| Glossary of PA Terms - O
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Off-axis colouration *
OH, O/H, OHL, OHR *
Ohm's law *
One microphone technique *
One note bass *
Open back *
Open reel *
Operating system *
Operational amplifier *
Opposite prompt *
Opposite prompt side *
Optical interface *
Orange Book *
Out of phase *
Output impedance *
Output stage *
Output-to-output isolation *
Output transistor *
Output valve *
Over-under method *
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.
An abbreviation for 'output'.
(But see also OP.)
An interval of musical
corresponds to a frequency
ratio of 2. So, two
frequencies are said to be an octave apart when one
frequency is twice (or, of course,
1⁄2) the other.
Two octaves would indicate one frequency 4 times the
other, three octaves 8 times, and so on.
used in PA work are usually
octave, 2⁄3 octave
or 1⁄3 octave types; very
occasionally 1⁄2 octave types are also seen.
These designations indicate the ratio between the
of any two adjacent frequency
bands of the equaliser:
a 1⁄2 octave is a ratio of
the square root of 2 (approximately 1.414),
1⁄3 octave is a ratio of
the cube root of 2 (approximately 1.26), and
2⁄3 octave is a ratio of
1⁄3 octave squared −
or alternatively the cube root of 4
Filters, such as those
used in crossovers,
typically have their slopes
specified in terms of decibels
For the musically minded, an octave is so-named because this
interval is reached at the 8th note of a tonic musical
scale. Between the lowest and highest of these 8 notes
are 7 intervals, made up of
5 tones and
Since a tone is a ratio of the sixth root
of 2, and a semitone is a ratio of the twelfth root of 2,
we can see that multiplying out these 7 intervals
(i.e. 21⁄6 x 21⁄6 x
21⁄6 x 21⁄6 x 21⁄6 x
21⁄12 x 21⁄12) gives a product equal
to a ratio of 2, i.e. one octave. See also
An abbreviation for 'optical digital interface'.
See ADAT optical.
An abbreviation for 'original equipment manufacturer'.
Describes a part or component
that is supplied with the
intention of it being incorporated into an item of equipment
by another manufacturer. As such parts are not intended for
direct supply to the end-user, normal requirements for
safety labeling, user instructions, etc. may be
An abbreviation for
Describes a point that is not located along the relevant
axis − usually of a
speaker. See also
the next definition.
occurs when a
picks up sound from a source that it is not directly aimed at,
or when a listener hears sound from a
speaker that is not directly
aimed at him or her.
In general, the most obvious effect of this colouration
is a reduction in treble
See also the previous definition and
Tone (1). Compare
OH (or O/H), OHL, OHR
An abbreviation for
'overhead left' or 'overhead right'.
Ohm (1), Ohms
The unit of electrical
and of impedance; the ratio of
the voltage, measured in volts, divided by the current,
measured in amps).
Its symbol is Ω, the Greek capital letter Omega.
(Note: Due to font conversions, on some
web pages this symbol may be displayed by your browser as a
'W' or a 'V'; it should look like a horseshoe shape with a
flat base.) A thousand ohms is called a kilohm (kΩ),
and a million ohms a megohm (MΩ).
For calculations involving resistance, see the
How do I calculate ...?
question on the FAQ page.
A manufacturer of PA equipment,
notably of speakers and
The rule that states (in simplified form) that the
amount of current that
flows through a resistance
can be calculated by dividing the
voltage across the
resistance (measured in volts) by the value of the
resistance (measured in ohms);
this gives the value of the current in
amps. Or, by rearrangement
of this formula, that the
voltage dropped across
a resistance can be calculated by multiplying the
value of the resistance by the current flowing through it.
Or, similarly, that the value of a resistance can be
calculated by dividing the voltage across it by the
current flowing through it. This rule (or 'law')
was named after its discoverer, Georg Ohm.
For other electrical calculations, see the
How do I calculate ...?
question on the FAQ page.
A slang abbreviation for 'omni-directional' −
see the next definition.
Describes something that behaves identically in
all directions, or, more realistically, that behaves
similarly in all directions. The abbreviated form 'omni'
is sometimes used.
Most commonly, the term is
used in relation to a
whose sensitivity is
essentially unaffected by the direction from which
sounds arrive at it.
That is, one that picks up sounds from all directions
equally well, regardless of which way it is pointed.
Such mics have limited application in
PA systems, because of the
increased likelihood of pick-up of sounds other than
the one(s) intended to be picked-up by each
particular mic (see Leakage), and the
increased likelihood of
feedback. However, such
issues may be of reduced significance if the
intended sound source is at a high
level and the
mic is positioned sufficiently close to it
− e.g. a headset
mic. Omni-directional mics are also useful
in some recording applications.
See the Microphones page
for more detail. Compare
The term is also used to describe an
antenna that has an
essentially identical reception response to radio signals
arriving at it from any direction, or that transmits
essentially equally in all directions. Compare
Describes a point that is located anywhere along the
relevant axis of equipment such
as directional microphones
and speakers − nearly
always it is the axis of maximum
sensitivity that is
being referred to. Such equipment
is generally best used on-axis, in order to obtain optimum
sensitivity and to
colouration. Sensitivity figures are
usually quoted as on-axis values.
See also End-firing and
Describes a facility provided by the item of equipment
in question. For example, in relation to a
mixer, 'onboard effects'
refers to effects
facilities provided internally by the mixer.
See also Signal
Describes an interconnection that is operating as an
− especially one which operates over a
cable that is suitable for
balanced operation, or
when the interconnection was intended to operate in
balanced mode but is failing to do so because of a fault.
Such an interconnection may also be described as
'single-ended'. Typical faults that may cause this
condition are a disconnection of one of the
legs of a
balanced line or a
from one of the legs to
e.g. due to a problem in a cable
In the case of
one-legged operation due to a fault on a fully
balanced interconnection, a reduction in
6 dB will usually
result, as compared to normal operation.
Increased hum (or other
also be evident, as a result of the absence of
rejection. If the fault is a short circuit, or a
partial short circuit, then on some types of interconnection
a reduction in bass may
occur, resulting in a 'tinny'
sound. In the event of a disconnection fault on one leg
of a quasi-floating
(servo balanced) interconnection, the results can be
unpredictable in some cases.
See also Pair.
A PA technique, especially favoured
by some Bluegrass artists for use in very small venues, in
which the musicicans control
the mix by varying their relative
distances from a single
microphone. In practice this
usually means that most of the musicicans are essentially
unamplified for most of the time: only the musicican who is
playing the lead part for a
particular section of the song is close enough to the
microphone to be amplified significantly − though may
be joined for some sections by another musicican for
a 'lead duet'. Usually, in such set-ups, there is no
Also called 'single microphone technique'.
See also Microphone
One note bass
Describes a very deficient bass
reproduction system in which all bass notes appear to be
reproduced at essentially the same
pitch. This effect is most
usually caused primarily by poor quality bass
speakers, but problematic
room resonances and poor
speaker placement can also be contributary factors.
See also Sub-bass.
An abbreviation for
(But see also O/P.)
An abbreviation for
See Live (2).
The condition of no current
flow, usually arising as a result of no
load being connected to
an output of an item of
equipment. It is therefore also known as a 'no load'
condition, and is often quoted in a specification
of the voltage
level that is
to be expected at that output − especially in
interconnections, when the value of the expected load
impedance is usually
not clearly defined. An example would be in the case
of the output level of a
Alternatively referred to as an
Describes a 2-terminal
component that is no longer
able to pass any current
because of a fault. For example, a
driver with a burnt-out
Or, describes a cable or
other interconnection that has developed a disconnection
in one or more of its
that are intended to minimally
exclude the room sound.
These types are generally used only when it is
necessary to be able to hear the room sound at the
same time as the sound from the headphones.
Closed back types
are generally preferred by engineering personnel,
especially when working in high ambient
Computer software which manages the overall operation
of the computer hardware (memory, disk storage, display,
and provides facilities to support the operation of
software. Examples are Microsoft Windows®
Abbreviation OS. See also
Often abbreviated to 'op-amp', a versatile
circuit (IC) electronic
component that provides
a large amount of gain and which
has two inputs of opposite
polarity (usually referred
to as the non-inverting, or '+', and the inverting,
or '−', inputs), so making it very convenient for use
in circuits which
feedback. Its applications are very numerous, but
The required behaviour of the overall circuit is
obtained by the connection of other components,
and capacitors, to the
IC types containing a single op-amp, two
op-amps (dual) and four op-amps (quad) are readily
available. Many different types offer different
performance in terms of available gain,
supply voltage and
and other significant parameters. Types commonly used
in audio applications
include those listed below. 'xx' indicates prefix letters
that vary according to the manufacturer, e.g.
RC, MC, NJM. One or more suffix letters may also be added.
Single: TL071, LF351, NE5534(A)
Dual: TL072, LF353, xx4558, xx4560, xx4580, NE5532,
LM833, MC33078, NJM2068MD
Quad: TL074, LF347, MC33079
Although most types follow a standard convention for
pin allocations, it should be noted that in general a
failed op-amp should be replaced only with one of the
same (correct) type. Types that offer improved performance
will not necessarily operate satisfactorily if the
circuitry has not been designed to accommodate that type.
This is especially true in the case of circuits where
the op-amp is being operated
or distortion circuits) or
outside of its intended or typical operating conditions
(e.g. at a low supply voltage, as in some
battery-powered applications such as
An important distinction is between JFET and bipolar types;
this refers to whether the input
transistors of the device
are junction FETs or
or Opposite prompt side
A term used in theatre to refer to
Commonly abbreviated to OP or OPS, respectively. Compare
An abbreviation for
See the next definition.
An interface which uses
pulses of light, rather than electrical
current, as the means to
convey a digital audio
or digital video
signal. The light is
conveyed by an extremely thin flexible glass
"light pipe" called an optical fibre.
The optical fibre is
such as the TOSLink
connector or the 3.5 mm optical connector.
Warning − laser radiation hazard: never look
into an un-mated
optical fibre connector.
See also AES3-ie.
A device which can be used to provide
in a signal path,
which can be useful in the avoidance of
earth loops. It operates
by means of optical coupling between a light-emitting
diode (LED) and a photo-transistor, these two devices
being enclosed within a single light-proof
Note that, in contrast to a
transformer (which also
provides galvanic isolation), no
power is coupled through
an opto-isolator. Rather, a
DC power source is needed to
supply the photo-transistor and the
circuitry which is usually necessary to restore the
original signal. An opto-isolator is always used at the
receiving end of MIDI
See CD standards.
A term describing the complexity of a
filter. The simplest type
of filter is described as 'first-order', the next as
'second-order', and so on. It is rare to find
analogue filters that
are more complex than fifth-order. One of the main
advantages of high-order filters is a more
rapid transition between the
and the stopband −
An abbreviation for the French national broadcasting system,
'Office de Radiodiffusion − Television Francaise'.
Usually refers to the stereo
microphone recording technique
developed by them, in which two
are positioned with a spacing of 17 cm between the microphone
diaphragms, and with their
axes at an angle
of 110º. This technique attempts to emulate the stereo cues
used by the human ear to perceive directional information
in the horizontal plane, by using a spacing similar to the
distance between the human ears and an angle which simulates
the shadowing effect of the human head.
This technique gives a wider stereo image than
the X-Y technique,
while still preserving good
See also NOS (1),
Mid-side pair and
a pictorial comparison of stereo microphone techniques.
(To view the image full-size in Explorer,
hover your mouse over the image and click on the green
'expand' icon that appears in the bottom right-hand
corner. Or, click when a magnifying glass containing a
An abbreviation for
A general term for repetitive change, usually
of constant frequency,
occurring either continuously or for a sustained
period of time. Or, the production of a continuous
signal having a
usually of constant frequency.
Oscillation occurs intentionally in an
but may also occur unintentionally whenever
feedback is present in combination with sufficient
For example, in the case of
acoustic feedback or
in any other situation of unintentional connection or
from a point in an amplified
signal chain to an
earlier point in that chain. Unsustained oscillation
is referred to as ringing
A device in which
is arranged to intentionally occur in order to
generate a signal
solely by 'artificial' means − usually
electronic. Such a device can be used to generate an
tone of defined
for test and/or alignment purposes, and may be a separate
item of equipment or an
onboard facility of more
complex equipment such as a mixer.
The term should strictly be used only of a generator
of signals having a
and not of signals having a random waveform, such as
An oscillator may be created by
feeding back a
signal from the output
of an amplifier
to its input,
that is in-phase
with the signal that is already there;
this is called 'positive
See also SOL.
An item of test equipment that allows the
waveform of a
signal to be visually
examined on a display screen. An important factor in
the choice of an oscilloscope is its
bandwidth, which in
practice relates to the upper limits of signal
frequency that can be
accurately displayed. Some models are suited only to the
signals, whilst others can easily handle very much higher
frequency signals. The more complex types provide
facilities such as multi-channel operation,
delayed timebases, and on-screen digital readouts.
Early types of oscilloscope used a
CRT as the display device and
were therefore referred to as CROs,
but most modern types utilise LCD
displays instead. See also
An abbreviation for the
Occupational Safety and Health Administration, the
organisation responsible for matters of health and
safety at work in the
USA. This includes issues relating to
See also HSE.
Out of phase
Describes the situation in which two
signals do not have exactly
the same phase,
i.e. there is some phase difference
between them. This means that the
voltages (or currents)
of the two signals do not reach their maximum values at the
same point in time; they do not rise and fall in exact step
with each other.
Note that the term 'out of phase' is a valid description of
two signals with a phase difference of 180 degrees,
i.e. that are in
anti-phase, and in fact
the term is sometimes used with the intention
of referring to that specific situation, rather than to
any other phase difference. More usually, though,
in this case the intention is to refer to refer to
the situation of two signals having opposite
polarity, e.g. as
encountered in a balanced
interconnection − though it is strictly incorrect
to refer to this as a phase difference.
It is also sometimes used to refer to the situation in
which sound from a single
source reaches two different
two microphones) at
different points in time, usually due to different
path lengths. Or, to the situation in
which sound from two or more sources
two speakers) reaches
the same point at different points in time
(usually for the same reason). Note that, in such cases,
the extent to which the sound waves are out of phase with
each other will be different for each
In an AC
circuit, when the
current are out of phase
then the average power
dissipated is not
the RMS voltage multiplied by
the RMS current, because the voltage and current
are not fully 'working in step' with each other;
this situation is sometimes referred to as a
'non-unity power factor'.
See also Phase reversal
and Polarity reversal.
Describes a facility not provided within the item of
equipment in question, but rather provided by an external
item. For example, in relation to a
mixer, 'outboard processing'
refers to signal
processing performed by equipment external to the mixer.
An alternative term for a
socket. The term is
most commonly used to refer to sockets supplying
A connection point, e.g. on an item of equipment,
intended to be used for supplying a
signal to some other destination,
typically to an input connection
of another item of equipment (or, rarely, back to an
input of the same item of equipment). Or, a similar connection
point of a stage, module,
or component internal to an item of equipment.
Outputs of modules
processing systems may be virtual
connection points, physically unidentifiable in the hardware.
Or, the signal
that is obtained from such a connection point.
The term is sometimes used quantitatively of a signal in
reference to its level,
e.g. "How much output is that equipment giving?"
output may be classified in many ways, e.g. as
unbalanced (though several
different types of balanced output exist),
line level or
low impedance or
Often abbreviated to 'O/P'.
See also Connector,
Signal chain and
the next definition. Compare
The effective series
impedance of a
output connection on an item
of equipment. This is a measure of the extent to which
the level of the output signal
will drop as more current
is taken from the output, by the connection of a lower
value of load impedance.
For example, if the output impedance of an output is
1 kilohm, then a current
of 0.1 mA would cause the
to fall by 0.1 volts (see Ohm's law).
For examples of typical output impedance values see
Note, however, that the output impedance figure and maximum
output voltage cannot be
used alone to determine the maximum current that may be
drawn from an output,
because other factors may limit the
maximum available output current to a lower value.
For example, an
may be quoted as having an output impedance of
0.1 ohms and a maximum
output voltage of 40 V − but the amplifier is very
unlikely to be able to supply
Output impedance may also be referred to as 'source impedance'.
impedance and Drive (1).
The circuitry, within
an item of equipment (especially a power amplifier),
which delivers current
to the output
connection(s) of the
equipment. See also
Stage (2). Compare
The degree to which problems externally imposed on an
output of an item
of equipment affect the output
signal provided at other
outputs of that same equipment. Typical problems imposed
on outputs include
and interference originating from sources such as
the equipment to which the output is connected,
earth loops or
(RFI) picked-up by the interconnecting
cable. For example,
when two outputs of the same signal are simply connected in
parallel within the
equipment then no output-to-output isolation whatsoever
is provided. Output-to-output isolation is often of
particular interest in cases where it is important
that a signal sent to one destination is not adversely
affected by externally-imposed problems affecting
a copy of the same signal
being sent to other destination(s).
See also Distribution
A transistor that is
used in the output stage
of an item of equipment. When the output stage is intended
to deliver a substantial amount of
power, such as in the case of a
power amplifier or
backline amplifier, then the
final output transistors will be
The output stages of power amplifiers have at least 2
output transistors − some have as many as 16.
When replacing output transistors in most types of
power amplifiers, it is essential to check and if
necessary adjust the
See also the
section on the
Amplifiers and Speakers
A valve that is
used in the output stage
of an item of equipment. The output stages of
usually have either 2 or 4 output valves, though
a few types have just one. Some common types of output
valves used in guitar amplifiers are EL84, EL34, 6L6, 6CA7,
KT66, KT77 and 5881. Such valves are available in different
variants, and different manufacturers' versions
may have different characteristics (and so provide different
When replacing output valves it is essential to fit the
correct type, and in cases where the amplifier has a
bias adjustment it is highly
recommended to have the amplifier re-biased −
for further information see
The very last section of a song; the section that
concludes it. Compare Intro.
to digital conversion, the situation in which the
of the sampled analogue
to be converted exceeds the 0 dB FS value; i.e. the
maximum value that can be correctly represented in the
domain. Effectively, an over is an occurrence of 'digital
See also Digital gain
An alternative term for 'soft knee' − see
to digital conversion.
A specific method of
cables (otherwise known
as 'wrapping' them) for storage or transport. In this
method, alternate turns of the coil are made with an
opposite twist; this provides a substantially reduced
risk of the cable becoming tangled when the coil is
pulled straight on next usage. Coiled cables may be
conveniently secured using
An effect unit, usually
used with electric guitars, which simulates the sound of an
Usually a pedal.
Or, a facility, incorporated within the
of a guitar amplifier, enabling such an effect to be
produced without the internal
Strictly, describes an item of equipment, such as an
or speaker, that is
being supplied with an excessive
signal to one of its
inputs, causing the equipment
to operate in a manner outside of its normal or intended
behaviour or to be subject to abnormal stresses.
However, in practice the term is also used when internal
part(s) of that equipment
(such as an internal
stage or power amplifier)
are in a state of overload
as a result of the way its own controls are adjusted,
even though its input signal level may be normal.
A typical symptom of equipment being overdriven is
signal distortion or
Overdriving a speaker may result in its being permanently
Although overdriving equipment is in general an
undesirable mode of operation,
an overdriven guitar amplifier can give a particular
type of sound that may be considered desirable −
See also Speaker
Short for 'overhead
which is positioned above the sound source to be
picked up. Most usually encountered in the miking of
drum kits, where such microphones are usually of the
variety and are used to pick up the cymbals and the
general 'ambience' of the kit. Often abbreviated to
'OH' or 'O/H'.
As two such microphones are usually
used, positioned to the left and right of the kit,
they are frequently referred to as 'OHL' and 'OHR'.
Overhead microphones are also often used for
choirs and for orchestras, though in these cases
they are usually suspended on their cables rather
than being stand-mounted.
An undesirable condition within an item of equipment,
which occurs when the equipment is handling a
(or in the case of a
level exceeds the maximum
that the equipment is capable of handling normally.
Overload can potentially occur in any type of equipment
other signal processing circuits such as
It can also occur in
and in transducers such as
drivers and microphones.
Overload generally occurs for one or more of the
- Application of too high a signal level to an
of the equipment.
- Inappropriate settings of the equipment's controls
(e.g. a gain control set
- Connection of an inappropriate
load (usually one of
too low an impedance)
to an output of the
With reference to the first two of the above reasons,
note that common usage of the term (in PA
work) is much more general than what is
suggested by the term itself, i.e. it is not in any
way limited to issues related to equipment loading.
The usual result of overload is
of the signal. This distortion usually takes the form of
the production of harmonics,
which can introduce high-frequency
components to the signal at levels far exceeding its normal
Overload can also result in severe damage
to equipment such as
and speakers, especially
if the overload condition is prolonged. Some possible
serious consequences include:
Overheating of power amplifiers, with possible damage.
Overheating of driver
voice coils, with
Rapid destruction of high-frequency drivers
from the unusually high level of high-frequency
harmonics present in the signal. (Note that such damage
can occur as a result of overload occurring much
earlier in the system, for example in a
of driver cones or
resulting in mechanical damage to the driver.
Damage to passive
mechanisms, resulting in further reduced sound quality
or loss of operation.
Operation of power amplifier
resulting in loss of operation (until reset).
Damage to speaker cables
Overload that occurs within equipment circuitry
may also be referred to as 'clipping', because its
effect on the waveform of
an analogue signal is
typically that the positive and negative extremes of
the waveform are flattened, as if they had been clipped
off. Therefore, equipment having indicator lights
intended to show that an overload condition is
present (or being closely approached) are often
marked with the legend 'Clip'.
When the flattening commences abruptly at a certain point in the
waveform, and no further increase in
voltage is possible
beyond that point, the condition is referred to as
'hard clipping'. When the flattening occurs more
gradually, giving rounded corners to the flattened part(s)
of the waveform, the condition is then referred to as
The hard clipping effect is often due to the inability of
the equipment to handle (or provide) a signal of a larger
voltage than the DC
voltage ('power rails') internal to
the equipment, therefore a hard clipping condition may be
described as the signal (or the equipment) 'hitting the
rails'. In the most extreme case of clipping, the waveform
takes on a shape approaching that of a
In the case of a
signal, overload occurs when the
peak signal level exceeds
0 dB FS; such an occurrence
is referred to as an over.
It may occur during
to digital conversion if the converter is supplied
with an analogue signal of excessive level, or during
digital signal processing (equalisation, mixing, effects,
etc.) if excessive levels are generated.
It results in hard clipping, because (by definition) it is
not possible for levels greater than 0 dB FS
to be represented digitally.
In order to avoid this, the
nominal signal level
is usually kept well below this value.
See also Headroom and
− mains power
When applied to mains
distribution facilities, the term 'overload' refers to a mains
that exceeds the rated
maximum loading of the
cabling making up the
arrangements, at one or more points. To avoid the
dangerous overheating, fire, and/or
damage to equipment which may result,
devices such as a fuses
and MCBs are employed
as necessary to cut off the flow of current in the
event of a serious overload.
The rating of these devices must
be correctly co-ordinated with the rating of the
relevant cables and
connectors, for example
in accordance with BS 7671.
Note that an RCD does
not provide protection against overload.
See also Distro,
safety on the Safety page.
Compare Short circuit.
to digital conversion, the situation in which the
is significantly more than twice the maximum
frequency that is required
to be properly captured from the
signal − usually
at least four times that frequency. This provides the
advantage that the analogue
filter requires a less steep
and/or may have a higher
resulting in an improved
within the frequency range of interest.
When the resulting digital
signal is to be down-converted to a lower sampling
frequency, it must first undergo digital
above half the new sampling frequency, which may require
a steep-slope filter − however such filters having
an acceptable phase response are more readily
implemented in the digital
The chemical reaction of oxygen with another element,
resulting in the formation of an oxide of that element.
Usually refers to the reaction of the oxygen in the air
with the surface of an electrical
of copper cable
conductors and of connector
contacts. This is highly undesirable, as the oxide layer
may cause a high
resistance connection which in turn can cause overheating
(in high-current connections),
intermittent connections and
distortion (due to the
non-linearity of the
See also the next definition and
Copper that has been processed during manufacture to
its oxygen content. Excessive oxygen content in a copper
conductor is claimed
by some to noticeably
degrade the quality of signals
passed through it (especially when the oxygen is present in
the periphery of long cables).
(Due to inductive
effects, high frequency
currents tend to travel
more in the outer regions of a conductor than in its
central core.) The claimed effect of the oxygen has
some scientific rationale, in that copper oxide is a
Often abbreviated to 'OFC'.
See also SPOFC and
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This page last updated 18-Mar-2017.