Glossary of PA Terms - D
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The glossary pages provide definitions for over 2680 PA-related terms and abbreviations. 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.
In the list below, the most commonly looked-up terms are in bold, lighting-specific terms are in pink, and video-specific terms are in orange.
D-sub or D-SUB * D-Type * DA * DAB * DAC * Dais * Daisy-chain * Damping * Damping factor * Danger * Dante * DARS * DAT * Data compression * Data rate * DAW * dB, dBV, dBv, dBu, dBm, dB FS, dB TP, dB SPL, dB (A) and dB (C) * DB * dB per octave * DB-9 * DB-15 * DB-25 * DBO * DC * DC protection * DCA * DCI * DCP * DDA * DDC * DDL * DE-15 * De-emphasis * De-esser * De-mate * De-rating * De-regulated frequency * De-rig * Dead * Dead blackout * Dead spot * Death capacitor * Decay * Decibel * Decibels * Deck * Decoder * Delay * Delay speaker * Delays * Demodulation * Demodulator * Demultiplexer * Demultiplexing * Depth * DEQ * Desk * Destructive interference * Destructive solo * Detent * DI * DI box * DI output * Diaphragm * Dielectric * Differential amplifier * Differential drive * Differential input * Diffraction * Diffuse field * Digital * Digital amplifier * Digital audio * Digital audio workstation * Digital black * Digital cliff * Digital gain * Digital mixer * Digital multicore * Digital signal processing * Digital snake * Digital stagebox * Digital to analogue convertor * Digital video * Dim * Dim out * Dimmer buzz * Dimmer fizz * Dimmer noise * Dimmer pack * Dimmer rack * Dimming * DIN * DIN 45405 * DIN 45412 * DIN 45573 * DIN 45596 * DIN 651 * DIN noise weighting * Diode * 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 * DisplayPort * Dissipation * Distance factor * Distant pickup * Distortion * Distributed public address * Distribution amplifier * Distribution board * Distro * Diversity * Divided pickup * Djembe * DL * DLNA * Dly * DMM * DMS-59 * DMX * Dog box * Dolby A * Dolby B * Dolby C * Domain * Dome antenna * Dosemeter * Dosimeter * Double balanced cable * Double bass * Double blind test * Double insulated * Double micing * Double miking * Double normalling * Double termination * Down * Down-stage * Downfill * Downfills * Downward expander * DPP * Drain * Drain wire * Draw * Dress * Drive * Driver * DRM * Drop-out * Drum cage * Drum screen * Dry * Dry hire * Dry joint * DSD * DSM * DSP * DSUB or Dsub * DTRS * DTS * DTT * Dual concentric * Duck tape * Ducking * Duct tape * Dummy load * Duplex * Duty cycle * DV * DVD * DVI * DVM * DVR * Dynamic equaliser * 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.
Alternative generic names for any of a number of different types of connector having a 'D-shaped' shell surrounding either 2, 3 or (rarely) 4 parallel rows of pins (in the male version). 'sub' is short for subminiature, as at the time of its introduction this was considered to be a compact form of connector. In the UK, 'D-Type' was initially the most common term used for it, but 'D-sub' is now more common.
These connectors are now most commonly encountered in a 2-row 25-pole form as an 8-channel balanced audio connector (usually wired to the DTRS standard), and in a 3-row ('high-density') 15-pole form as a 'VGA' connector. Other variants, incorporating differently-shaped signal earth pins, are used for DVI interconnections.
D-sub connectors are formally identified by a code of the form Dx-n, where x is a letter indicating the size of the shell and n is a number (of one or two digits) indicating the number of poles (not including the shell, i.e. the number of pins or holes). This may be followed by the letter M or F to indicate the gender. For example, DE-15M is a male VGA connector (though these are sometimes incorrectly referred to as DB-15M) and DB-25F is a female DTRS connector.
Various sizes were once in common use for low-speed serial data interconnections such as RS-232 (until the advent of higher-speed serial interconnections such as USB and IEEE 1394). Initially the DB-25 was most commonly used for this purpose, but for most applications was then largely superseded by the DE-9. See also VGA and DTRS.
An abbreviation for 'distribution amplifier'.
An abbreviation for 'digital audio broadcasting', referring to the broadcast of public digital radio services. In the UK these broadcasts are in the frequency range 217.5 to 230 MHz, but some other countries use other frequencies.
An abbreviation for 'digital to analogue convertor'.
A raised platform area or small stage, from which lectures, presentations, etc. are delivered. See also Lectern.
A method of interconnecting several items of equipment such that an output of one item feeds the input of a second item, the output of that one feeds a third item, and so on in succession. Or, where several, usually identical, items of equipment (generally their inputs) are each connected at various points along the length of a common set of cable conductors by means of a parallel cable junction at the location of each item, but outside of it. In the first-mentioned scenario, 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 between the connectors inside the equipment (as in the case of several daisy-chained speakers fed from a single amplifier output), 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, where many daisy-chained lanterns may be separately addressed through a single control cable.
- Where the chained equipment 'outputs' provide a processed version of their inputs, as in the case of daisy-chained serial effects units. This interconnection arrangement is commonly used with guitar pedals.
See also Distribution amplifier.
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).
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.
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.
A trademarked name for a protocol for passing real-time multi-channel digital audio over an IP network, developed by Audinate (external link, opens in a new window). The audio interconnections provided by the network may be managed by a computer connected to it. Compare AES50 and AES67.
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.
See Compression (2).
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 App, Plug-in, Accelerator, Platform (2) and SAC.
dB, dBV, dBv, dBu, dBm, dB FS, dB TP, dB SPL, dB (A) and dB (C)
An abbreviation for 'distribution board'.
An abbreviation for 'double bass'.
dB per octave
DB-9, DB-15, DB-25
An abbreviation for 'dead blackout'.
An abbreviation for 'direct current', a current or voltage that 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). In general, DC is obtained from AC by means of a rectifier. Compare AC.
See Speaker protection.
An abbreviation for 'digitally controlled amplifier' − see VCA.
An abbreviation for 'Digital Cinema Package', a file standard for digital video projection systems, orginally developed by Digital Cinema Initiatives to use with their systems. Several files are required to make up the package for a particular film, providing information for the video and audio content and for the associated data needed by the projection system.
An abbreviation for 'Disability Discrimination Act', previous UK legislation requiring the provision of reasonable adaptations for disabled persons, where services are being provided to the general public. An example of such an adaptation is the provision of assistive listening facilities where the understanding of speech (or song) is fundamental to the service(s) being provided. In October 2010 the DDA was repealed in England, Scotland and Wales, as its provisions are now covered in similar manner by the Equality Act. It still applies, however, in Northern Ireland. Assistive listening 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 setting). However, other assistive means are sometimes appropriate, such as systems using infra-red or radio, speech-to-text display services, or sign-language interpretation facilities. See also BS 8300.
An abbreviation for 'display data channel', a communication path between a computer system and display equipment, defined by VESA and used to carry information such as screen resolution. This enables the computer system to provide an appropriate format of video information to the display. In the case of a DE-15 connector (also known as a VGA or HD15 connector), the DDC is carried on pins 12 (data) and 15 (clock) in a format similar to the I2C (inter-IC) communication protocol, with pin 10 as signal earth. The actual information passed over the DDC is known as EDID.
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.
The formal designation for a 15-pin high-density D-sub connector, commonly referred to as a 'VGA connector' or an HD15 connector. 'DE-15' may be followed with 'M' or 'F', denoting male or female types.
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.
The process of applying a lower rating than the usual value to an item of equipment or to a component. This may, for example, be necessary as a result of the item being operated under adverse environmental conditions such as an elevated ambient temperature. If in doubt as to whether or not any de-rating is necessary in a particular case, consult the manufacturer.
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.
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.
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. Compare Shadow.
In vintage Class II (i.e. unearthed) mains-powered equipment, a internal capacitor connected between the incoming mains supply and the metallic chassis of the equipment, usually for interference-suppression purposes. This name was applied because, at the time that such equipment was produced, a type of capacitor was employed that could fail short-circuit, making the equipment chassis live at mains voltage with potentially fatal consequences. This scenario was particularly evident in the case of vintage guitar amplifiers, as a guitarist using such an amplifier with a failed capacitor could, on contact with earthed items such as a microphone, receive a fatal or severe electric shock. Therefore, it is strongly recommended that all vintage equipment be internally checked and if necessary suitably modified to eliminate this deadly hazard. Note that regular PAT tests are insufficient in this case, as the capacitor may fail between tests (or even during use).
Modern equipment instead utilises safety capacitors in this position; these are specifically designed to avoid the hazard posed by the vintage capacitors.
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.
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.
A slang term for the floor, often referring to the stage floor.
An effects unit that provides delay-based effects such as echo, reverberation, chorus and flanging. Or, a term for such effects themselves. Such effects are most commonly provided either by comprehensive outboard delay effects units or by the multi-purpose onboard effects facilities of mixers. Almost all delay units now function digitally, and are sometimes known by the abbreviation 'DDL' (for 'digital delay line'). See also Pedal and ADT.
A dedicated unit, or a feature of speaker management equipment, whose purpose is to provide a delayed version of a signal for feeding to "secondary" speakers that are situated a considerable distance in front of the main front-of-house speakers, so that the sound heard from the secondary speakers is closely synchronised with the sound heard from the main speakers.
The signal delay provided must be adjusted to match the additional delay undergone by the sound from the main speakers in travelling through the air to listeners situated beyond the location of the secondary speakers, as compared to the delay undergone by the sound reaching those listeners from the secondary speakers. As the exact value of that additional delay will vary for different listening positions, some compromise is necessary and usually the unit is adjusted so as to provide the best result for the majority of listeners. It is usually considered that best results are obtained by aiming to ensure that, for most listeners situated beyond the secondary speakers, the sound from the main speakers arrives very slightly before that from the secondary speakers, as this helps in the correct psychoacoustic location of the original sound source (see Haas effect). Approximately 3 milliseconds of delay is required per metre of distance between the main and secondary speakers (see Speed of sound). Use of a delay unit would not normally be considered for distances between main and secondary speakers of less than about 10 metres. See also the next definition, Propagation time, Time alignment and Haas effect.
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'. See also Time alignment.
See the previous definition.
An abbreviation for 'dynamic equaliser'.
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.)
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, Polarity reversal and Comb filter. Compare Constructive interference.
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.
An abbreviation for 'direct inject'. See 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.
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 (which is only rarely a necessity).
DI boxes are most commonly single channel units, but two and four channel types are also available.
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.
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.
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. See also Small diaphragm and Large diaphragm.
Usually refers to the insulating material between the two plates of a capacitor. Common dielectric materials for non-polarised capacitors are ceramic, polyester (PET), polypropylene (PP), and (to a lesser degree) polyphenylene sulfide (PPS). Polarised electrolytic types utilise an electro-chemically created dielectric of aluminium oxide. For further details see Capacitor.
The term is also used to refer to the internal insulation between the inner and outer conductors of a coaxial cable or connector − particularly one of defined characteristic impedance, generally used in impedance-matched interconnections.
An amplifier circuit or stage that amplifies the difference between the signals present on its two input connections (or 'legs'), neither of which are permanently connected to signal earth. The input connections are generally referred to as '+' and '−' or as 'Hot' and 'Cold' respectively. The term most usually refers to the circuit that provides the balanced operation of an electronically balanced balanced input. See also Operational amplifier.
The type of signal provided by a balanced signal output that drives both legs. Or, a term used to describe the nature of such an output, by reference to the type of signal it provides. Note, however, that not all such outputs behave in the same way under various conditions of loading or faulty interconnections. For further information see Balanced.
An alternative term for a balanced signal input. Such inputs are either electronically balanced or transformer balanced. Electronically balanced inputs operate using a differential amplifier. For further information see Balanced.
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.
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.
Describes anything which functions by means of numbers, particularly when it is desired to contrast it with alternative methods of funtioning that operate without numerical tecniques (see Analogue).
Properly speaking, digital equipment is equipment that processes, stores or conveys information that is represented numerically. 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 (audio, video, etc.) is stored as a sequence of numbers. The storage media may be a computer memory or hard drive, a removable disc (CD, Mini disc or DVD) or a legacy tape system such as DAT.
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 appropriate only when re-timing is provided).
In reference to audio power amplifiers, which by definition provide an analogue output suitable for driving passive speakers, no such equipment as a 'digital amplifier' exists according to the strictest definition of 'digital'. Although the audio level represented by a digital signal may be increased by numerical means, within the limits of the coding scheme (see Digital gain), such an increased-level digital signal cannot directly drive a passive speaker. In order to drive a speaker, a power amplifier's primary function is to produce an analogue signal of sufficiently high voltage and sufficiently high available current, and since this cannot be achieved through numerical processing techniques alone, it is at best misleading to refer to any power amplifier as one that provides 'digital amplification'.
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 an internal PWM or PDM switching signal which drives the output stage power transistors. The high-level switched signal so produced is then subjected to analogue filtering within the amplifier, in order to produce an analogue signal suitable for driving speakers. 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.
Describes an audio signal or recording in which the audio information is represented in digital form (i.e. numerically) rather than in analogue form. Or, describes equipment that processes, records or plays back audio using digital processing or recording techniques.
In a digital audio signal, 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 contains a sequence of binary numbers, and the variations in the value of those numbers indicates the variations in the regular sample values of instantaneous voltage that make up the sound waveform being represented by the digital signal (according to some agreed coding scheme). In the simplest coding scheme, sometimes referred to as linear PCM, each number proportionally represents the voltage value of a single sample. The level of digital signals is usually expressed in dB FS.
In a digital audio recording, the audio information is stored as a sequence of binary numbers, typically including additional bits to provide for error detection and recovery and for auxillary data such as track identity and copyright information. The storage media may be a computer memory or hard drive, a removable disc (CD, Mini disc or DVD) or a legacy tape system such as DAT.
See also Digital mixer, AES3, SPDIF, SDIF, Analogue to digital conversion, DSP, Full scale, Over, Latency, Compression (2), Dolby, DTS, 5.1, 7.1, MP3, AAC, Pulse code modulation, Streaming, Float (1), FIR, IIR and FFT.
Digital audio workstation
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.
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.
A mixer that functions using digital techniques. Usually the term refers to a mixer in which all mixing, routing 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. For example, it may be provided by an app on a mobile device that is wirelessly linked to the processing equipment.
- As the signal does not pass through the control potentiometers and routing selectors, noise levels are reduced and potentiometer wear and contamination are less of an issue.
Although some pre-amplification facilities are often provided with the mixer, most types provide for integration with digital multicore ('digital snake') systems in which each channel's signal is pre-amplified and converted into digital form by an equipment rack located at the stage. See also Fat channel, Motorised fader and Analogue to digital conversion. Compare Analogue mixer.
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, and usually also in the reverse direction (for returns), taking the place of a conventional multicore. The 'send' signals are converted from analogue to digital at a digital stagebox (often in racked equipment at the stage end of the link), which then multiplexes them for sending to the mixer location via a high-speed data link, typically using a UTP or fibre-optic cable and a protocol such as AES50. Digital multicores are nearly always used in conjunction with a digital mixer, in which case the mixer end of the cable is usually connected directly to the mixer. An alternative arrangement is to use an analogue multicore from the stage to a digital monitor mixer, and then a digital multicore to link the monitor mixer to the FOH mixer, in which case a conventional analogue stagebox is used on stage. (In theory, the digital multiplex signal from a digital stagebox could be demultiplexed at the mixer location and converted back to individual analogue channels for connection to an analogue FOH mixer, but this would be a very unusual arrangement.)
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 mic pre-amps to enable the direct connection of wired mics to the stagebox. The latter arrangement is preferable (and more common), as facilities can then be provided for controlling the gain setting of each pre-amp remotely from the mixer. In this case input pads may be manually controlled, or be automatically switched-in at low gain settings, to allow the connection of line-level sources to the same inputs. See also Analogue to digital conversion and CAT 5.
Digital signal processing
A stagebox that employs a digital interconnection to the associated mixer, which is usually a digital mixer. Digital stageboxes are active units, incorporating microphone pre-amplifiers and analogue to digital convertors for the inputs and digital to analogue convertors and line output amplifiers for the returns from the mixer. The interconnection with the mixer is commonly an Ethernet connection utilising the AES50 standard.
Digital to analogue convertor
Describes a video signal or recording in which the video information is represented in digital form (i.e. numerically) rather than in analogue form. Or, describes equipment that processes, records or plays back video using digital processing or recording techniques. Examples of digital video interfaces are DVI, HDMI and SDI.
Dimmer buzz, Dimmer fizz, Dimmer noise
Alternative terms for the interfering noise, originating from lighting dimmers, that is sometimes picked up by audio equipment and heard as a background buzzing or 'fizzing' sound. This should not happen to any significant degree if good quality audio and dimming equipment and balanced audio interconnections are used, and proper earthing practice is followed.
Dimmer pack, Dimmer rack
An item of equipment, or a rack of several such items, whose purpose is to provide a dimming function, e.g. for incandescent stage lighting, typically under DMX control. Dimmer packs usually provide independent dimming for a group of dimmed output circuits (often either 3 or 4), each dimmed output being under the control of a different DMX channel. They usually operate using phase-angle control.
An alternative term for ducking.
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 superseded 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):
A shell-less 2-pole version (with a central flat pin [− pole] and an offset round pin [+ pole]) was similarly used for domestic equipment speaker connections. The standard-sized 180º 5-pole variety is now used for MIDI interconnections. See also IEC and CCIR.
A now defunct standard for the measurement of noise in radio receivers and similar equipment, but is sometimes still quoted to refer to A-weighting. The current standard used for specifying A-weighting is IEC 61672. See also Weighting.
See IEC noise.
See Phantom power.
An early standard for sound level meters, but sometimes still quoted to refer to A-weighting. DIN 651 is alternatively designated 'IEC 651'. The current standard for sound level meters and the specification of A-weighting is IEC 61672-1. See also Weighting.
DIN noise weighting
An electronic component that conducts current very well in one direction but very poorly in the other. Diodes have a multitude of uses in electronic circuits, but the most common application is within rectifier circuits. Diodes are usually semiconductor devices, but they are also available in the form of valves. Special types of diode include zener diodes and LEDs. 'Normal' diodes are usually rated in terms of their maximum continuous forward current and their maximum reverse voltage (under specified operating conditions).
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).
An American term for a 'direct inject' box − see DI box.
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.
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 − on some mixers this is panel-switchable while others provide that choice by means of an internal switch or jumper. 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.
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.
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.
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).
- For microphones having a single direction of maximum sensitivity see Uni-directional.
- For microphones having two directions of maximum sensitivity see Bidirectional microphone.
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, Dispersion, 60x40, Exponential horn, Beamwidth and Line array.
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.
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.
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 incandescent 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. Compare Incandescent lamp and LED.
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.'
Different types of speaker, especially those equipped with HF horns, are designed to provide different angles of dispersion, sometimes referred to as 'beamwidth'. A speaker with large dispersion angles (horizontally and/or vertically) 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 more accurately direct its sound to a target area some distance away, and is therefore described as a 'long throw' speaker. The nominal dispersion angles of a speaker are usually specified in the format 'horizontal x vertical' − for more information see 60x40. See also Directivity and Line array.
A digital video and audio interface standardised by VESA. It is used on some computers and on some compact mobile devices such as smartphones and tablets. It is available in two sizes, 'standard' and 'mini'. Adaptor cables are available to enable interconnection with HDMI compatible display equipment and other standards. Compare Lightning connector.
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. into 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, lamps, 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.
The power that is dissipated as heat within electrical components or equipment must be adequately removed, e.g. by means of natural air circulation, heatsinks and/or forced cooling, in order to avoid an unacceptable rise in temperature which may result in damage and/or the operation of a thermal cut-out.
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.
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:
For example, this means that a super-cardioid microphone, when pointed at a sound source and placed 1.9 times further away from it than an omni-directional microphone, will pick up about the same ratio of direct to reverberant sound as the omni-directional microphone. See also Critical distance (1) and Radius of reverberation.
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 (see also Breakup).
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.
The overall amount of distortion that is present in a signal, or that is introduced into a signal by a specific item of equipment, is usually indicated by a THD value.
Distributed public address
See Public address.
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 (see Output-to-output isolation).
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.)
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.
A slang abbreviation for 'distribution', usually relating to equipment and cabling for the distribution of mains power. See also the previous definition and MDU, IEC, CEE-form, Socapex, Powerlock, Snaplock, Camlock, BS 7909, Power conditioner and Sparky.
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.
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.
A medium-sized percussion drum of African origin, typically having a skin of either 10" (25 cm) or 12" (30 cm) diameter and a body that narrows from the skin downwards towards a half-way point, and then widens again.
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.
An abbreviation for Digital Living Network Alliance®, an organisation that sets standards for the interoperability of networked consumer audio-visual equipment. Their website is www.dlna.org (opens in a new window).
An abbreviation for 'delay'.
A space-saving digital video interface standard, allowing two display monitors (or projectors) to be connected (e.g. to a computer) to equipment through a single connector. Typically, it is used in conjunction with an adaptor cable that provides two separate connections to the two monitors − these connections may be VGA, DVI, HDMI, DisplayPort, etc.. The connector is a D-Type having 4 rows of 15 pin positions, but with one pin position blanked (so a total of 59 pins are used). 'DMS' is an abbreviation for 'dual monitor solution'. A hyphen should be used before the '59', but is sometimes omitted.
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.
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 is an abbreviation of 'digital multiplex'.
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 are designated for a second balanced data circuit, but are rarely used. The DMX standard does not permit them to be used to carry power. Some DMX-compatible equipment is equipped with 3-pin XLRs of the same type as used for audio interconnections, even though the DMX standard does not permit this. In this case, pins 1, 2 and 3 serve the same functions as on the 5-pin connector (i.e. pin 3 is '+', the opposite of audio usage) − though some equipment has pins 2 and 3 reversed.
Regardless of whether 5-pin or 3-pin connectors are used, 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. However, some devices connected to DMX links may communicate bi-directionally − see RDM.
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).
A slang term for a loom box.
Dolby® 5.1, Dolby® 7.1
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.
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.
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.
See Helical antenna.
A device that records the total exposure to a particular quantity received by a person over a given period, usually one working day. In PA work the term usually refers to an audio dosimeter, sometimes referred to as a 'noise dosimeter' or a 'personal sound exposure meter' (PSEM). These are devices that are worn in order to record the wearer's total daily exposure to sound, so as to assess the potential for damage to hearing. See also NIHL, Leq, Time-weighted average, SEL, Audiology, Tinnitus, SNR (2), SNHL, and Acoustic Safety on the Safety page.
Double balanced cable
An alternative name for star quad cable.
A musical instrument; the lowest-pitched member of the violin family, most often encountered in the strings section of a classical orchestra but also used in other settings such as jazz bands and swing bands. See also Bass (2).
Double blind test
A test of equipment, usually a comparison test such as an A-B test, in which both the person evaluating the equipment and the person operating or supervising the test are kept unaware of the identity of each item of equipment during the test. Compare Blind test.
See Class II.
Double micing, Double mic'ing, Double miking
- To capture different tonal qualities of a musical instrument or a backline speaker. In this case, the two mics will typically be pointed at different parts of the instrument or speaker. In addition, different types of microphones may be used, and/or be placed at different distances. The outputs of the mics will then be mixed (somtimes with a phase reversal), as required.
- Mics will sometimes be placed on both sides of a lectern but only one actually used; this is to help reduce plosive effects by discouraging the talker from speaking directly on-axis into a mic.
- To provide redundancy, in case of failure of one of the mics (most commonly used in the case of important broadcasts and for speeches by VIPs). In this case only one of the mics will usually be made active at a time, in order to avoid comb filtering effects.
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.
See Gone down.
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.
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.
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.
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).
As a verb, to cause a flow of current or power in a circuit that connects a source of electrical energy to a load, usually with reference to the typical or maximum current or power requirements of the specified load. In this context, the source referred to is most likely to be the mains supply, a power unit, a battery, or a signal output of an item of equipment.
An abbreviation for 'dress rehearsal'.
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.
Drive (2) [Verb]
To supply a signal from one item of equipment to another. 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.
A slang term for "operate" an item of equipment, as in "Do you know how to drive this effects unit?"
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 (or tweeters). See also 2-way, Concentric, Transducer and Ferrofluid.
In computers and computer-controlled equipment, a small item of software that enables the operating system (or, in the absence of an operating system, the main part of the software) to communicate with a particular hardware device, either internal to the computer equipment or connected to it. Strictly, 'driver' is short for 'device driver', though the full term is rarely used.
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.
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.
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.
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.
An abbreviation for 'direct stream digital', a term describing a particular format for audio storage or interconnection in which audio data is represented in a pulse-density modulated (PDM) form. (More academically speaking, it is a delta-sigma modulation scheme.) This is the format in which audio is represented on an SACD disk.
It can be considered to occupy a position half-way between an analogue and a digital representation of the original audio. It has similarities with an analogue signal in that the pulse density is directly representative of the audio signal − simply low-pass filtering a DSD signal will produce the analogue waveform, without any 'decoding' being involved. It has similarities with a digital signal in that it is a representation of constant-rate samples and that the instantaneous signal voltage has only two states. However, it should be noted that, unlike PCM or other such codes (but rather like PWM), DSD is not a true digital format as the sample values are not represented numerically.
DSD signal interconnections are not normally encountered in PA work, being limited to SACD conversion, mastering and duplication equipment, where a physical interface in the form of SDIF is usually employed.
An abbreviation for 'digital signal processing' or 'digital signal processor'.
An abbreviation for 'Digital Tape Recording System', originally referring to a range of Tascam 8-track tape-based digital audio recorders (now obsolete). However, the term is still in use to refer to the recording format used by those machines and also to refer to the connection scheme used for the 25-way D-sub (DB-25) connector on those machines, carrying 8 balanced analogue audio channels, which became an informal standard for 8-channel analogue audio interconnections between multi-channel items of equipment such as pre-amplifiers, analogue to digital convertors and recording systems. This standard is now formalised by AES59. For details of the internal wiring for DTRS DB-25 connectors (and also for a 4-channel bi-directional digital interconnect) see this PDF file on the Tascam website: DB-25_Pinout (external link, opens in a new window). See also SAC (2).
A trademarked name for a series of digital audio encoding systems, most frequently used in conjunction with video programme data (such as on DVDs or within an HDMI signal) as an alternative to Dolby® encoding. Like Dolby, it supports 5.1 and 7.1 systems, though the 5.1 system is the one most commonly referred to. It was developed by Digital Theater Systems Inc. of the USA (also known as 'Dedicated to Sound'), now renamed DTS Inc. See also Codec.
An abbreviation for 'digital terrestrial television', referring to digital TV transmissions that are broadcast from land-based transmitters at UHF frequencies. Such transmissions are of relevance to PA systems primarily as regards possible conflicts between the frequencies they use and those that are used by radio microphones and in-ear monitoring (IEM) systems. For further information see 'Wired or Radio' on the Microphones page. See also LTE.
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.
The intentional temporary reduction of a signal level for a period during which another signal has precedence. Alternatively referred to as 'dimming'. For example, in a DJ console, the music level may be automatically reduced while the DJ speaks. Or, in a studio the normal monitoring feed may be reduced in level while a producer or recording engineer speaks to the performers. 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.
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.
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.
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.
An interconnection or interface that is able to pass information in one direction only may be described as simplex, but this term is rarely used. See also I/O.
The proportion of time, usually expressed as a percentage, that an item of equipment or a component is actively in operation. For example, a 25% duty cycle refers to the item being actively operational for a quarter of the time. Duty cycle limitations are imposed on some types of equipment (such as some motors, generators and transformers) − usually to avoid an unacceptably high operating temperature being reached as a result of the power being handled or delivered by the equipment. Therefore, maximum duty cycle figures are frequently dependent upon power levels and ambient temperature.
An abbreviation for 'digital video'.
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.
An abbreviation for 'digital video interface'. An inconnection 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. Three basic variants are available, with three different arrangements of pins on the connector: DVI-D (digital only), DVI-A (analogue only) and DVI-I (integrated digital and analogue). The digital capability may be provided with a 'dual-link' facility, supported by a central group of pins which are missing in the 'single-link' variant. The analogue capability is supported by 4 pins surrounding the flat earth pin, which are missing in the DVI-D variant (pin 8 is also used for the analogue capability). DVI-D and DVI-A are not compatible with each other, but display devices equipped with either of these input variants may be connected to a DVI-I output. Likewise, a DVI-I input may be connected to DVI-D or DVI-A sources. There is also a 'mini DVI' connector, but it is rarely encountered. See also HDMI.
An abbreviation for 'digital voltmeter' − see DMM.
Dynamic equaliser (Dynamic EQ)
An item of equipment, or a facility of one, that provides equalisation which 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 varies automatically within manually-set limits. Such an equaliser can be arranged to provide many different results, including de-essing. A common application is in the boosting of bass (and sometimes treble) frequencies at low levels, in order to compensate for the reduced sensitivity of the ear at these frequencies (see Loudness (2)). Frequently abbreviated to 'DEQ'.
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).
The name is a shortened form of 'electrodynamic microphone', which indicates that sound causes a movement of electrons through the microphone capsule (when connected to a load). Compare Condenser microphone.
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 LRA, Peak, Metering and Compression (1).
When describing an item of equipment, in general terms its dynamic range is a measure of the extent of variation in level that the equipment is able to handle (without further manual adjustment, once correctly set up), while maintaining an acceptable signal-to-noise ratio and an acceptable degree of distortion. Numerically, it is the difference (in decibels) between the noise floor of the equipment and the signal level at which unacceptable distortion (e.g. clipping) would occur. (Note that this means that specified equipment dynamic range figures are meaningless unless details of the noise measurement method, such as any weighting applied, are given, along with the relevant distortion figure.) Put another way (using figures in decibels), dynamic range is the sum of the signal-to-noise ratio and the headroom. In regard to digital equipment, see also the comments on this term under Quantisation noise.
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This page last updated 29-Jan-2021.