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  Glossary of PA Terms - U

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The glossary pages provide definitions for over 2680 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.

In the list below, the most commonly looked-up terms are in bold, lighting-specific terms are in pink, and video-specific terms are in orange.

U * Ábar * UDF * ÁF * UHD * UHF * UI * Ultrasonic * Unbalanced * Understudy * Unfinalise * Uni-directional * Uniform directivity * Uninterruptible power supply * Unison * Unity gain * Unity power factor * Universe * Unloaded * Unplugged * Unpowered mixer * Unpowered speaker * Unregulated frequency * Unregulated power supply * Unscreened cable * Unshielded cable * Unterminated * Unweighted * Up-stage * ÁPa * Upper mid-range * Upright piano * UPS * Upscaling * Upward masking * Upward scaling * US terminology * USB * USITT * Utility output * UTP * UXGA

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.

U (1)
An abbreviation for 'units', meaning a unit of vertical measurement on a 19 inch rack system. One U is equal to 1.75 inches (approximately 44.5 mm). Note, however, that the mounting holes in the rack system are at a smaller spacing than this. These units may be referred to as 'rack units', but this term should not be abbreviated to 'RU' as the correct abbreviated form for these units is 'U'.

U (2)
An abbreviation for 'unity gain', sometimes used as a panel marking to indicate such a setting of a control (e.g. on some Mackie mixers).

U (3)
A symbol for voltage, sometimes used in formulae in place of 'V'. Note that 'U' should never be used as an abbreviation for 'volts', the unit of voltage.

Ábar
A microbar, a unit of pressure equal to one millionth of a bar, or to one million pbar. The Ábar is a unit sometimes used for quoting sound pressure levels (SPL), as the bar is very much too large for that purpose. One Ábar is equivalent to 0.1 Pa. See also Pascal and dB SPL on the Decibels page.

UDF
An abbreviation for 'universal disk format', a format specification for data file storage on optical media, usually compact disk.

ÁF
An abbreviation for 'microfarad', one millionth of a farad. A common unit for the measurement of capacitance. See also nF and pF.

UHD
An abbreviation for 'ultra high definition'. Refers to video displays or projection that provides images of very high resolution, such as 4K and 8K. Compare HDTV. For information on the most common video resolutions see VGA.

UHF
An abbreviation for 'ultra high frequency'. Refers to radio frequencies in the range 300 MHz to 1000 MHz, used by many types of radio microphones, radio instrument systems and in-ear monitoring systems. For more details, see the radio mic information on the Microphones page. Compare VHF.

UI
An abbreviation for 'uuser interface' − the complete set of controls, indicators and displays on an item of equipment.

Ultrasonic
Describes a sound whose frequency is above the generally accepted audio-frequency range but is below radio frequencies. That is, sounds in the frequency range 20 kHz to around 100 kHz. Compare Infrasonic.

Unbalanced
Describes a signal interconnection arrangement having only one un-earthed conductor. The 'return' current path is via a signal earth conductor, often the screen of the interconnecting cable. The screen therefore forms the 'cold' conductor of the circuit.

Unbalanced interconnections can be vulnerable to a number of problems, particularly when the interconnection is used to carry low-level signals. Such problems include:

For these reasons, this type of interconnection is suitable only for use with moderate lengths of cable (up to around 5-30 metres, depending upon the interconnection's impedance and signal level). For low-level signals (such as in microphone interconnections) or for long lengths of cable, balanced interconnections are much preferred.

An unbalanced interconnection is sometimes referred to as 'one-legged' or 'single-ended', particularly when operated over a cable that is suitable for balanced interconnections. See also Coaxial cable, Line and Ground-compensated. (A table comparing the most common types of balanced interconnections and balanced-to-unbalanced interconnections is provided under the 'Balanced' entry.)

Understudy
A person who learns the part of a principal performer, so as to perform that part in the event of the principal performer being unwell or otherwise unavailable. Or, any person in training for a particular role or duty, such as Sound Engineer.

Unfinalise
The process of erasing the table of contents (TOC) on a re-writable audio compact disc (CD-RW), in order to allow additional tracks to be recorded. This operation cannot be performed on a recordable compact disc (CD-R). Compare Finalise.

Uni-directional
Describes a microphone that is more sensitive to sound at the front than at the sides or the back. The most common types of uni-directional microphone are subcardioid, cardioid, super-cardioid, hyper-cardioid and rifle. See the Microphones page for more detail. Compare Bidirectional and Omni-directional.

Uniform directivity
See Constant directivity.

Uninterruptible power supply
An item of equipment that is able to provide a continued supply of operating power, up to a given level of load, to items of mains-powered equipment during failure of the mains supply of a given maximum duration. Usually abbreviated to 'UPS'. Under normal conditions, the protected load equipment takes its mains power via the UPS equipment. Most types of UPS operate using internal rechargeable batteries, which must be in good condition in order for the UPS to function as specified. Frequently a UPS will incorporate a power conditioning function.

Except in public address systems that are used for safety-critical announcements, a UPS would not usually be employed to provide uninterruptible power to a complete PA system. However, it may be advantageous to employ one in conjunction with certain types of associated equipment, such as show control computers. Some types of UPS are intended to provide power only for sufficient time to allow computer systems to be shut down gracefully; these types often provide a data connection to the computer, so that shut down may be performed automatically within the required time.

Unison
The situation in which two or more instruments are playing notes of the same pitch, or in which two or more voices are singing at the same pitch. The term is generally only used when this applies to a reasonably lengthy sequence of consecutive notes. Compare Harmony.

Unity gain
The situation in which, between two specified points in a system, there is overall neither any gain (amplification) nor loss (attenuation). That is, the gain between these two points is 0 dB − a multiplying factor of one (a value otherwise known as 'unity'). On some mixers (e.g. Mackie), the position of a control which corresponds to a unity gain condition is marked 'U'.

Unity power factor
Describes the situation in which, in an AC circuit, the current and the voltage are in phase with each other. In this situation the power factor has a value of one (a value otherwise known as 'unity'). The term is generally used only in reference to the supply and utilisation of mains power.

Universe
See DMX.

Unloaded (1)
Describes a signal output or an electrical supply source that has no load connected to it. See also Open-circuit (1) and Unterminated (2). Compare Loaded (1).

Unloaded (2)
Describes a speaker enclosure or cab that has no drivers fitted. Compare Loaded (2).

Unplugged
Apart from the obvious meaning, describes a musical performance or recording in which the sound mix is relatively uncluttered; in particular being largely devoid of added effects and in which heavy drums, sustained or overdriven lead guitar and electronic keyboard fills are usually absent. The term is most often applied in the context of a solo artist (often with an acoustic guitar) or vocal group, and derives from the intention of producing an overall sound that appears similar to what might be expected of the artist(s) concerned if giving an entirely unamplified live performance (of vocals and acoustic instruments) in a suitably small venue.

Unpowered mixer
A mixer that does not incorporate power amplification facilities in the same unit. Compare Powered mixer and Mixer-amplifier.

Unpowered speaker
See Passive speaker.

Unregulated frequency
See De-regulated frequency.

Unregulated power supply
A power supply that is designed to provide output(s) whose voltage(s) may vary significantly, even under normal operating conditions. Compare Regulated power supply.

Unscreened cable
A cable that does not incorporate a screen. For example, most speaker cables are unscreened. Also called 'unshielded cable'. See also UTP. Compare Screened cable.

Unshielded cable
See Unscreened cable.

Unterminated (1)
Describes a cable, or a circuit (or line) within a cable, that has no physical termination, i.e. that is neither wired to a connector nor directly to any equipment.

Unterminated (2)
Describes an output or a line that has no terminating impedance at its final destination. In the case of an impedance-matched interconnection, this is an unsatisfactory situation. In the case of a voltage-matched interconnection, however, this is a standard condition for specificiation of a signal output level (commonly quoted in dBu or in mV) − also known as an open-circuit condition. Compare Unloaded.

Unweighted
Describes a level measurement to which no weighting has been applied.

Up-stage
Further away from the audience − towards the 'back' of the stage. So-called because of the slight upwards incline ('rake') in this direction on a theatrical stage. (This is the origin of the term 'to up-stage someone'.) Compare Down-stage.

ÁPa
A micropascal, a unit of pressure equal to one millionth of a Pascal (Pa). The ÁPa is a unit sometimes used for quoting sound pressure levels (SPL), as the Pascal is rather large for that purpose. One ÁPa is equivalent to 10 pbar. An SPL of 20 ÁPa is equivalent to 0 dB SPL. For further details see Pascal. See also dB SPL on the Decibels page.

Upper mid-range
See Higher mid-range.

Upright piano
An acoustic piano that has its strings and sound board fixed vertically. Compare Grand piano.

UPS
An abbreviation for 'uninterruptible power supply'.

Upscaling
The process of converting the format of a signal (or of a set of similar or related signals) to a format that is capable of carrying higher quality versions of that type of signal. This does not increase the quality of the signal being converted, but merely makes it compatible with other signals in the higher quality format. Such compatibility may be required for example, to allow several signals in that format to be handled or processed together. (It should be noted that the higher quality format may in some cases be less resilient than the original one, e.g. to being carried over long distances, or to the presence of a particular type of interference.) Upscaling is usually performed by a scaler. It is sometimes referred to by the fuller term 'upward scaling'.

Upward masking
A specific form of the masking phenomenon, in which a sound masks another sound which is at a significantly higher frequency.

Upward scaling
See Upscaling.

US terminology
See American terminology.

USB
An abbreviation for 'universal serial bus' a standardised digital interface for the interconnection of computers and peripherals. The original '1.0' specification is now superceded by the '2.0' version. USB connections are made using cables made for the purpose, equipped with either 'Type A' or 'Type B' connectors. See also IEEE 1394 and IEEE 802.11.

View USB A image  View USB B image

USITT
An abbreviation for 'United States Institute of Theatre Technology'. See also ABTT and ALD.

Utility output
A 'general purpose' line-level output of a mixer, provided to supply auxiliary items of equipment such as an induction loop amplifier. Utility outputs are often stereo, but may be mono.

UTP
An abbreviation for 'unshielded twisted pair', a type of cable containing one or more twisted pairs and no integral screen (also called a shield). It is mostly used to interconnect computers (and some other computer-related equipment) in computer networks using an Ethernet protocol. It typically contains four twisted pairs and is terminated with RJ45 connectors.

This type of cable is now commonly used between 'senders' and 'receivers' that convey audio and/or video programme signals over moderate distances. For example, it is commonly used in VGA extender systems. Another common application is in digital multicore systems.

It is important to use the correct 'category' of UTP cable, to suit the speed of the computer network or whatever other equipment it is used with. For example CAT 3 cable is suitable for use at a bit-rate of only up to 10 Mbit/s (e.g. 10Base-T Ethernet), whereas CAT 5 supports up to 100 Mbit/s (e.g. 100Base-T Ethernet). A popular category is CAT 5e; this is an enhanced version of CAT 5 having a nominal bandwidth of 350 MHz and is capable of carrying 100Base-T over longer distances or 1000Base-T over short distances. See also CCA. Compare STP.

UXGA
An abbreviation for 'ultra extended graphics adaptor'. A standard interface for the connection of display equipment (such as monitors and projectors) to computers. The standard gives a resolution of 1600 pixels horizontally and 1200 pixels vertically. The aspect ratio is 4:3. See also VGA, SVGA, XGA, SXGA, WXGA and DVI. The most commonly encountered standards, their resolutions and aspect ratios are tabled under the entry for VGA.

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This page last updated 26-Sep-2017.