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Glossary of PA Terms - O

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The glossary pages provide definitions for over 2270 PA-related terms. If you can't find the term you are looking for, or would like any of the existing definitions to be expanded, please email me − likewise of course if you find any errors in the links etc. Use of this information is conditional upon acceptance of the Disclaimer on the PAforMusic home page.

O'clock * O/P * Octave * ODI * OEM * OFC * Off-axis * Off-axis colouration * OH, O/H, OHL, OHR * OHLS * 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 * Orange Book * Order * ORTF * OS * Oscillation * 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, 1⁄2 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 (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 (approximately 1.587).

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. 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 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). Compare Beaming.

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

OHLS
See LSF.

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). For calculations involving resistance, see the How do I calculate ...? question on the FAQ page.

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. 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.

Omni-directional
Describes a microphone 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 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 effects' refers to effects facilities provided internally by the mixer. See also Signal processing. 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. 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 short circuit from one of the legs to signal earth, e.g. due to a problem in a cable connector.

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 hardware (memory, disk storage, display, etc.) and provides facilities to support the operation of the applications software. An example would be Microsoft Windows. Abbreviation OS. See also Platform (2).

Operational amplifier
Often abbreviated to 'op-amp', a versatile integrated 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 utilise negative feedback. Its applications are very numerous, but include small-signal amplifier circuits, active filters and equalisers. The required behaviour of the overall circuit is obtained by the connection of other components, particularly resistors and capacitors, to the op-amp. 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, frequency response, slew rate, noise, supply voltage and current requirements and other significant parameters.

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 interconnections.

Orange Book
See CD standards.

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. Or, click when a magnifying glass containing a '+' appears.)

OS
An abbreviation for 'operating system'.

Oscillation
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 repetitive waveform, usually of constant frequency. Oscillation occurs intentionally in an oscillator, but may also occur unintentionally whenever positive feedback is present in combination with sufficient gain. For example, in the case of acoustic feedback or in any other situation of unintentional connection or coupling from a point in an amplified signal chain to an earlier point in that chain. Unsustained oscillation is referred to as ringing or resonance.

Oscillator
A device in which oscillation 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 audio-frequency tone of defined frequency and level 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 repetitive waveform and not of signals having a random waveform, such as pink noise.

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 feedback'. 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 (see Ohm's law).

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). Compare Input impedance.

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 value 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 Digital gain and 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, such as an amplifier or speaker, 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) − signal
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 distribution facilities, the term 'overload' refers to a mains current being drawn that exceeds the rated maximum loading of the supply system at one or more points. To avoid the dangerous overheating, fire, and/or damage to equipment which may result, circuit protection devices such as a fuses and MCBs are employed as necessary. 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 Power supply and Electrical safety on the Safety page. Compare Short circuit.

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 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 oxide's resistance). See also the next definition and Tinned conductor.

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 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 semiconductor material whose non-linearity can introduce distortion. Often abbreviated to 'OFC'. See also SPOFC and Audiophile.

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