RCD.

Ground isolator
See Earth isolator.

Ground lift
Another name for earth lift.

Ground loop
Another name for an earth loop.

Ground sense
See Ground-compensated.

Group
A facility of a mixer that allows the level of the signals from a number of selected channels to be adjusted as a set, by one or more group faders. Other facilities may also be provided, such as group PFL, group muting, etc.. There are two kinds of groups: audio groups and VCA groups.

In an audio group (also called a 'sub-group' or a 'sub-mix', because the group mix is usually a subsidiary component of the main mix), the audio signals from the channels selected to make up that group are mixed together onto a group bus. The resulting sub-mix may then be controlled in level by the group fader(s), and is usually available at a group output. An insert point is often also provided for it, to enable the group to be passed through an outboard serial effects unit before being returned to the mixer to be added into the main mix.

Note that, as the audio group is mixed after the channel pan controls, two group buses and two group faders are normally used to allow stereo operation − one for Left and one for Right. By convention, Left buses/faders are usually odd-numbered, and Right buses/faders even-numbered. (When an additional mono output is provided, to feed a Centre speaker, an additional fader may be provided for that mix − see LCR (1).)

When using outboard parallel effects units fed from channel post-fade auxiliary sends, a potential difficulty arises with audio groups: adjusting the channel fader keeps the effect level in proportion to the direct (dry) level, but adjusting the group fader alters the balance of effects and dry signal. This is because a group fader adjustment alters the direct signal level without a corresponding change to the effect level (unless the effect return is via a channel in the same group).

VCA groups are possible only on VCA-equipped mixers, and are generally found only on the larger types − but are becoming a more popular feature. They operate in an entirely different way to audio groups, in that the group fader does not carry an audio signal but rather 'remotely' controls the level of post-fade signal passing through each of the selected channels (in addition to the 'local' control provided by the channel fader). This 'remote control' is achieved by a DC control signal that is applied to a VCA (voltage-controlled amplifier) on each channel. So, no separate sub-mix of the signals from the 'grouped' channels exists, and therefore cannot be output from the mixer or processed via an insert socket. When more than one VCA group fader is assigned to a particular channel, all of the assigned VCA group faders have control over the level of signal passing through that channel.

VCA grouping does not suffer from the potential effects-imbalance problem experienced with audio groups, because a VCA group fader affects the level of the post-fade auxiliary sends as well as the level reaching the main (or audio group) mix bus. Also, because the group level adjustment takes place before the channel pan controls, only a single group fader is needed − even for LCR mixes.

Because each of these two methods has its own merits, mixers which provide the VCA grouping facility usually also support audio grouping (but often to a lesser degree than mixers without VCA groups). See also Matrix.

Group bus
A mixing bus on which the signals from selected channels are summed into an audio group mix. Group buses are usually paired into Left and Right buses. For example, if a channel is routed to audio group '5-6', this means that the proportion of the channel's signal that is panned to the Left is fed to bus number 5, and the proportion panned to the Right is fed to bus number 6. In a stereo system, these two group buses will usually be routed to the Left and Right main-mix buses, respectively. (In mixers with LCR audio groups, a third main-mix bus is also required, for mixing the mono centre signals.)

Group delay
Strictly, a measure of the slope of the phase response of a system over a specified frequency range. That is, a measure of how rapidly the phase shift through the system changes as the applied frequency changes through that range.

A system in which the phase response changes linearly with (linear) frequency over a particular frequency range (that is, a system having a linear phase response over that frequency range) will have a constant group delay over that frequency range, which is a good thing in high-quality audio systems.

Group delay usually remains fairly constant throughout regions of constant frequency response (e.g. well within the passband of a filter), but can change very significantly where the frequency response is rapidly changing (e.g. around a filter's cut-off frequency or frequencies). This has important consequences for the design of audio crossovers and in the design and use of equalisers.

In general, group delay does not refer to the absolute signal delay time through a system, but to the amount of delay experienced by a 'group' of similar-frequency signals relative to that experienced by signals of different frequency. However, provided that the group delay is reasonably constant over a particular frequency range, and that the frequency response is also reasonably constant over that range, then the group delay is the actual time delay that a 'group' of signals occupying only that frequency range will experience on passing through the system. See also Minimum phase.

Group fader
See Group.

GRP
An abbreviation for 'group'.

Guitar pickup
The part of a guitar which detects the vibration of the strings (and body) and converts this vibration into an audio signal. See also Humbucker.

Guitar processor
An item of equipment that provides signal processing for guitar signals.

Gun microphone
Another name for a rifle microphone.

Guru
A slang term for a person who is an acknowledged expert in a particular field. Most often applied to computer software experts. See also Tech-head.

Gyrator
An electronic circuit that uses semiconductors to simulate an inductor. It is usually constructed using an operational amplifier.

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This page last updated 10-Nov-2009.