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Glossary of PA Terms - D

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The glossary pages provide definitions for over 2270 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.

D-Type * DA * DAC * Dais * Daisy-chain * Damping * Damping factor * Danger * DARS * DAT * Data compression * Data rate * DAW * dB, dBV, dBv, dBu, dBm, dB FS, dB SPL, dB (A) and dB (C) * DB * dB per octave * DBO * DC * DC protection * DDA * DDL * De-emphasis * De-esser * De-regulated frequency * De-rig * Dead * Dead blackout * Dead spot * Decay * Decibel * Deck * Decoder * Delay * Delay speaker * Delays * Demodulation * Demodulator * Demultiplexer * Demultiplexing * Depth * Desk * Destructive interference * Destructive solo * Detent * DI * DI box * DI output * Diaphragm * Dielectric * Differential drive * Differential input * Diffraction * Diffuse field * Digital * Digital amplifier * Digital audio workstation * Digital black * Digital cliff * Digital gain * Digital mixer * Digital multicore * Digital signal processing * Digital snake * Digital to analogue convertor * DIN * DIN 45405 * DIN 45412 * DIN 45573 * DIN 45596 * DIN 651 * DIN noise weighting * Dip * DIP switch * Direct box * Direct contact * Direct current * Direct inject * Direct output * Direct radiator * Direct sound * Directional cable * Directional microphone * Directivity * Dirty * Discharge lamp * Discrete circuit * Dispersion * Dissipation * Distance factor * Distant pickup * Distortion * Distribution amplifier * Distribution board * Distro * Diversity * Divided pickup * DL * DMM * DMX * Dolby A * Dolby B * Dolby C * Domain * Double balanced cable * Double insulated * Double normalling * Double termination * Down * Down-stage * Downfill * Downfills * Downward expander * Drain * Drain wire * Dress * Drive * Driver * DRM * Drop-out * Drum cage * Drum screen * Dry * Dry hire * Dry joint * DSM * DSP * Dual concentric * Duck tape * Ducking * Duct tape * Dummy load * Duplex * DVD * DVI * DVM * DVR * Dynamic equalisation * Dynamic microphone * Dynamic range * Dynamics * Dynamics processor

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.

D-Type
A generic name for any connector with a 'D-shaped' shell, surrounding the pins of the male version. However it is especially applied to types having 2 different-length rows of pins, popular in 9-, 15- and 25-pole versions and once in common use for low-speed serial data interconnections (until the advent of higher-speed serial interconnections such as USB and IEEE 1394). See also VGA.

View D-type (9 pole) image

DA
An abbreviation for 'distribution amplifier'.

DAC
An abbreviation for 'digital to analogue convertor'.

Dais
A raised platform area or small stage, from which lectures, presentations, etc. are delivered. See also Lectern.

Daisy-chain
A method of interconnecting several items of equipment such that an output of one item feeds the input of the next, and so on in succession. The term is used of two distinct cases:

• Where the chained equipment 'outputs' provide an identical copy of the signal applied to their inputs: either by a simple parallel connection of the connectors inside the equipment (as in the case of speaker interconnections), or via a buffer circuit (as in the case of MIDI interconnections, where such outputs are usually labelled 'THRO'). Another example of this case is in DMX lighting control feeds.
• Where the chained equipment 'outputs' provide a processed version of their inputs, as in the case of serial effects interconnections.

Damping
A reduction of something over a period of time − especially a deliberate reduction of resonance, reverberation, echo, etc. that would otherwise have continued for a longer period. For example, the damping control on an echo effects unit controls how rapidly the echoes die away. Likewise, a pillow may be placed inside a kick drum to reduce resonances. The amount of damping of a resonant system is often specified by a Q value − see Q (3).

Damping factor
A measure of how well the motion of the cone of a speaker is controlled by the power amplifier driving that speaker. If the damping factor is poor (a low number), the speaker cones will continue to vibrate significantly after a movement instructed by the amplifier, and this will considerably impact on sound quality. For 'Hi-Fi' applications a sensible target value is 50, although in less critical PA applications values of around 15 to 20 would usually be considered acceptable and even as low as 10 may be tolerated. A very high value (several hundred) may be quoted for an amplifier, but usually it is the gauge and length of the speaker cables that have the most effect.

The overall value is calculated by adding the output impedance of the amplifier to the round-trip resistance of the speaker cable, and then dividing the speaker impedance by the result of that sum. For example, if an amplifier having an output impedance of 0.02 ohms is connected to an 8 ohm speaker by a cable with a total resistance of 0.05 ohms then the damping factor is 114. But if the cable resistance were 0.35 ohms then the damping factor would be only 22. (As a guide, the round-trip resistance of 2.5 mm² cable is approximately 0.015 ohms per metre length of cable, whilst for 4 mm² cable the figure is approximately 0.01 ohms per metre length). Note that for the calculation to be accurate at treble frequencies, the inductance of the cable must be taken into account as well as its resistance. See also Series.

Danger
A significant likelihood of injury or death arising from a hazard. N.B. This definition of the term may differ from officially recognised definitions. See also Risk. Compare Safety. For further information on safety see the Safety page.

DARS
An abbreviation for 'digital audio reference signal', an AES3 signal used for synchronisation of digital audio equipment. It is usually a digital black signal.

DAT
An abbreviation for 'digital audio tape', a digital tape recording standard which uses a 'mini cassette' tape format and a helical scan technique. The normal sampling rate for 2-track (stereo) recording is 48 kHz, and the normal bit depth is 16 bits. However, other sampling rates are possible such as 44.1 kHz (for CD compatibility) and 32 kHz (for 4-track reduced-bandwidth operation). See also Analogue to digital conversion.

Data compression
See Compression (2).

Data rate
See Bit-rate.

DAW
An abbreviation for 'digital audio workstation', a computer-based system for multi-track audio recording and the subsequent digital mixing and processing of the recorded material. See also Application, Plug-in, Platform (2) and SAC.

dB, dBV, dBv, dBu, dBm, dB FS, dB SPL, dB (A) and dB (C)
An abbreviation for 'decibel'. The letter(s) following ‘dB’, when present, indicate the reference level or weighting. For full information and value conversion calculators see the Decibels page.

DB
An abbreviation for 'distribution board'.

dB per octave
See Roll-off.

DBO
An abbreviation for 'dead blackout'.

DC
An abbreviation for 'direct current', a current or voltage which does not reverse its polarity (and usually has a substantially steady value). This type of current is produced by batteries; alternatively it may be converted from the mains supply by using a power supply unit (PSU). It is used for powering small items of equipment such as microphones (see phantom power) and guitar pedals, and as a means of remote control (especially in analogue lighting systems). Compare AC.

DC protection
See Speaker protection.

DDA
An abbreviation for 'Disability Discrimination Act', UK legislation requiring (amongst other things) the provision of assistive listening facilities where speech-based services are provided to the general public. Such facilities are most often provided by means of an induction loop system, because most types of personal hearing aid have built-in loop receivers (see T-position). However, other assistive means are sometimes appropriate, such as systems using infra-red or radio.

DDL
An abbreviation for 'digital delay line', a delay unit that functions digitally. (The term 'delay line' originates from an early analogue delay technique, in which a long line was used to provide signal delay.

De-emphasis
See Pre-emphasis.

De-esser
An item of equipment used to reduce sibilance. Its basis of operation is the provision of compression that is effective only at high audio frequencies (e.g. greater than 4 kHz), and/or the provision of dynamic equalisation.

De-regulated frequency
A radio frequency (e.g. for use by radio microphones or in-ear monitoring equipment) that is free for use without any licensing requirement. As the use of these frequencies is relatively uncontrolled, interference from other users can sometimes be a problem. Sometimes referred to as an 'unregulated frequency'. For further details see 'Wired or Radio' on the Microphones page. See also Channel 70, ISM and ETS. Compare Regulated frequency.

De-rig
To dismantle a system (such as a PA system or a stage lighting system) that was erected temporarily, specifically to meet the requirements of a particular event. See also Load-out. Compare Rig.

Dead (1)
Describes acoustics having a short reverberation time. Compare Lively.

Dead (2)
Describes a cable, a mains supply point, etc., that is not presently energised with electrical power.

Dead (3)
A slang term describing equipment that is faulty, usually to such an extent that it cannot be used at all. See also Gone down.

Dead blackout
A point during a performance when all lighting that may safely be switched off, is switched off, for dramatic effect. This includes any back-stage lighting that may leak onto the stage or into the auditorium. However, all emergency lighting and fire exit signs must remain on. Often abbreviated to 'DBO'. Compare Blackout.

Dead spot
A point in space where a wanted sound cannot be heard as well as it can be heard at surrounding points; typically due to physical obstructions or to the effects of destructive interference between the sound from two different sources (e.g. two speakers) or between two sound paths (e.g. the paths of direct and reflected sound). The locations and depth of dead spots are likely to be heavily influenced by the location and orientation of the speakers and, where reflected sound is involved, by the acoustics of the space.

Decay
Informally, the period of time during which a sound dies away to silence. In this context, the notes produced by many musical instruments (especially undamped stringed instruments such as the guitar) have a long decay time, in comparison with their attack time. However, as a formal parameter in the definition of sound envelopes the precise meaning of the term is different − see ADSR.

This term may sometimes be used to refer to the release control of a compressor.

Decibel
A unit of relative measurement of effective power, on a logarithmic scale. For a comprehensive explanation and value conversion calculators see the Decibels page. See also Gain, Loss, Phon and Sone.

Deck
In general, an item of equipment for the recording and/or playback of audio or video material. However, the term is frequently used as an abbreviation for a record deck, alternatively known as a turntable. See also Vinyl and Reel-to-reel.

Decoder
See Multiplex.

Delay
An effect unit (described under Echo). Such a unit may also be used to provide a delayed version of a signal to "secondary" speakers that are situated a considerable distance in front of the main speakers, so that the sound heard from the secondary speakers is synchronised with the sound heard from the main speakers − the latter having been delayed in travelling through the air to reach the location of the secondary speakers. Such a use would not normally be considered for inter-speaker distances of less than about 10 metres. Approximately 3 milliseconds of delay is required per metre of distance between the main and secondary speakers (see Speed of sound). Almost all delay units now function digitally, and are known by the abbreviation 'DDL' (for 'digital delay line'). See also the next definition, Haas effect and ADT.

Delay speaker
A speaker that is fed with a delayed version of a signal, as compared to the signal fed to the main speakers. When a number of such speakers are being referred to, this term is sometimes shortened to 'delays'.

Delays
See the previous definition.

Demodulation
The reverse process to modulation.

Demodulator
A device that performs the reverse process to modulation.

Demultiplexer
A device that separates out the component signals of a multiplex.

Demultiplexing
The process of separating out the component signals of a multiplex.

Depth
The extent to which an effect such as chorus or phase impacts upon the sound.

Desk
Short for 'mixing desk' − see Mixer. (In wider usage, may also be used to refer to a lighting control desk, or to any form of control panel in which the controls are mounted on a horizontal or slightly inclined surface.)

Destructive interference
The phenomenon whereby certain sounds seem to disappear, or to significantly decrease in level, at a particular position in a room (or indeed outdoors), even though there is no physical obstacle in the way. This is caused by the sound having arrived at this position along two different paths − e.g. one a direct path and the other a reflected one − such that the two versions of the sound are of similar level but opposite phase. They therefore effectively cancel each other out, and no sound (or only a much reduced one) is heard.

Since, for a given amount of delay, the phase relationship varies with frequency, usually only certain parts of a complex sound will seem to disappear at a given point in the room − but other parts may disappear at other points. See also Dead spot, Anti-phase and Polarity reversal. Compare Constructive interference.

Destructive solo
See Solo.

Detent
A feature on a control (rotary or slider types) which defines a specific position in its travel by requiring a slightly increased mechanical force in order for the control to be moved from that point. Most usually, this feature is encountered as a 'centre detent', a single detent that is provided at the half-way point in the travel (e.g. on equaliser, pan and balance controls). Occasionally controls may be provided with many detents, giving a 'click-click-click' feel as the control is operated.

DI (1)
An abbreviation for 'direct inject'. See DI box.

DI (2)
An abbreviation for 'digital input', used to identify panel connectors having that function. See also AES3.

DI box
A device, usually used on stage, which allows an unbalanced signal source (such as a keyboard or a combo line output) to be fed to a balanced low impedance input, typically a 'microphone input' of a mixer. The DI box also provides a high impedance input, so that high impedance sources such as passive guitar pick-ups can be connected directly to the DI box if required. Facilities are usually provided to help avoid earth loop problems, and provision is usually made for adjustment to the signal level (often by means of a switchable attenuator), to allow a range of source levels to be catered for. DI stands for 'direct inject', so called because the instrument, guitar pre-amplifier, etc. is connected 'directly' to the PA system rather than its sound being picked up by a microphone.

To avoid the need for use of a DI box, some instrument amplifiers incorporate a DI facility and so provide a balanced "DI Output" connector (usually a male XLR).

DI boxes may be passive or active. Passive types require no power source, and achieve the required impedance conversion and isolation of the input and output signal earth connections by means of a transformer. Active types contain electronic circuitry which requires power (supplied from an internal battery, an external mains power unit or by phantom powering). Active types are able to provide a higher input impedance than passive ones and also have the advantage that their input impedance is unaffected by the capacitance of the balanced line or by the impedance of the load(s) that the line is connected to − these factors may be of benefit when connecting high impedance passive guitar pick-ups without using a pre-amplifier. However, many active types fail to provide complete electrical isolation (galvanic isolation) between the input and output (only rarely a necessity).

Note that use of a DI box is essential when connecting an unbalanced source to a balanced input that provides phantom power, or to a balanced line that carries phantom power, as to make a direct-wired connection (e.g. using an adaptor plug or cable) would expose the source equipment to the phantom power voltage, which may cause serious damage to that equipment.

DI output
An output that is intended to be connected directly to a low impedance balanced input, such as a mixer 'microphone input' or to a stagebox input, allowing the source signal to be 'directly injected' into that equipment. DI outputs are most often found on combos, heads and pre-amplifiers; occasionally on keyboards and rarely on guitars. The connector is usually a male 3-pole XLR, and is connected to the PA system using a balanced cable (such as a microphone cable). Caution: Always check the manufacturer's instructions before applying phantom power to a DI output. See also Emulated line output.

Diaphragm
A surface that is intended to capture the motion of sound waves, and so move back and forth in sympathy with them. Or, a surface that is intended to create sound waves by its own back and forth motion. The former usage of the term is the most common, and applies when the term is used to refer to the internal part of a microphone that vibrates in sympathy with the sound waves entering the microphone. The latter meaning applies in reference to a driver − specifically referring to the internal part of a horn driver that vibrates in sympathy with the signal applied to the driver, so creating sound waves. Other types of drivers usually employ a conical diaphragm, which is therefore more commonly referred to as a cone. In either case, it may also be called a membrane. Usually, a diaphragm is very thin, and is circular in shape.

Dielectric
Usually refers to the insulating material between the two plates of a capacitor. Common dielectric materials for non-polarised capacitors are polystyrene, polycarbonate, polyester, polypropylene, and ceramic. Polarised electrolytic types utilise an electro-chemically created dielectric of aluminium oxide.

The term is also used to refer to the internal insulation between the inner and outer conductors of a coaxial cable − particularly one of defined characteristic impedance, generally used in impedance-matched interconnections.

Differential drive
See Balanced.

Differential input
See Balanced.

Diffraction
The phenomenon whereby sound waves bend around objects they encounter on their path. This is a frequency-dependent effect, because it only occurs to any significant degree when the wavelength of the sound is larger than the physical size of the object. The result is that treble sounds are readily blocked by substantial obstacles in their direct path, while bass sounds are not. Compare Reflection. See also Refraction.

Diffuse field
Describes locations at which indirect sound energy from a particular sound source predominates over the direct sound energy from that source. Also known as the reverberant field and as the ambient field (though some would argue that there are subtle differences between the exact meaning of the three terms). Within the diffuse field, the inverse square law does not apply. Note that within the diffuse field the direction of the sound source may still be clearly identifiable to a listener, because of the Haas effect. Compare Free field. See also Radius of reverberation, Critical distance and Critical frequency.

Digital
Describes anything which functions by means of numbers.

A digital audio signal is one in which the value of some parameter of the signal (usually its voltage) makes rapid transitions between just two states, which respectively represent a '0' and a '1'. This sequence of 0's and 1's forms a sequence of binary numbers, and the variations in the value of those numbers correspond to the variations in instantaneous voltage that make up the sound waveform being represented by the digital signal, according to some agreed coding scheme. The level of digital signals is usually expressed in dB FS.

Properly speaking, digital equipment is equipment that processes, stores or conveys information in digital form. Sadly, the term is much misused, being sometimes applied to equipment that is not truly digital according to the above definition, but that merely incorporates a form of internal or external control making use of digital (i.e. numerical) techniques − or even that is simply claimed to be of a quality suitable for use with truly digital equipment.

A digital recording is one in which the programme material (e.g. music) is stored as a sequence of numbers. The storage media may be tape (see DAT), removable disc (CD, Mini disc or DVD) or a hard drive. See also Digital mixer, AES3, SPDIF, DMX, Analogue to digital conversion, DSP, Full scale, Over, Latency, Compression (2), Pulse code modulation, Streaming, Float, FIR, IIR and FFT. Compare Analogue.

Digital amplifier
Properly speaking, an amplifier that accepts a digital input signal and outputs a higher-level (and possibly re-shaped and re-timed) version of that same digital signal. More usually, such an amplifier is referred to as a repeater or a regenerator (though the latter term is strictly only appropriate when re-timing is provided).

In reference to audio power amplifiers, which by definition produce an analogue output suitable for driving passive speakers, no such equipment as a 'digital amplifier' exists according to the strictest definition of 'digital'. This is because the basic function of any amplifier is to provide an output corresponding to its input signal but at an increased voltage level, or with an increased available power level, and no amplfier is able to achieve this through numerical techniques alone. (See, however, Digital gain.)

Nevertheless, the term 'digital amplifier' is commonly misused to refer to a Class D power amplifier (otherwise known as a 'switching amplifier'), some types of which accept a digital input signal and/or make use of digital processing techniques to produce their internal PWM or PDM switching signal which drives the power stage transistors. For more information on that type of amplifier see Pulse width modulation. For a list of common amplifier classes, see Amplifier Classes on the Amplifiers and Speakers page.

Digital audio workstation
See DAW.

Digital black
A digital signal that represents silence. Named by analogy with video signals, in which black is the lowest possible level of luminance.

Digital cliff
A term used to describe the effect of decreases in the quality of a digital signal, for example, decreases in its signal-to-noise ratio or increases in the amount of jitter present. If the signal quality progressively decreases from 'good' towards 'poor', the effect on the perceived quality of the conveyed information (e.g. programme material) is initially very slight, but at a specific signal quality value the perceived quality of the conveyed information abruptly reduces dramatically.

This is in marked contrast to the effect of an equivalent progressive reduction in the quality of an analogue signal, which is a gradual reduction in the perceived quality of the conveyed information. The term 'digital cliff' arises from the cliff-like shape of a graph of conveyed information quality against digital signal quality.

Digital gain
A control that, using numerical processing techniques, operates on a digital signal internal to an item of equipment so as to enable adjustment of the digital signal level at that point. Such a change in digital level does not in any way affect the voltage level of a digital signal passed over electrical interconnections between items of equipment − it affects only the numerical values conveyed by that signal. When the digital signal is eventually converted to an analogue one, a corresponding change in the voltage level of that analogue signal will occur (in the absence of other factors affecting it).

As with other kinds of change in signal level, when adjusting digital gain care must be taken to avoid increases that would bring the digital level to the point of clipping during signal peaks or transients. In the case of a digital signal, the clipping point (0 dB FS) is determined not by the maximum signal voltage accommodated by the processing circuitry, but rather by the format of the digital signal at that point. To avoid the possibility of clipping, adequate headroom must be maintained at all points throughout the entire signal chain, during both analogue and digital processing.

Note that a digital gain control is unable to correct for a substantially misadjusted analogue Gain control at the pre-amplifier (i.e. prior to analogue to digital conversion). Too little pre-amp gain will result in a poor signal-to-noise ratio, while too much will result in clipping at the pre-amp or during subsequent processing. See also Over.

Digital mixer
A mixer that functions using digital techniques. Usually the term refers to a mixer in which, after pre-amplification, each channel's signal is individually converted into digital form and all mixing and other signal processing within the mixer is performed digitally. This arrangement has a number of important advantages, including:

• Many complex combinations of settings can be electronically saved and restored at the touch of a button. This is especially useful when several different acts are to use the same set of mixer inputs during a show, or when a show such as a musical has many different scenes. (This is why the saved sets of settings are often called 'scenes'.)
• Digital signal processing can be conveniently be employed to enable the incorporation of functions such as compression, noise gating, reverb, delay and far more comprehensive equalisation facilities than are generally provided on an analogue mixer. This greatly reduces the need for outboard processing equipment.
• A common set of controls (which are often display-screen based) can be used for adjustment of any channel; on mixers handling a large number of channels this greatly reduces the area of the control surface and increases reliability.
• If required, the control surface can more readily be located remotely from the signal processing equipment.
• As the signal does not pass through the control potentiometers, noise levels are reduced and potentiometer wear and contamination are less of an issue.

Digital multicore, Digital snake
A system that uses digital technology to provide a multi-channel audio link for signals from the stage to a mixer location, taking the place of a conventional multicore. The signals are converted from analogue to digital at the stagebox (or in racked equipment at the stage end of the link), and are then multiplexed for sending to the mixer location via a high-speed data link, typically using a UTP or fibre-optic cable. At the mixer end, the digital multiplex signal is either demultiplexed and converted back to individual analogue channels for connection to the mixer in the usual way, or, in mixer-integrated systems, the multiplex is accepted directly by the digital mixer.

Typically such systems provide either line-level inputs for connection of line-level sources such as instruments, radio microphone receivers and separate wired microphone pre-amps, or else they incorporate microphone pre-amps to enable the direct connection of wired mics to the stagebox. See also Analogue to digital conversion and CAT 5.

Digital signal processing
Signal processing performed by digital means. Frequently abbreviated to 'DSP'. See also Float.

Digital to analogue convertor
An item of equipment, or a device within an item of equipment, that converts a digital audio or video signal into an analogue one. Often abbreviated to 'DAC'. Compare Analogue to digital conversion.

DIN
An abbreviation for Deutsches Institut für Normung (sometimes incorrectly written as Deutsches Institute für Normalung), a German organisation which defines many types of industrial standards, including electrical and audio-visual equipment standards.

In the context of connectors, the abbreviation is most frequently used to identify a round multi-pole connector, available in many different configurations from 2-pole to 8-pole, including miniature versions (mini-DIN).

The 4-pole mini-DIN connector is frequently used for S-video interconnections.

The standard-sized 3-pole and 180º 5-pole ('5-pin A') versions of this connector were once commonly found on domestic audio equipment, for mono and stereo respectively. (These are now largely superceded by the phono connector.) The usual pin allocations for the stereo version were as follows (in clockwise order, looking at the front of the socket):

• 1 - Left input
• 4 - Right input
• 2 - Signal earth
• 5 - Right output
• 3 - Left output

View DIN (5-pole 180º) image

DIN 45405
See Weighting.

DIN 45412
See Weighting.

DIN 45573
See IEC noise.

DIN 45596
See Phantom power.

DIN 651
See Weighting.

DIN noise weighting
See Weighting.

Dip
Same as Cut. See also Ducking.

DIP switch
An electrical component that incorporates several sub-miniature 2-position switches in a single very compact device. The switches are so close together that they require a small tool to operate them, so applications of this component are limited to settings that are very infrequently changed, such as 'installation' settings. Depending on the application, the individual switches may have separate functions or may be used to set up a numerical code such as an address. Such codes are effectively set up in binary, but may be recorded or displayed as hexadecimal or decimal values. A common use for DIP switches is for setting the address of addressable items of equipment such as DMX lighting fixtures. DIP stands for 'dual in-line package'; this relates to the component having two parallel rows of pins that connect it to the circuit board below (these are not visible on an installed switch).

Direct box
An American term for a 'direct inject' box − see DI box.

Direct contact
In electrical safety, the potentially lethal situation where a person comes into contact with a conductor that is intended to be live at a dangerous voltage (e.g. at mains voltage). This situation is protected against by the presence of insulation or by the conductor being securely enclosed; these measures are referred to as 'basic protection' against electric shock. Supplementary protection against direct contact may be provided by a suitable RCD, but this must never be the sole means of protection against direct contact. See also PAT. Compare Indirect contact.

Direct current
See DC.

Direct inject
See DI.

Direct output
A line-level output of a mixer, provided on each channel, which allows the pre-amplified signal on that channel to be supplied to other equipment such as a multi-track recorder or another mixer. The direct-out signal may be pre-EQ or post-EQ. Channel feeds for live recording of professional performances are more usually obtained by use of a multi-channel microphone splitter. See also Insert. Compare Split output.

Direct radiator
Describes a speaker in which the driver(s) are located at the front face of the enclosure, and create sound by acting directly on the air in front of the enclosure. This contrasts with the use of horns, diffractors and other devices that are sometimes employed to couple a driver to the air surrounding its enclosure. Note that a full-range speaker may utilise a mixture of coupling methods; in the case of low and medium-powered PA speakers a direct-radiating woofer and a horn-loaded HF driver is commonly employed. See also Baffle.

Direct sound
Sound that has travelled on a single essentially straight-line path from its source to the listener or to a microphone − specifically without having undergone any reflection along that path. Compare Indirect sound. See also Free field, Diffuse field and Radius of reverberation.

Directional cable
Strictly, no such thing. It is claimed by some audiophiles that some types of cable may give better performance when an audio or video signal is passed through it in a particular direction − that is, with a specific end of the cable connected to the signal source. Considering the cable alone (i.e. without connectors), there is no technical justification for such a claim because, as such signals are AC, the current flows equally in both directions (when averaged over time) − regardless of the direction of signal flow.

However, when considering a cable with connectors attached, directional factors may be introduced. This may be through obvious means such as different connector types at each end (e.g. male and female XLRs), or by more subtle factors such as differences in the way that the two connectors are wired (for example, screened cables which have their screen connected at one end only − see Telescoping shield and Pseudo-balanced).

Directional microphone
See Uni-directional and Bidirectional microphone.

Directivity
The extent to which a speaker, or one of its drivers, has narrowed dispersion angles. A narrower dispersion angle results in the emitted sound energy being more concentrated and so (potentially) provides an increased on-axis sound level and therefore a corresponding increase in the sensitivity of the speaker.

The directivity may be expressed as a Q value or as a value in decibels, either of which may be referred to as the directivity index. This index value indicates the increase in on-axis sound level as compared to the level that would have been obtained (at the same distance) if that speaker had been radiating the same total sound energy spread equally in all directions; this value is usually frequency-dependent.

So, it can be seen that a long throw speaker has a higher directivity than a short throw one. The correspondence between (circular) dispersion angles and the Q and decibel expressions of directivity is illustrated in the table below.

Note that the Q value of directivity index is not in any way associated with the Q values that are used to describe the degree of damping of a driver's or speaker's resonance. See also Constant directivity, Exponential horn, Beamwidth and Line Array.

Dirty (1)
Describes a signal or other electrical interconnection (including an earth connection or a power source) that is contaminated with noise. Or, describes a signal or sound in which distortion is significantly present − whether undesirably so or deliberately introduced. See also Power conditioner, Filter and Technical earth. Compare Clean.

Dirty (2)
Describes an electrical contact (especially the mating surface of a connector conductor or the track or wiper of a potentiometer) that is physically contaminated with material that impairs the quality of the electrical contact, typically causing a high resistance connection (with the possible effect of overheating in some situations), unreliability of the connection, and/or the introduction of distortion or noise. In the latter event, the contact or potentiometer may be described as 'noisy'. In the case of metallic contacts, a frequent cause of such contamination is oxidation.

Discharge lamp
A lamp in which the electric current flows through a gas enclosed within the lamp envelope, rather than through a metallic filament. In constrast to filament lamps, discharge lamps cannot be operated direct from the supply: the luminaire or fixture incorporates 'control gear' specifically designed to suit the appropriate type and power rating of the lamp. The control gear provides a high initial voltage to start the discharge and 'ballast' to limit the current once the discharge is established. Common types of discharge lamp are low and high pressure mercury vapour and low and high pressure sodium vapour; most types cannot be operated from dimmer-controlled supplies. Discharge lighting can sometimes be a source of interference affecting PA systems.

Discrete circuit
An electronic circuit that is constructed using a number of individual components connected together, often by means of a PCB. Compare Integrated circuit.

Dispersion
The 'spreading out' of sound from a source; the extent to which the sound from a speaker effectively covers the area in front of it. As the distance from the source increases, the sound will have spread its energy over a larger area and so the sound pressure level decreases − this happens according to the 'inverse square law.'

A speaker with large dispersion angles (horizontal and/or vertical) will spread its sound over a larger proportion of the total area in front of it, even when the distance between the speaker and the target area is quite small − it is therefore described as a 'short throw' speaker. A speaker with small dispersion angles is able to accurately direct its sound to a target area some distance away, and is therefore described as a 'long throw' speaker. See also Directivity.

Dissipation
The conversion of electrical power into heat (or light) within a circuit component or by an item of equipment. Or, the total amount of electrical power absorbed from the circuit by a component, or drawn from the supply by an item of equipment − minus any power that is usefully converted into forms other than heat or light (e.g. sound or motion) or that is passed on in electrical form.

• In the case of non-transducer components such as resistors and transistors, all of the power absorbed from the circuit by that component is dissipated by the component as heat.
• In the case of transducer components such as speaker drivers, motors, etc., some power will always be dissipated by the component as heat (as no transducer is 100% efficient). The remainder of the power absorbed from the circuit is passed on in the form into which it is converted.
• In the case of electrical equipment that passes on no substantial amount of power in electrical form (such as an unpowered mixer), essentially the whole of the power drawn from the supply is dissipated by the equipment as heat (and/or light).
• In the case of electrical equipment that does pass on a substantial amount of power in electrical form (such as a power amplifier or a transformer), the proportion of the power drawn that is dissipated by the equipment as heat will be determined by the efficiency of the equipment.

In theory, purely reactive components (i.e. capacitors and inductors) dissipate no power. However, as in practice these components possess some inherent series resistance, they do dissipate some power − though usually only very little.

Distance factor
A value describing the extent of directionality provided by a directional microphone. The distance of such a microphone from a sound source must be this many times greater than the distance of an omni-directional microphone in order to provide the same ratio of direct sound to natural reverberation as would be obtained using the omni-directional microphone. Typical distance factors for various polar responses are as follows:

• 1.7 for cardioid
• 1.9 for super-cardioid
• 2.0 for hyper-cardioid
• 1.7 for bidirectional

Distant pickup
Describes the placement of a microphone at a distance of around 2 feet (60 cm) or more from a sound source. Compare Close pickup.

Distortion
A modification of a signal, caused by the equipment handling it, in which new frequencies are added that are related in some way to the original signal but which themselves were originally not present to any significant degree. This is usually undesirable. A distorted sound or signal is sometimes described as dirty, whereas one that is free from distortion may be described as clean.

Distortion may be caused when the amplification or processing circuits within equipment are overloaded by excessive signal levels. Such an overload situation is most usually due to equipment being incorrectly adjusted or wrongly connected, and in extreme cases results in signal clipping. In this type of distortion, harmonics (primarily odd harmonics) of the frequencies originally present are added. At the point of clipping, the level of these odd harmonics dramatically increases. Such distortion may be avoided by ensuring that adequate headroom is maintained throughout the entire signal chain (see Gain structure). An essential element in this is the correct adjustment of pre-amplifier gain controls. It is also necessary to be sure that any condenser microphones used are not subjected to excessive sound levels (some are fitted with pad switches).

There are also other types of unwanted distortion, that are not caused by overload. Two possible sources of this are faulty cable connections (see High resistance connection) and defective equipment (notably worn or dirty signal-carrying switches and potentiometers). Some specific types of distortion are intermodulation and crossover distortion.

Sometimes distortion may be deliberately introduced, for example in order to produce a richer or harsher sound from an instrument such as an electric guitar. To allow the extent and type of distortion to be carefully controlled, this would normally be done using an effect designed for the purpose (see Pedal), but may often be produced simply by increasing the gain of the guitar amplifier so as to deliberately cause signal overload.

It should be noted that any item of electronic equipment will always introduce at least a small amount of distortion, but with good quality equipment, correctly used and adjusted, this is usually too small to be noticeable. It is only when the distortion reaches unacceptable (that is, audible) proportions that the equipment is said to be "distorting" the signal.

Distribution amplifier
An amplifier that provides independent multiple outputs for each input that it has, so as to enable one or more sources to (each) supply its signal to several destinations. An important feature is the isolation between its outputs, which means that if a fault occurs on the feed to one of the destinations then the feed to the other destinations will be unaffected.

Use of a distribution amplifier is essential when multiple destinations are to be supplied with the same impedance-matched signal such as video or digital audio, in order to avoid double termination of the feed. (The only exception to this is where the equipment allows for a daisy-chaining of the signal, as in the case of DMX lighting control and some video equipment.)

A distribution amplifier intended for use with the radio-frequency signals picked-up by an antenna is frequently referred to as an antenna distribution unit, or ADU.

Note that the term 'amplifier' could be considered to be strictly incorrectly applied here, because the equipment often provides no voltage gain. However, the term is justified by the fact that, in total across all its outputs, the equipment is generally able to supply more current than would be available from the source that feeds it.

Distribution board
An item of equipment containing an arrangement of mains power circuit protection devices such as MCBs and RCDs. Commonly abbreviated to 'DB'. See also the next definition.

Distro
A slang abbreviation for 'distribution', usually relating to equipment and cabling for the distribution of mains power. See also the previous definition and IEC, CEE-form, Socapex, Powerlock, BS 7909, Power conditioner and Sparky.

Diversity
In general terms, describes something that is separated, or spread out. In relation to its application to radio microphone receivers, the term generally refers to an arrangement in which the radio signal is picked up on two aerials (or antennae), each connected to a separate set of receiving electronics − the audio output of the unit is obtained from the aerial which is giving the best quality signal at any moment in time, or else is a combination of the results obtained from both aerials. These types of receivers are much less prone to drop-outs than types having a single set of receiving electronics. The aerials are generally best set at between + and − 30 to 45 degrees from the vertical, i.e. spreading apart at between 60 and 90 degrees to each other.

Some single-channel receivers are equipped with two aerials, even though they have only a single set of receiving electronics − these may perform a little better than receivers with only one aerial, but fall far short of the performance obtained from 'true diversity' receivers. See also the 'Wired or Radio' section on the Microphones page.

Divided pickup
A guitar or bass pickup that provides a separate output signal from each string of the instrument. These multiple signals may then be processed separately, or may be used as separate controlling inputs to a synthesiser.

DL
An abbreviation for 'double layer' (or 'dual layer'). A type of recordable DVD that incorporates two layers of information on the same side of the disc, giving nearly twice the information capacity (8.5 GB as compared to 4.7 GB for the single-layer type). See also Blu-ray.

DMM
An abbreviation for 'digital multimeter'; a multimeter that provides its readout of measured quantities via a digital (i.e. numeric) display. Such multimeters are frequently equipped with an 'autoranging' facility, which largely avoids the need to manually select the appropriate value-range of the quantity to be measured (although the type of quantity − e.g. voltage, current or resistance − must still be manually selected). DMMs are sometimes referred to as DVMs (digital voltmeters), because the very earliest types were able to measure only voltage. See also AVO, Megger and Calibration.

DMX (DMX-512)
A standardised digital control system interface for stage lighting and similar equipment, originally developed by USITT and capable of carrying up to 512 channels of data per control cable (termed a DMX 'universe').

DMX equipment is set up, at the time of installation, with a specific address that allows the controller to direct commands to each particular item of equipment. The address is the channel ID of the first channel in the set of channels implemented for each item. The addresses of equipment are typically set up using DIP switches.

DMX uses balanced interconnections, and most usually 5-pin XLR connectors. The cable screen connects to pin 1, 'Data −' to pin 2 and 'Data +' to pin 3. The shell is normally left unconnected. (Pins 4 and 5 have no standard use.) Control signal sources are equipped with female connectors, and destinations with male connectors − note that this is the opposite way round to audio XLR connections.

The electrical interface standard is EIA-485 (previously known as RS-485), and the bit-rate is 250 kbit/s asynchronous. This high rate means that a suitable type of cable must be used to avoid deterioration of the signal quality, especially if long distances are involved − under perfect conditions the maximum theoretical transmission distance is 1 km, but 500 m is a more practical limit. The cable must have a characteristic impedance of 120 ohms and must be daisy-chained from equipment to equipment − not branched along its length. Only the last item of equipment in the chain must have its termination switch in the 'On' position (or have a 120 ohm terminating plug fitted if it has no such switch). DMX is an abbreviation of 'digital multiplex'.

Dolby A
A trademarked name for the original tape noise reduction scheme from Dolby Laboratories, intended for analogue audio recording on reel-to-reel tape. It operates by dividing the audio spectrum into several frequency bands, and applying appropriate compression to each band during recording. To achieve the reduction in noise, the corresponding expansion must be applied during playback (failure to do so will result in incorrect reproduction of the recorded material). See also the next two definitions.

Dolby B
Similar in principle to Dolby A, but simplified for use with consumer cassette recorders. It applies compression to the higher frequencies only. It gives an improvement in signal-to-noise ratio of about 10 dB at high frequencies. See also the next definition.

Dolby C
An improved version of Dolby B. It gives an improvement in signal-to-noise ratio of about 20 dB at high frequencies. Recordings made using Dolby C must be played back using Dolby C.

Domain
A realm (or 'world') of operation, or of significance. For example, "signal processing in the digital domain" refers to the processing of a signal while it is in digital form, i.e. digital signal processing.

Double balanced cable
An alternative name for star quad cable.

Double insulated
See Class II.

Double normalling
See Normalling.

Double termination
An undesirable condition in an interconnection that is meant to be impedance-matched, caused by the connection of an output to two (or possibly more) terminated inputs. The effect is that the overall load impedance is no longer equal to the source impedance and to the characteristic impedance of the cable, causing reflections of the signal back down the cable which may seriously impact upon the quality of the interconnection. Additionally, a double termination will result in the received signal level being too low. Where an impedance-matched source needs to feed several destinations, typical solutions are use of a distribution amplifier or, where applicable, use of a daisy-chained signal routing in which the input terminations are disconnected at all but the last item in the chain.

Down
See Gone down.

Down-stage
Further towards the audience − towards the 'front' of the stage. So-called because of the slight downwards incline ('rake') in this direction on a theatrical stage. Compare Up-stage.

Downfill, Downfills
The additional sound that is required in order to provide fill at the very front of the audience, provided by downwards-angled speakers − typically located beneath a line array. Employing such speakers (referred to as downfills) means that the curvature of the line array can be reduced, enabling it to be more effective in its mid- and long-throw coverage. (This is especially relevant when the line array is flown high.) Note that the downfills are not part of the line array proper, and would usually be driven by a separate amplifier. Compare Front-fill.

Downward expander
See Expander.

Drain
In mains power distribution, describes a connector that is wired (ultimately) to power-using equipment. It may be a fixed connector or may be attached to a cable. The mating connector is described as a source type. See also Distro.

Drain wire
An uninsulated conductor that is sometimes included inside the screen of a screened cable (e.g. a coaxial cable), and in electrical contact with it, in order to ensure a continuous low-resistance screen conductor path throughout the cable length and to facilitate termination of the screen to the cable's connectors. A drain wire is usually provided only in cables that employ a foil screen or a semiconductor screen (not in cables employing a screen that is formed of braided or lapped copper strands).

Dress (1)
An abbreviation for 'dress rehearsal'.

Dress (2)
To arrange cabling (especially when fixed, e.g. to racking) in an orderly fashion.

Drive (1)
[Noun:] A signal, especially in reference to the requirements of an input, or to the capabilities of an output, of an item of equipment. For example, "There's insufficient drive for this effects unit", or "This amplifier provides adequate drive for those speakers". See also Sensitivity. Compare Feed.

Or, [Verb:] To supply a signal to an item of equipment. For example, in a particular signal chain, a mixer may drive an active crossover, which drives the power amplifiers, which in turn drive the speakers.

Drive (2)
A slang term for "operate" an item of equipment, as in "Do you know how to drive this effects unit?"

Driver
The part of a speaker that performs the conversion of electrical power into sound waves. This conversion is usually achieved by means of a voice coil causing the motion of a cone or diaphragm. PA speakers often contain several drivers − typically some combination of woofers, mid-range drivers and horns. See also 2-way, Concentric, Transducer and Ferrofluid.

DRM
An abbreviation for 'digital rights management'. Any scheme intended to prevent the use of an item of software or other digitally recorded information (e.g. music) from being used other than by a legitimate user. See also SCMS SDMI and HDCP.

Drop-out
A momentary or intermittent interruption in normal operation.

In recording, a short disruption in the played-back information, caused by a physical anomaly of some kind on the recording surface. In digital recording, may be compensated for (either completely or partially) by the operation of error correction.

In signal transmission systems (especially radio systems), a loss of acceptable signal for a short duration or under specific circumstances (for example, at a particular location of a radio microphone). See also Diversity.

Drum cage, Drum screen
An acoustic screen, usually made of a transparent plastic such as Plexiglas®, that is placed around a drum kit and drummer to reduce the on-stage sound level and to reduce leakage of drum sound into other instrument microphones and vocal microphones. Preferably these screens are partially lined with sound-absorbant panels − often just at the lower front, the sides and the back. Some types have a 'lid', preferably also lined.

Dry
Describes a sound that is free from effects, especially from reverberation. See also Clean. Compare Wet.

Dry hire
Hiring of individual items of equipment from a hire company, on a 'self assemble and operate' basis, as compared to hire (usually of a complete system) that includes the personnel to assemble and operate it. See also Normalise. Compare Wet hire.

Dry joint
A soldered electrical joint of poor quality, resulting in problems such as an intermittent signal, crackles, distortion, inconsistent operation, etc. Often the problems are not apparent until months, or even years, after the joint was made.

When making your own soldered joints, the two most important factors in avoiding dry joints are to ensure that both parts being joined are sufficiently heated, and that they remain completely stationary whilst the joint is cooling.

DSM
An abbreviation for 'Deputy Stage Manager'. See also SM and ASM.

DSP
An abbreviation for 'digital signal processing' or 'digital signal processor'.

Dual concentric
See Concentric.

Duck tape
A trademarked name for a particular brand of waterproof adhesive tape, intended for general-purpose use. It is sometimes used as gaffer tape, though is not as strong as the gaffer tape usually used in the entertainment industry. Note that only tape that is specifically intended for use as an electrical insulating tape should be used for that purpose. Use of the silver-coloured variety of Duck tape is not recommended for use on cables − see Gaffer tape for further information. Often confused with duct tape, because of the similar-sounding name.

Ducking
The reduction of a signal level for an interval of time, during which another signal has precedence. For example, in a DJ console, the music level may be automatically reduced while the DJ speaks. Another possible application would be in a public address system, where the musac level may be reduced during announcements. Ducking may be achieved by passing through a compressor the signal that is to be controlled, and applying the controlling signal to the compressor's side chain input.

Duct tape
An airtight (and often waterproof) adhesive tape, usually silver-coloured, intended for sealing the joints of air-conditioning and ventilation ducts. However, the name has come to be used to refer to any kind of strong adhesive tape. Often confused with Duck tape, because of the similar-sounding name. Note that only tape that is specifically intended for use as an electrical insulating tape should be used for that purpose. As duct tape is intended for permanent application, when removed it may leave a residue or cause damage to the surface finish. High-quality gaffer tape is recommended for general PA-related applications. Further, use of the silver-coloured variety of duct tape is not recommended for use on cables − see Gaffer tape for further information.

Dummy load
A load connected to an amplifier for test purposes, in order to make test-bench measurements such as maximum power output and distortion levels in a consistent manner and without creating high sound levels. The simplest dummy load consists of a resistor of the appropriate impedance and having a suitably high power rating. However, more sophisticated versions attempt to simulate the frequency-dependence of the load's impedance value.

Duplex
Describes an interconnection or interface (particularly a communications link) that is able to pass information in both directions. Often sub-categorised as follows:

• Half-duplex: Information may be conveyed in both directions, but not simultaneously.
• Full-duplex: Information may be conveyed in both directions simultaneously.

DVD
Supposedly an abbreviation for 'digital versatile disc', but really more of a name in its own right. Nevertheless, a disc which is able to support a variety of digital audio and video recording formats. Its capacity is 4.7 GB (single layer). See also DL and Blu-ray.

DVI
An abbreviation for 'digital video interface'. A standard for digital and analogue interconnections between a computer and display equipment (monitor screens and projectors), providing an improvement in image quality over the earlier analogue-only VGA/SVGA standard. Several variants are available, including DVI-I, DVI-D and DVI-A. See also HDMI.

View DVI connector image

DVM
An abbreviation for 'digital voltmeter' − see DMM.

DVR
An abbreviation for 'digital video recorder'. See also PVR.

Dynamic equalisation (Dynamic EQ)
Equalisation that automatically varies according to the level of the signal being equalised. The most common arrangement is that the centre frequency and Q of the equalisation are set by manual adjustment, and the extent of the cut and/or boost automatically varies within manually-set limits. Such an equaliser can be arranged to provide many different results, including de-essing.

Dynamic microphone
A type of microphone in which sound is converted to an electrical signal by causing the vibration of a conductor in a magnetic field, so inducing a voltage in the conductor. Most dynamic microphones that are used in PA work are of the 'moving coil' type − this means that the conductor is in the form of a coil of wire which is attached to the diaphragm. This coil is called the 'voice coil'. (Another, less common, kind of dynamic microphone is the ribbon microphone.)

As this process generates sufficient signal level for direct connection to a PA system, no amplification of the signal is required within the microphone. This type of microphone is most useful for close pickup applications such as lead vocals, guitar amplifiers, etc. (see the Microphones page for more information).

Dynamic range
When describing a sound or an audio signal, dynamic range refers to the difference in level between the quietest and loudest periods of the programme, usually measured in decibels. See also Peak, Metering and Compression (1).

When describing an item of equipment, its dynamic range is a measure of the extent of variation in level that the equipment is able to satisfactorily handle (without further manual adjustment, once correctly set up). Numerically, it is the difference (in decibels) between the noise floor of the equipment and the level at which unacceptable distortion (e.g. clipping) would occur. So, it is the sum (in decibels) of the signal-to-noise ratio and the headroom. However, in regard to digital equipment see also the comments on this term under Quantisation noise.

Dynamics
A general term for the variations in level that occur in a signal or in a sound. For more information see Dynamic range.

Dynamics processor
A signal processing unit that modifies the dynamic range of a signal, such as a compressor, expander, limiter, noise gate or de-esser. See also Compander and Digital signal processing.

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This page last updated 09-Feb-2012.