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Glossary of PA Terms - O

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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.

O'clock * O/P * Octave * ODI * OEM * OFC * Off-axis * Off-axis colouration * OH, O/H, OHL, OHR * Ohm * Ohm's law * Omni-directional * On-axis * Onboard * One-legged * One microphone technique * One note bass * Op-amp * Open * Open-circuit * Open back * Open reel * Operating system * Operational amplifier * Optical-fibre * Optical interface * Opto-isolator * Order * ORTF * OS * Oscillator * Oscilloscope * Out of phase * Outboard * Output * Output impedance * Output stage * Over * Over-easy * Over-sampling * Overdrive * Overdriven * Overhead * Overload * Oversampling * Overtone * Oxidation * Oxygen-free copper

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.

O'clock
See Hours.

O/P
An abbreviation for 'output'.

Octave
An interval of musical pitch that 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.

Graphic equalisers used in PA work are usually octave, 2/3 octave or 1/3 octave types. These designations indicate the ratio between the centre frequencies of any two adjacent frequency bands of the equaliser: a 1/2 octave is a ratio of the square root of 2, and a 1/3 octave is a ratio of the cube root of 2. (A 2/3 octave can't easily be expressed as a root − it's a ratio of 2 raised to the power of 2/3.)

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 2 semi-tones. Since a tone is a ratio of the sixth root of 2, and a semi-tone is a ratio of the twelfth root of 2, we can see that multiplying out these 7 intervals (i.e. 2^1/6 x 2^1/6 x 2^1/6 x 2^1/6 x 2^1/6 x 2^1/12 x 2^1/12) gives a product equal to a ratio of 2, i.e. one octave. See also Third, Fifth and Cent.

ODI
An abbreviation for 'optical digital interface'. See ADAT Optical.

OEM
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 relaxed.

OFC
An abbreviation for 'oxygen-free copper'.

Off-axis
Describes a point that is not located along the relevant axis − usually of a microphone or speaker. See also the next definition. Compare On-axis.

Off-axis colouration
Colouration that occurs when a uni-directional microphone 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 frequencies. See also the previous definition and Tone (1).

OH (or O/H), OHL, OHR
An abbreviation for 'overhead', 'overhead left' or 'overhead right'.

Ohm
The unit of electrical resistance, of reactance and of impedance; the ratio of voltage to current (i.e. 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).

Ohm's law
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. This rule (or 'law') was named after its discoverer, Georg Ohm.

Omni-directional
Describes a microphone whose sensitivity is essentially the same to sounds arriving at it from any direction. That is, it picks up sound equally well from all directions. See the Microphones page for more detail. Compare Uni-directional and Bidirectional.

On-axis
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 minimise off-axis colouration. Sensitivity figures are usually quoted as on-axis values. See also End-firing and Side-addressed. Compare Off-axis.

Onboard
Describes a facility provided by the item of equipment in question. For example, in relation to a mixer, 'onboard processing' refers to signal processing facilities internal to the mixer. Compare Outboard.

One-legged
Describes an interconnection that is operating as an unbalanced interconnection − 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 (e.g. a disconnection of one of the legs). Such an interconnection may also be described as 'single-ended'. In the case of one-legged operation due to a fault on a fully electronically balanced interconnection, a reduction in level of 6 dB will usually result, as compared to normal operation, and increased hum or other noise may also be evident (as a result of the absence of common mode 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. See also Pair.

One microphone technique
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 sound engineer. Also called 'single microphone technique'. See also Microphone technique.

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.

Op-amp
An abbreviation for 'operational amplifier'.

Open (microphones)
See Live (2).

Open-circuit (1)
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 voltage-matched 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 microphone. Alternatively referred to as an unloaded or unterminated condition. Compare Termination (2).

Open-circuit (2)
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 voice coil. Or, describes a cable or other interconnection that has developed a disconnection in one or more of its conductors.

Open back
Describes headphones 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 sound pressure level environments.

Open reel
See Reel-to-reel.

Operating system
Computer software which manages the overall operation of the computer (memory, disk storage, display, etc.) and provides facilities to support the operation of the applications software. An example would be Microsoft Windows. Abbreviation OS.

Operational amplifier
An integrated circuit 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 utilise negative feedback. Often abbreviated to 'op-amp'.

Optical-fibre
See the next definition.

Optical interface
An interface which uses pulses of light, rather than electrical current, as the means to convey a digital audio or video signal. The light is conveyed by an extremely thin flexible glass "light pipe" called an optical fibre. The optical fibre is terminated in optical connectors 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.

Opto-isolator
A device which can be used to provide galvanic isolation 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 component. 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 amplification circuitry which is usually necessary to restore the original signal. An opto-isolator is always used at the receiving end of MIDI connections.

Order
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 passband and the stopband − see Slope.

ORTF
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 cardioid microphones 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 mono information. See also NOS, A-B pair, Mid-side pair and Microphone technique.

View 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.)

OS
An abbreviation for 'operating system'.

Oscillator
A device which generates a signal of which there is no original 'natural' source; the signal is generated solely by electronic means. Such a device is usually used to generate a tone for test and/or alignment purposes, and may be a separate item of equipment or an onboard facility of complex equipment such as a mixer. The term should strictly be used only of a generator of signals having a repetitive waveform and not of signals having a random waveform, such as pink noise.

An oscillator may be produced 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 feedback'. This situation may occur unintentionally (for example in the case of acoustic feedback, or other unintentional connection or coupling from a point in an amplified signal chain to an earlier point in that chain), resulting in unwanted oscillation. See also SOL.

Oscilloscope
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 display of audio-frequency signals, whilst others can easily handle much higher frequency signals. The more complex types provide facilities such as multi-channel operation, digital storage, delayed timebases, and on-screen digital readouts. See also Calibration.

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 instantaneous 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.

In an AC circuit, when the voltage and 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 '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. Compare In-phase and Anti-phase.

Outboard
Describes a facility not provided by the item of equipment in question. For example, in relation to a mixer, 'outboard processing' refers to signal processing performed by equipment external to the mixer. Compare Onboard.

Output
A connection point, on an item of equipment, intended to supply a signal from that equipment to an input connection of some other item of equipment (or, rarely, back to an input of the same item of equipment). Or, the signal that is supplied from such a connection point. Electrically, an audio output may be balanced or unbalanced (though several different types of balanced output exist). Often abbreviated to 'O/P'. See also Connector, I/O, Drive (1), Level, Signal chain and the next definition.

Output impedance
The effective series impedance of a signal 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 voltage to fall by 0.1 volts.

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 amplifier 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 400 A.

Also referred to as 'source impedance'. See also Characteristic impedance and Drive (1).

Output stage
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 Input stage.

Over
In analogue to digital conversion, the situation in which the instantaneous magnitude of the sampled analogue signal to be converted exceeds the 0 dB FS value; i.e. the maximum value that can be correctly represented in the digital domain. Effectively, an over is an occurrence of 'digital clipping'. See also PPM.

Over-easy
An alternative term for 'soft knee' − see Knee.

Over-sampling
See Analogue to digital conversion.

Overdrive
An effect unit, usually used with electric guitars, which simulates the sound of an overdriven amplifier. Usually a pedal. Or, a facility, incorporated within the pre-amplifier section of a guitar amplifier, enabling such an effect to be produced without the internal power amplifier clipping.

Overdriven
Describes an item of equipment (usually an amplifier) that is in a state of overload. Strictly the term ought to be used only when an excessive signal level is applied to an input of the equipment, but in practice it is also used when some other part(s) of the item (such as an internal power amplifier) is overloaded as a result of the way its own controls are adjusted.

Overhead
Short for 'overhead microphone' − a microphone 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 condenser 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.

Overload (1) − audio
An undesirable condition of an item of equipment, which occurs when the equipment is handling a signal (or in the case of a microphone, a sound) whose level exceeds the maximum that the equipment is capable of handling normally. Overload generally occurs for one or more of the following reasons:

• Application of too high a signal level to an input of the equipment.
• Inappropriate settings of the equipment's controls (e.g. a gain control set too high).
• Connection of an inappropriate load (usually one of too low an impedance) to an output of the equipment.

The usual result of overload is distortion 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 high-frequency content.

Overload can also result in severe damage to equipment such as power amplifiers 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 probable damage.
• Rapid destruction of high-frequency drivers (horns), resulting 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 pre-amplifier.)
• Excessive excursion of driver cones or diaphragms, resulting in mechanical damage to the driver.
• Damage to passive crossovers.
• Operation of speaker protection mechanisms, resulting in further reduced sound quality or loss of operation.
• Operation of power amplifier thermal cut-outs, resulting in loss of operation (until reset).
• Damage to speaker cables and connectors.

Overload may also be referred to as 'clipping', because its effect on the waveform of an analogue signal is 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 point in the waveform, and no further increase in level is possible beyond that point, the condition is referred to as 'hard clipping'. When the flattening effect is more gradual or 'rounded', the condition is referred to as 'soft clipping'. 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 power supply 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 square wave.

In the case of a digital signal, overload occurs when the peak signal level exceeds 0 dB FS; such an occurrence is referred to as an over. In order to avoid this, the nominal signal level is usually kept well below this value. See also Headroom and Power compression.

Overload (2) − mains power
When applied to mains power facilities, the term 'overload' refers to a mains current being drawn that exceeds the rated capacity of the supply at some point within the supply system. This may result in dangerous overheating, fire, and/or damage to equipment, especially if any part of the supply system is not appropriately protected by a fuse or MCB. Note that an RCD does not protect against supply overload. See also Power supply and Electrical safety on the Safety page.

Oversampling
In analogue to digital conversion, the situation in which the sampling frequency is significantly more than twice the maximum frequency that is required to be properly captured from the analogue signal − usually at least four times that frequency. This provides the advantage that the analogue anti-aliasing filter requires a less steep slope and/or may have a higher cut-off frequency, resulting in an improved phase response 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 filtering to attenuate frequencies 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 domain.

Overtone
See Harmonic.

Oxidation or Oxidisation
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 conductor, especially of copper cable conductors and of connector contacts. This is highly undesirable, as the oxide layer has a high resistance and can cause overheating (in high-current connections), intermittent connections and distortion (due to the non-linearity of the oxide's resistance).

Oxygen-free copper
Copper that has been processed during manufacture to significantly reduce its oxygen content. Excessive oxygen content in a copper conductor is claimed by some audiophiles 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 a scientific basis in that copper oxide is a semiconductor material. Often abbreviated to 'OFC'.

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This page last updated 10-Nov-2009.