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Glossary of PA Terms - D |
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The glossary pages provide definitions for over 1900 PA-related terms. If you can't find the term you are looking for, or would like any of the existing definitions to be expanded, please email me − likewise of course if you find any errors in the links etc. Use of this information is conditional upon acceptance of the Disclaimer on the PAforMusic home page.
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 per octave * DBO * DC * DC protection * DDA * DDL * 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 black * Digital cliff * 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 * Direct box * Direct contact * Direct current * Direct inject * Direct output * Direct radiator * Direct sound * Directional cable * Directional microphone * Directivity * Dirty * Discrete circuit * Dispersion * Dissipation * Distance factor * Distant pickup * Distortion * Distribution amplifier * Distro * Diversity * DL * DMM * DMX * Dolby A * Dolby B * Dolby C * Domain * Double balanced cable * Double insulated * Double normalling * Double termination * Down * Down-stage * Downfill * Downward expander * Drain wire * Drive * Driver * DRM * Drop-out * Drum screen * Dry * Dry hire * Dry joint * DSP * Duck tape * Ducking * Duct tape * Dummy load * Duplex * DVD * DVI * DVM * DVR * Dynamic microphone * Dynamic range * 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.
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 a copy of their inputs: either by a simple parallel connection of the connectors (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 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.
Danger
A significant likelihood of injury or death
arising from a hazard.
N.B. My definition of the term may
differ from officially recognised definitions.
See also Risk.
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
word length
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).
DAW
An abbreviation for 'digital audio workstation', a
computer-based system for multi-track
audio recording and
the subsequent digital
processing of the recorded material.
See also Application
and Plug-in.
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 see the
Decibels page.
dB per octave
See Roll-off.
DBO
An abbreviation for
'dead blackout'.
DC
An abbreviation for 'direct current', a
current
or voltage which has a
substantially steady value, and
does not reverse its polarity.
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 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, but other means such as infra-red or radio are
sometimes appropriate.
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-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).
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. 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 Go 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.
It may be a result of the location or
orientation of the speakers,
or of the acoustics of
the space, or a combination of these factors.
See also
Destructive
interference.
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 see the
Decibels page.
See also 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
'record deck'.
See also
Reel-to-reel.
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.
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
See Capacitor.
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 a location 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 and
Critical distance.
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) jumps between just two states, which respectively represent a '0' and a '1'. The sequence of 0's and 1's forms a sequence of binary numbers, and variations in the value of these numbers corresponds to variations in the sound being represented by the signal, according to some agreed coding scheme.
Properly speaking, digital equipment is equipment which handles 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 which merely incorporates a form of internal or external control which makes use of digital (i.e. numerical) techniques − or even which is simply claimed to be of a quality suitable for use with truly digital equipment.
A digital recording is one in which the information (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 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 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 further signal processing
within the mixer is performed digitally. This arrangement
has a number of important advantages, including:
- 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.
- 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'.)
- 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 associated
on-stage racked equipment), 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 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
digital multiplex is accepted directly by the 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 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
DIN 45596
See Phantom
power.
DIN noise
weighting
See Weighting.
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 the screen connected at one end only).
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.
| Angle | Q | dB |
|---|---|---|
| 40 | 33.2 | 15 |
| 60 | 14.9 | 11.7 |
| 90 | 6.8 | 8.3 |
| 120 | 4 | 6 |
| 150 | 2.7 | 4.3 |
| 180 | 2 | 3 |
| 360 | 1 | 0 |
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 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 mains
power connection) that is contaminated with
noise.
(But may occasionally be used to describe a signal or sound
into which distortion
has been 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 high a
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.
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 another form
(usually heat, light or sound) by a
circuit component or
by an item of equipment.
In the case of transducer
components such as drivers,
motors, lamps, etc.,
some power will always be dissipated as heat (as no transducer
is 100% efficient).
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 as heat.
The power that is dissipated as heat within electrical
equipment must be adequately removed by means of
heatsinks and/or
forced cooling,
otherwise an unacceptable rise in temperature will occur
and damage may result, 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 have some series resistance, they do dissipate some amount of power, though often only a 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,
related in some way to the original signal but themselves
originally not present to any significant degree,
are added to it.
This effect is usually undesirable, being caused by overload of (or within) the equipment. Such overload is generally due to the equipment being incorrectly adjusted, misused, or faulty. In this type of distortion, harmonics (primarily odd harmonics) of the frequencies originally present are added.
Sometimes distortion may be deliberately introduced, particularly in the case of an electric guitar signal, to increase the 'raunchiness' of the sound. To allow the extent and type of distortion to be carefully controlled, this would normally be done using an effect designed for the purpose, or by a specific facility of the guitar amplifier, but may often be achieved simply by increasing the gain of the amplifier.
As regards unwanted distortion, although the term is most frequently used to refer to modification of a signal due to overload of equipment, there are other types of unwanted distortion that are not caused by overload. In particular, see Intermodulation and Crossover distortion.
It should be noted that any item of electronic equipment will always introduce 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
which 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.
Distro
A slang abbreviation for 'distribution', especially of
mains power.
See also IEC,
CEE-form,
Socapex and
Power conditioner.
Diversity
In general terms, describes something that is
separated, or spread out. In relation to its application
to radio microphones,
see the 'Wired or Radio'
section on the Microphones page.
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').
It uses a balanced
connection, 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 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-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
Speakers located beneath
a line array, giving
coverage for the very front of the
auditorium.
Employing such speakers 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 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).
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 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).
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. Some types have a 'lid'.
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.
DSP
An abbreviation for
'digital
signal processing' or 'digital signal processor'.
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 insulating tape
should be used for that purpose. Further,
use of the silver-coloured variety of Duck tape
is not recommended for use on
cables. 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 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 insulating tape
should be used for that purpose.
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.
DVM
An abbreviation for 'digital voltmeter' − see
DMM.
DVR
An abbreviation for
'digital
video recorder'.
See also PVR.
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).
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.Dynamic
range
When describing a sound or an audio
signal,
the difference in level
between the quietest and loudest periods of the
programme,
usually measured in
decibels.
See also Compression.
When describing an item of equipment, a measure of the degree to which the equipment is able to satisfactorily handle a signal of varying level (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
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 10-Nov-2009.