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Glossary of PA Terms - C

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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.

C-ducer * C-form * C-weighting * Cab * CABA * Cable * Cable gauge * Cable tie * Cabled microphone * Cabling harness * Cabling loom * Caged nut * Calculations * Calibration * Cam * Camlock * Can * Cannon * Cans * Capacitance * Capacitive coupling * Capacitor * Capacitor microphone * Capsule * Cardioid * Carrier * Cartridge * Cascading * Cassie * CAT 5 * CAT 5e * CATV * CAV * CCA * CCIR * CD * CD standards * CEE-form * CENELEC * Cent * Centre frequency * Centre tap * Channel * Channel 38 * Channel 69 * Channel 70 * Channel strip * Chapter * Characteristic impedance * Charge * Chase * Chassis * Chebyshev * Checksum * Cherrypicker * Chip * Choke * Chord * Chorus * Chroma * Chrominance * Cinch * Circuit * Circuit breaker * Circuit diagram * Circuit protection * Circuit protective conductor * Circumaural * Clarity * Class I * Class II * Classes of amplifiers * Classes of filters * Clean * Clean earth * Clean feed * Clearcom * Click * Click track * Cliff effect * Clip microphone * Clipping * Clock * Close pickup * Closed back * Closed box * Cluster * CLV * CMRR * CNMAT * Co-channel interference * Coax * Coaxial * Coaxial cable * Code * Codec * Coding scheme * Coincident-microphone technique * Cold * Cold standby * Color bars * Color code * Coloration * Colour bars * Colour code * Colouration * Column speaker * Comb filter * Combo * Common impedance coupling * Common mode choke * Common mode interference * Common mode rejection ratio * Comms * Compander * Competent person * Component video * Composite video * Compression * Compression driver * Compression ratio * Compressor * Concentric * Concert pitch * Condenser microphone * Condition report * Condom * Conductor * Conduit * Cone * Conga * Connector * Console * Console tape * Constant directivity * Constant Q * Constant voltage * Constructive interference * Consumer * Contact microphone * Control surface * Convection-cooled * Cord * Core * Corner frequency * Coulomb * Coupler * Coupling * CPC * CPU * Crank * Crash * CRC * Creative leakage * Crest factor * Crew * Crisp * Critical band * Critical distance * Critical frequency * CRO * Cross-fader * Cross-fill * Crossover * Crossover distortion * Crossover frequency * Crosstalk * CRT * Crystal-controlled * CSA * CTS * Cue * Current * Current dumping * Curve * Cut * Cut-off frequency * CVS * Cyc * Cycle * Cyclic redundancy check * Cyclorama

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.

C-ducer
A registered brand name for a capacitive tape contact microphone, ideally suited to piano and other acoustic string instruments. So-named by an abbreviation of 'capacitive transducer'. For further details see www.c-ducer.com.

C-form
See CEE-form.

C-weighting
See Weighting.

Cab
Short for 'cabinet'. An enclosure, especially a speaker enclosure, however the term is most often used to refer to the entire speaker (complete with drivers etc.), rather than just the enclosure itself. See also Front-of-house, Backline, Monitor, Stack and Box.

Cable
A flexible means of carrying signals or power, for the purpose of interconnecting or powering items of equipment. In common usage, the term is often used to refer to an item which is equipped with a connector at one or both ends, in which cases the type of connector may be used to provide a limited description of the cable (as in 'an XLR cable', 'a jack cable', etc). Strictly, however, the term should be applied to the flexible component only, without its connectors.

It usually refers to a means of electrical interconnection, in which case it generally contains several conductors that are insulated from one another. However, very heavy duty power interconnections may utilise several separate cables that each contain only a single heavy-gauge conductor. The insulated conductors within a cable are usually referred to as the cores of the cable. The term 'cable' may also refer to a means of optical interconnection, in which case it usually contains only a single optical fibre. A cable may be referred to as a 'lead' or as a 'cord' (USA term), especially if not lengthy, but it should not be referred to as a wire.

It is very important to use the appropriate type of cable for each application. In particular, for safety reasons the maximum voltage and current limitations of the cable must be observed. Further, different types of cable are constructed differently and have different values of resistance, inductance, capacitance and characteristic impedance. The amount of current that can be safely carried by a cable is determined by the gauge of the cable conductors and by other factors such as the type of insulation it employs and the ambient temperature when in use. See also Balanced, Unbalanced, Coaxial cable, Star quad cable, Multicore, Installation cable, Triaxial cable, Figure-of-8 cable, AES3-IE, MIDI, DMX, UTP, STP, SCART, AWG, Pair, Screen, Sheath, Cable tie, Oxygen-free copper, Voltage drop, CCA, LSF and TOSLink.

Cable gauge
See Gauge.

Cable tie
A narrow plastic strip which is placed around a bundle of cables to keep them together. They are available in many different lengths and widths. To secure the tie around the cable, the narrow end of the strip is fed through the eye at the other end, where a ratchet device engages with 'one-way' serrations along the length of the strip, allowing the tie to be tightened to the required degree as the free end is pulled. They may also be used to secure cables to fixed objects, but not overhead or where any appreciable weight of cable is involved.

There are two types: non-releaseable ties are used in permanent and semi-permanent installations (such as internal rack wiring), and releasable ties are used to keep coils of cable neat and tidy during storage and transit. The non-releaseable types are commonly referred to as 'zip ties'.

This use of releasable cable ties (or alternatively Velcro ties) is far superior to the 'industry standard' method of wrapping a few turns of PVC tape around the coil, for many reasons including:

• The ties may be re-used time and time again.
• The avoidance of a consumable item is more environmentally friendly.
• It does not rely on several rolls of PVC tape (one for each person coiling) being available at the times & locations of need.
• When the coil is unwound and connected, its tie (of appropriate length) may be conveniently stored around one end of the cable ready for use when it is time to coil it up again.
• The cost of keeping tape supplies topped-up is avoided.
• The problem of disposing of the sticky pieces of tape removed from coils is avoided.
• Persons uncoiling cables do not need to carry knives to cut tape.
• There is no risk of damaging the cable when removing the ties.
• Pieces of improperly removed tape, and/or sticky patches of adhesive residue, do not get left on the cable.
• Like PVC tape, releasable ties are available in several colours for identification purposes (if required).

Cabled microphone
See Wired microphone.

Cabling harness, Cabling loom
See Loom.

Calculations
For general audio and electrical calculations see this entry on the FAQ page. For decibel calculations see the Decibels page.

Calibration (1)
The process of ensuring that indicating devices (such as level meters, multimeters and oscilloscopes) give readings of the required accuracy, and/or that controls have an effect that is suitably consistent with their markings. It is usual for professional test equipment to be calibrated at regular intervals (e.g. annually) to ensure that it gives correct readings. See also Tolerance.

Calibration (2)
The particular intended nominal relationship between the readings given by indicating devices such as level meters and the values that such readings relate to. For example, "A 0 dB reading on that level meter corresponds to a +4 dBu output level." Or, the particular intended nominal relationship between the markings of control settings and the values that such markings relate to. For example, "A setting of 10 on that control gives 30 dB gain." Or, a general term for the scale markings themselves, of indicators or controls. For example, "This level meter is calibrated from −50 to +20 dB" or "This control is calibrated from 0 to 10."

Cam
A slang term for a Camlock connector − see the next definition.

Camlock
A range of single-pole mains power connectors intended for applications where a higher current rating is required than that provided by CEE-form connectors. Rated at up to 400 amps. WARNING: Electric shock risk! These connectors have accessible conductors and are intended to be used only by qualified persons. They must not be handled, mated or de-mated whilst the circuit is live. In common with other types of single-pole power connectors, all poles of the circuit must be mated before the circuit is energised: the poles should mated in the sequence Earth first, then Neutral, finally the Phases − and be de-mated in the reverse sequence. The labelling and colour-coding of the connectors must conform to the up-to-date standards of the country concerned. Compare Powerlock and Snaplock.

Camlock (400A) image

Can
A slang term for a simple lantern, especially a PAR can. See also Cans.

Cannon
(Frequently mis-spelled 'Canon'.) See XLR.

Cans
A slang term for headphones. Note that many professional mixers require headphones that have an impedance of a much higher value than those intended for domestic 'HiFi' use. Typically headphones of at least 200 ohms may be required, whilst domestic types are generally less than 50 ohms. Or, the plural of 'can'.

Capacitance
The property of a pair of conductors in close proximity, which causes a changing electric charge on one conductor to affect the charge on the other one. This property is measured in farads. It enables the storage of electric charge, and also enables the flow of an alternating current to occur between the conductors, even though there is no physical contact between them (and therefore no possibility of any flow of direct current). More importantly, the higher the frequency of the current, the greater the influence between the conductors. Therefore, this property can be used to construct filters, selectively passing some frequencies and blocking others.

The capacitance between conductors may be increased by increasing their area or by reducing the distance between them, and when such a component is deliberately formed it is called a capacitor.

The capacitance which exists, undesirably, between the conductors of a cable (or other wiring) may be a cause of high-frequency attenuation in the cable, especially in high impedance interconnections. It may also be responsible for crosstalk resulting from capacitive coupling between the conductors of different circuits. See also Reactance. Compare Inductance.

Capacitive coupling
The phenomenon whereby a signal that is present on one conductor impresses itself to some degree on a nearby conductor because of the capacitance which exists between the two conductors. Such coupling is usually undesirable, and is normally avoided by use of screened conductors and balanced interconnections. Also known as 'electrostatic coupling'. See also Crosstalk. Compare Inductive coupling and Common impedance coupling.

Capacitor
An electrical component whose purpose is to introduce capacitance into a circuit, typically either with the intention of passing higher frequency AC currents more readily than lower frequency ones, or of blocking the passage of DC current.

It consists of two layers of conducting material (often a metallic foil), separated by a very thin insulating layer called the dielectric; these layers are often wound into a tight spiral to save space. In PA work, capacitors are an essential component of passive crossovers. The capacitance value (in picofarads) of some types of capacitor is indicated by a colour code.

When replacing capacitors there are other considerations besides ensuring that the replacement has the correct nominal value of capacitance and tolerance. It must also have a voltage rating at least as large as the one to be replaced, and be of the correct type. Polarity, temperature rating and/or temperature coefficient may also be important.

There are many different types of capacitors, each designed to suit different applications. The types differ mostly in their dielectric material, common materials for non-polarised capacitors being polystyrene, polycarbonate, polyester, polypropylene, and ceramic. Where large values of capacitance are required in a small space, polarised electrolytic types are often used; these utilise an electro-chemically created dielectric of aluminium oxide. Polarised electrolytic types must always be connected with the correct polarity as well as having an adequate voltage rating. Failure to observe these requirements may cause the capacitor to explode. In passive crossovers the absence of a DC voltage makes polarised electrolytic types unsuitable, so special non-polarised electrolytic types (also called bi-polar electrolytics) are often used instead. See also Inductor and Resistor.

Capacitor microphone
Another name for a condenser microphone.

Capsule
An internal part of a microphone. The capsule is the specific part that converts sound energy into an electrical signal. (Other parts typically include the body, connector, switch, built-in equalisation, etc.) Some types of microphones (notably some radio microphones) have a detachable capsule. A choice of capsules offering different polar responses may be available, enabling the microphone to be configured for the required application. Sometimes called a cartridge. See also Transducer.

Cardioid
Describes the polar response of a uni-directional microphone whose sensitivity to sound gradually decreases from the front to the back. Its useful angle of acceptance (measured from side to side) is around 130º. Its minimum sensitivity to sounds is at an angle of 180º, measured from the front axis (i.e. is on the rear axis). The name arises from the heart-shaped pattern of the sensitivity, when plotted as a polar response graph. See the Microphones page for further information. See also Subcardioid, Super-cardioid and Hyper-cardioid.

View uni-directional polar responses illustration

Carrier
In information transmission, the signal (usually at radio-frequency) which is modulated by the baseband information signal, in order to carry the information through the transmission medium.

Cartridge
See Capsule. See also Vinyl.

Cascading
The process of interconnecting two or more mixers such that one or more mix output(s) from one mixer are supplied as input(s) to another mixer. This arrangement may be used to effectively create one large mixer from several smaller ones, but is also used for other purposes.

Cassie
A slang term for a casual stage-hand, i.e. a member of the stage crew who is not permanently employed by the venue (or, artist, company, etc.).

CAT 5, CAT 5e
An abbreviation for 'category 5' (or 'category 5e'), a UTP data communications cable with defined performance characteristics. Commonly used for computer networking applications, but also for point-to-point digital audio and video interconnections such as feeds to data projectors and in digital multicore systems. The original CAT 5 specification is now largely superceded by CAT 5e, which has an enhanced performance (see UTP).

CATV
An abbreviation for 'community antenna television', a television signal distribution system in which a common aerial serves several properties, or an abbreviation for 'cable television'.

CAV
An abbreviation for 'constant angular velocity'. Describes the rotation of a disc as being maintained at a fixed number of revolutions per minute (e.g. an LP vinyl disc). Compare CLV.

CCA
An abbreviation for 'copper-clad aluminium'. A material used as the conductor in some types of cable. This material is cheaper and lighter than pure copper, but gives a higher round-trip DC resistance for a given length of cable. The copper cladding provides a lower resistance to the high-frequency currents flowing in the cable, as these tend to flow close to the surface of the conductors ('skin effect'). This makes cable using this type of conductor popular for high frequency applications, e.g. UTP network cables.

CCIR
An abbreviation for the (French) name of the European organisation responsible for setting standards for audio equipment, measurements, etc. It is most often seen in noise specifications, to distinguish the weighting scheme specified by CCIR 468 from the 'A-weighting' scheme specified by DIN 45412 (or IEC 651). The CCIR weighting scheme is also known as DIN 45405. For further information see Microphone Noise Levels on the Microphones page. In analogue reel-to-reel tape recording, CCIR usually refers to a particular tape equalisation standard − compare NAB.

CD
An abbreviation for constant directivity (as well as for the obvious "compact disc").

CD standards
The standards for the content and format of the information contained on the most common types of CD are defined by various books that are identified by colours. The books most relevant to audio and video are:

• Red Book The standard for CD-DA (digital audio) discs. Masters for pressed production discs must conform to this standard.
• Yellow Book The standard for CD-ROM discs.
• Orange Book The standard for CD-R single-session (disc-at-once) and multi-session (track-at-once) discs. It also applies to CD-RW discs.
• Green Book The standard for CD-I (CD Interactive) discs and players.
• Blue Book The standard for CD+ and its variants. Similar to Orange Book, but with the restriction of one audio session plus one data session. The standard ensures that audio CD players will not try to interpret the data session as audio.
• White Book The video-CD (VCD and SVCD) standard. A standard for MPEG video on CD, now superceded by the DVD format.
• Scarlet Book The standard for SACD discs.

CEE-form
An informal term for a mains power connector of the form (i.e. style) originally specified by the CEE − the International Commission on Rules for the Approval of Electrical Equipment. As the CEE no longer exists, such connectors should more accurately be referred to as complying with the standard IEC 60309, adopted in Europe as EN 60309. In the UK, this is referred to as BS EN 60309 (formerly BS 4343). Sometimes incorrectly written 'C-form'.

IEC 60309 specifies a multitude of variations of this style of connector, according to the voltage, maximum current and frequency that they are intended to be used with. However for PA and lighting purposes there are three basic types of interest:

• 3-pole 110 volt, coloured yellow
• 3-pole 230 volt, coloured blue
• 5-pole 400 volt 3-phase, coloured red

N.B. The colours of these connectors have no relevance to the identifying colours of red, yellow and blue previously used in the UK for the line conductors of 3-phase supplies (now superceded by brown, black and grey).

For current ratings greater than 125 amps, single-pole connectors such as the Snaplock, Powerlock or Camlock types are usually employed.

The CEE no longer exists − after being renamed the 'International Commission for Conformity Testing of Electrical Equipment' its approvals scheme was integrated into the IEC in 1985 and is now operated by IECEE, the 'IEC System for Conformity testing and Certification of Electrotechnical Equipment and Components' (their website is www.iecee.org). See also Socapex.

View CEE-form (16A 230V) image

CENELEC
The abbreviated name of the European Committee for Electrotechnical Standardisation, a standards organisation for the European electronics industry. See also IEC.

Cent
An interval of musical pitch, 1200 of which make up one octave. In terms of frequency, a cent is therefore a ratio of 1200th root of 2, i.e. approximately 1.00057779. The cent is most commonly used in indications of tuning devices for musical instruments. In instruments tuned to sound equally 'true' in any musical key, there are a hundred cents to the semi-tone. The human ear is generally insensitive to tuning inaccuracies of less than 5 cents. See also Concert pitch.

Centre frequency
The frequency that lies in the middle of the passband of a bandpass filter, or that lies in the middle of the stopband of a bandstop filter. Or, the frequency that lies in the middle of range of frequencies that are affected by an equaliser having a peaking response.

In the case of symmetrical filter or equaliser, the centre frequency will be the frequency at which the filter or equaliser has the most effect. That is, the frequency at which a bandpass filter has least attenuation or a bandstop filter has most attenuation; or at which a peaking equaliser provides the most boost or cut.

Centre tap
A tap that is located mid-way along a winding of a transformer. By connecting a secondary winding centre tap to a common reference point (often earth), the transformer may be used to provide a pair of anti-phase outputs of equal voltage. A transformer having such a winding may be referred to as a 'centre-tapped transformer'. Most often encountered in conversions between unbalanced and balanced interconnections (see DI Box) and in power supplies. It is also used to provide a balanced mains supply. See also Balun.

Channel
One of several similar (or identical) possible signal paths through an item of equipment, or between items of equipment. Or, a potential such path. The term is typically used in two ways:

• To refer to a physical section of, or an independent signal route through, an item of equipment (or through part of an item of equipment), often having its own controls and/or connectors. For example, a channel strip of a mixer, handling the signal applied at just one of several possible inputs of the mixer. (A mixer will typically be equipped with 8, 12, 16, 24, 32 or 48 channels, enabling the independent handling of signals originating from that number of sound sources.) Or, one of several lines through a multicore. A power amplifier typically has two independent channels, sometimes four. (See also Side and P/C.)
  
  
• Less tangibly, to refer to one of several possible paths through some communication medium (such as radio waves), to a defined band of radio frequencies or to one of several independent information paths on a point-to-point digital interconnection. Examples are: "Try tuning the radio mic to a different channel", "That keyboard sends on MIDI channel 7", or "A single SPDIF digital signal carries both the left and right channels".

Channel 38
The designation given to the band of radio frequencies between 606 and 614 MHz; the new band allocated for licensed use by UHF radio microphone and in-ear monitoring systems (PMSE users). In practice, carrier frequencies will be restricted to the range 606.5 to 613.5 MHz; this range is expected to accommodate at least 8 simultaneous system channels without interference (when using suitable equipment). It is not yet clear whether the precise frequencies will be stipulated, or small sub-ranges licensed with the precise frequencies merely advised.

In the UK, PMSE usage of channel 69 is in the process of being migrated to channel 38 as part of the move to clear the so-called '800 MHz band' (790 to 862 MHz) for other services throughout much of Europe; this migration is currently intended to be complete by 2012. For further information on frequency allocations see the radio mic information on the Microphones page. See also JFMG and Regulated frequency.

Channel 69
The designation given to the band of radio frequencies between 854 and 862 MHz, which is currently allocated for licensed use by UHF radio microphone and in-ear monitoring systems (PMSE users). This band accommodates about 14 system channels, at specific designated carrier frequencies. The designation arises because this band occupies the next 8 MHz slot above the highest-numbered UHF television channel, channel 68 (846 to 854 MHz).

In the UK, PMSE usage of channel 69 is in the process of being migrated to channel 38 (606 to 614 MHz) as part of the move to clear the so-called '800 MHz band' (790 to 862 MHz) for other services throughout much of Europe; this migration is currently intended to be complete by 2012. For further information on frequency allocations see the radio mic information on the Microphones page. See also Regulated frequency, JFMG and the next definition.

Channel 70
The designation given to the band of radio frequencies between 862 and 870 MHz, the lower part of which is allocated for unlicensed use by UHF radio microphone and in-ear monitoring systems (PMSE users). In PA work the term usually refers to the part allocated for that use, which can accommodate up to 8 or 9 system channels with carrier frequencies from 863.1 to 864.9 MHz. However, many budget UHF systems are either fixed-frequency or offer a choice of only four frequencies in this range (the actual carrier frequencies used vary according to the system manufacturer and model).

The designation arises because this band occupies the next-but-one 8 MHz slot above the highest-numbered UHF television channel, channel 68. It is sometimes referred to as the 'ISM band'. Its use for PMSE applications remains unaffected by the move to clear the so-called '800 MHz band' (790 to 862 MHz) for other services throughout much of Europe. For further information see the radio mic information on the Microphones page. See also De-regulated frequency, ETS and the previous definition.

Channel strip
The set of controls and associated electronics that provide the initial signal processing for a single channel of a mixer. The controls are often arranged vertically, and usually provide, as a minimum, adjustments for gain, equalisation and level-adjustable routing. In mixers of a modular construction, the channel strips may be removed (either singly or in groups) for maintenance or repair, or for replacement by a compatible strip having different facilities. For further details of the facilities typically provided on a channel strip, see Channel Facilities on the Mixing Facilities page.

Chapter
An indexable sub-division of a DVD title.

Characteristic impedance
The 'natural' impedance exhibited (at relevant frequencies) by a particular 2-conductor component of an interconnection between equipment (especially by a cable or a connector) by virtue of the inherent inductance of the conductors and the capacitance between them. The values of these attributes are determined by the physical dimensions of the component and the construction materials used, so these are the factors that determine its characteristic impedance value. The characteristic impedance is unaffected by the cable length, and is not to be confused with the 'end to end' (or 'round trip') impedance of speaker cables and the like, which increases with length (see Gauge).

It is vital that cable and connectors of correct characteristic impedance are used in impedance-matched interconnections between equipment, especially where radio-frequency signals and/or long lengths of cable are involved. This is because when the characteristic impedance of the interconnecting path is equal to that of the input and output of the interconnected equipment, maximum signal transfer is achieved and troublesome signal reflections are minimised. The quality of an impedance match is indicated by a return loss figure. In practice there is some variation of characteristic impedance with frequency, but it is important only that the value is within acceptable limits over the frequency range of interest. Examples of impedance-matched interconnections are video, digital audio and DMX lighting control.

Video and unbalanced digital audio interconnections operate at a characteristic impedance of 75 ohms, whilst balanced digital audio interconnections operate at a characteristic impedance of 110 ohms and DMX lighting control interconnections at 120 ohms (balanced). Antenna connections of radio microphones are usually at either 50 ohms or 75 ohms (see Antenna distribution unit).

Note that, in an impedance-matched interconnection, when the far end of the cable is terminated in an impedance equal to the characteristic impedance of the cable, the value of impedance that would be seen between the two conductors at the near end will always be that same value − regardless of the length of the cable. (More theoretically, the characteristic impedance is the value of impedance that would be seen between the two conductors at one end of the cable if the cable was of infinite length − regardless of what, if anything, was connected at the far end.) Often abbreviated to just 'impedance'.

Charge
Stored electrical energy, measured in coulombs (or, in the case of a battery, in ampere-hours). A continuous flow of charge is what forms an electric current. The amount of energy available from a given amount of stored charge depends upon the voltage at which the charge is stored. (In fact voltage can be considered to be a measure of energy per unit charge − the value of the voltage is equal to the number of joules per coulomb.) See also Capacitance and Condenser microphone.

Chase
A pre-programmed repetitive sequence of lighting scene changes, usually occurring at a steady rate (which may be preset or adjustable).

Chassis
The physical framework or casing of an item of equipment, usually one made of metal. As the chassis is usually connected to either a signal earth or safety earth, or both, the term may be used to refer to an earth connection. See also Class I, Enclosure and Pin 1 problem.

Chebyshev
Describes a class of filter having a controlled amount of variation (or 'ripple') of attenuation over its passband. Named after the person first to document this design. Whilst ripple in the passband is undesirable, by tolerating it this class of filter is able to achieve a more abrupt transition between its passband and its stopband than classes which aim to provide the minimum possible amount of ripple. See also Bessel and Butterworth.

Checksum
An extra value that may be incorporated into stored or communicated data at regular intervals (or just at the start or end of the data), in order to facilitate subsequent checking of the data’s integrity.

Each checksum value is calculated from the preceding set of data values (since the previous checksum, if any) by the storing or sending equipment, according to a pre-defined addition-based method. The reading or receiving equipment then performs the same calculation on the data and compares the resulting checksum value with the read or received one; any discrepancy indicates an error in the data (or in the checksum). See also Cyclic redundancy check, Parity, Bit error rate and Error correction.

Cherrypicker
A powered device for lifting a person (or sometimes two) to a height in order to gain access to equipment such as flown speakers or lighting trusses. Use of such access equipment requires appropriate training and careful attention to safety. The term may be hyphenated, or written as two separate words. See also Fly and SWL.

Chip
A slang term for a silicon chip − see Integrated circuit.

Choke
See Inductor.

Chord
Several musically-related notes (usually either 3 or 4 of them), played at essentially the same time. See also Polyphonic.

Chorus (1)
An effect unit which modifies a signal in such as manner as to simulate the presence of several additional sources of the same (or similar) sound, offset slightly in pitch and in timing, as occurs in a group of vocalists singing in unison (see Chorus (2)).

Chorus (2)
A group of vocalists, often large in number, who sing together to complement, or to provide a backing to, the solo vocalist(s). Most often encountered in stage musicals.

Chrominance (Chroma)
The part of a colour composite video signal that carries the colour information, usually coded in some way (such as PAL or NTSC). The chrominance information includes the hue (shade) of the colours and their saturation (depth), but the brightness information is conveyed by the luminance signal. See also S-video and Colour bars.

Cinch
Another name for a phono connector, so-named after one of the first companies to manufacture it. (However, Cinch also manufacture many other different types of connector.)

Circuit (1)
A network of interconnected electrical or electronic components, designed to implement some specific required function. Such a network may be passive, for example, a frequency-selective filter formed by a circuit which contains only passive components such as inductors, capacitors and resistors, or may be active, i.e. contains also semiconductor (or valve) components requiring a source of power. See also PCB.

Circuit (2)
A signal path from one point to another, usually through some form of cable or communications link. Or, a path intended to carry power − usually a specific mains power path from the mains distribution board. When the path is physical, it may be termed a 'line'.

Circuit (3)
An abbreviation for 'circuit diagram'.

Circuit breaker
Short for 'miniature circuit breaker' − see MCB.

Circuit diagram
A diagram that illustrates the individual components used in a particular electrical or electronic circuit, and the details of the interconnections between them. Often abbreviated to just 'circuit'. In such diagrams, each type of component is represented by a standard symbol, and the interconnections are represented by straight lines. Circuit diagrams are alternatively (and perhaps more properly) known as 'schematics'. Compare Block diagram.

Circuit protection
A generic term for the device(s) such as fuses and MCBs protecting a particular mains supply circuit against the effects of excessive current, due for example to an overload or short circuit condition. It may also include devices protecting against excessive earth leakage, such as RCDs and RCBOs.

Circuit protective conductor
A safety earth conductor that is associated with a particular mains supply circuit, and provides the safety earthing for equipment connected to that circuit. Commonly abbreviated to CPC.

Circumaural
Describes headphones that are worn with the pads completely surrounding the outer part of the ears. Most closed back headphones are of this type. Compare Intraaural and Supraaural.

Clarity
A subjective measure of the degree to which individual components of a sound may clearly be distinguished within the overall sound. This is affected particularly by the quality with which the treble frequencies are reproduced, and their correct balance of level relative to lower frequencies. In relation to vocals (whether spoken or sung) it relates to the intelligibility of the words, which is heavily dependent upon the degree to which consonants can be clearly distinguished (see ALCONS). A sound with good clarity may be described as 'crisp'. See also Sibilance, Warm and Muddy.

Class I
[Pronounced "Class One".] Describes an item of mains-powered equipment that must have an adequate connection to a safety earth in order that it does not present an electric shock hazard. Any equipment that is not marked with the Class II symbol (a square within a square) must be assumed to be Class I.

For all Class I equipment that is intended to be connected to the mains by means of a flexible cable and plug (rather than being permanently wired into the installation), its safety earth connection must be made through the mains cable and mains plug of the equipment, so it is essential that a 3-core cable (live, neutral and earth) is used, fitted with a correctly wired 3-pin plug, and that any extension cables used also meet these requirements. (It is not safe for the earth connection to be obtained from other equipment through signal cables.)

Further, in order for the necessary shock protection to be provided, the mains socket outlet from which the equipment obtains its power must provide a satisfactory safety earth connection. See also Chassis and Indirect contact. Compare Class II.

Class II
[Pronounced "Class Two".] Describes an item of mains-powered equipment that does not require a connection to a safety earth. This type of equipment is fitted with supplementary or reinforced insulation, in order that it does not present an electric shock hazard. So, such equipment is also described as 'double insulated'. Any equipment that is not marked with the Class II symbol (a square within a square) must be assumed to be Class I.

As no safety earth connection is required, 2-core mains cables (live and neutral only) and 2-pin plugs may be used, but be sure that any such cables are supplying only Class II equipment. To be safe, 2-core extension cables are best avoided. (Note: A 2-core extension cable must never be fitted with a 3-pin plug and socket, as this may in the future mislead someone into thinking that it is a 3-core cable.) See also Indirect contact. Compare Class I.

Classes of amplifiers
See the Amplifier Classes section on the Amplifiers and Speakers page.

Classes of filters
See Bessel, Butterworth and Chebyshev. See also Linkwitz-Riley.

Clean
Describes a sound or a signal which is free from distortion, effects, etc. See also Dry. Compare Dirty.

Clean earth
See Technical earth.

Clean feed
An alternative term for mix-minus.

Clearcom
A trademarked name for a proprietary intercom system commonly used in auditoria to enable communication between different locations or technical personnel. It uses a 3-wire interface, connected using 3-pole XLR connectors, which has become a standard to the extent that other manufacturers now produce equipment that is described as "Clearcom compatible". See also Comms and Talkback.

Click
A regular clicking sound which the person whose responsibility it is to set and maintain the correct tempo of the music hears through headphones, to provide them with a reference beat. This person would typically be a musician (often a drummer), the Conductor, or the Musical Director. The click is either generated electronically (usually local to the person who hears it, under their control), or is played from a click track.

Click track
A track of a 2-track or multi-track music recording, containing only a click. It is used when the playback of the recording is to be accompanied by live musicians, to enable the live performance to be correctly synchronised with the recorded music. The click track will usually start a defined number of beats before the music track(s), to cue the live performer(s). See also Tempo.

Cliff effect
See Digital cliff.

Clip microphone
See Lavalier.

Clipping
Describes an amplifier or other signal processing equipment (or internal part of it) that is being overloaded to the extent that the positive and negative extremes of the signal waveform are flattened, as if they had been clipped off. A slang term for this is 'hitting the rails'. For a fuller explanation of clipping, see Overload.

Clock
Besides the obvious device for indicating the time of day, in technical terms this refers to any continuous timing reference signal of fixed frequency, usually used to synchronise the timing of sequential events or the transfer of digital information. See also Synchronous and Bit-stream.

Close pickup
Describes the placement of a microphone within a distance of around 2 feet (60 cm) of a sound source (often substantially less than this). When miking within the near field of a source, the exact position and angle of the microphone usually has a major effect on the balance and timbre of the sound picked up. See also Colouration. Compare Distant pickup.

Closed back
Describes headphones that are intended to largely exclude the room sound, so that the sound from the headphones may be heard more clearly and without distraction, at lower headphone sound pressure levels (SPL) than would be necessary with open back types in environments of high ambient SPL. Closed back headphones used by engineering personnel are usually of the circumaural type.

Closed box
See Sealed box.

Cluster
A closely-located group of speakers, often flown centrally. Usually each speaker in the cluster points in a different direction, so as to cover a different section of the audience in the horizontal plane and so provides a broad horizontal coverage. Note that, in contrast to a line array, the enclosures of a cluster are most often located side-by-side. See also Array, Stack, Trapezoidal and LCR (1).

CLV
An abbreviation for 'constant linear velocity'. Describes a scheme in which the rate of rotation of a disc is varied in such a way that, regardless of the part of the disc being accessed, the surface of the disc passes the head at a fixed speed. To achieve this, the disc must rotated at a smaller number of revolutions per minute as the head moves from near the centre of the disc towards its outer edge. (e.g. a Compact Disc starts at 500 r.p.m. and gradually reduces to 200 r.p.m. at the end.) The purpose of this arrangement is to enable information to be stored on the disc at the maximum possible density over the whole surface area of the disc, without having to vary the rate at which information is written to, or read from, the disc. Compare CAV.

CMRR
An abbreviation for 'common mode rejection ratio'.

CNMAT
The Center for New Music and Audio Technologies, a branch of the University of California. Their website is cnmat.berkeley.edu.

Co-channel interference
In any radio link (such as is used within radio microphone and in-ear monitoring systems), interference that is experienced as a result of the operation of some other radio-frequency equipment or broadcast, whose transmissions use (or overlap) the frequency range(s) used by the affected link. See also Channel.

Coax
An abbreviation for 'coaxial cable'.

Coaxial
Describes something in which two parts share the same axis. Most often used in reference to a type of cable (see the next definition) or to a connector suitable for such a cable. The term is also used as an informal description of a digital audio interface complying with the S/PDIF standard (or, less often, with the AES3-ID standard), which both use coaxial interconnecting cable, to distinguish that type of interface from the optical equivalent (e.g. TOSLink). The term is sometimes used as an alternative to the more usual term concentric, in reference to a physical arrangement of the drivers in a 2-way speaker (see Concentric for details).

Coaxial cable
An unbalanced cable in which a single insulated central conductor is completely surrounded by a conducting screen, which is then covered by an insulating sheath. It is so called because the central conductor and the screen are geometrically coaxial. The name is sometimes abbreviated to 'coax'. See also Triaxial cable, Dielectric and Drain wire.

Code (1)
An abbreviation for 'National Electrical Code'.

Code (2)
An alternative term for software.

Codec
An abbreviation for 'coder and decoder'. The equipment or software which converts a programme signal from one form to another. The term usually refers either to the conversion between analogue and a particular digital form, or to a change in digital form (e.g. data compression).

Strictly, a codec should be able to perform the conversion operation in both directions i.e. for coding and decoding purposes. In general computer terminology, however, the word is frequently used to refer to a piece of software which (in practice) is only ever used for decoding purposes, for the playback of encoded programme material, regardless of whether or not that software also has an encoding capability. In such computer-based applications, the 'signal' on which the codec operates is a flow of data between software components. See also Analogue to digital conversion and Modem.

Coding scheme
See Analogue to digital conversion.

Coincident-microphone technique
See X-Y pair and Mid-side pair.

Cold
In an unbalanced interconnection, describes a conductor which, in normal use, is at an insignificant voltage with respect to the general mass of earth; that is, an earthy conductor. Often used as the screen of the interconnection, but otherwise may be marked '−'.

In a balanced interconnection, describes the leg whose voltage becomes more negative as the original (unbalanced) signal or sound pressure becomes more positive. Sometimes referred to as the 'return' leg, or marked '−'. See also Pair. Compare Hot.

Cold standby
Describes redundant equipment that is available for use in the case of a failure, but is not switched on ready for use. For further information see Redundancy. Compare Hot standby.

Colour bars (Color bars)
A test image consisting of a range of standard colours, usually arranged as adjacent vertical bars, which is used for evaluating the colour performance of a video display or projection system. A video signal conveying such an image is frequently used in the absence of an actual programme signal, as a means of confirming the continued operation of a video signal path or processing equipment. ('Color' is the US spelling.) See also Chrominance and Grey scale.

Color code (Color code)
Colours are frequently used in many circumstances as a means of identification − see, for example, CEE-form and Phase (3). ('Color' is the US spelling.)

In electronics, the term 'colour code' specifically refers to the numeric coding scheme used to indicate the value of some types of resistor (and also some types of capacitor). Unfortunately there are several variants of this scheme, but in the simplest and most common one, four bands of colour are used − the first three indicate the nominal value and the fourth one indicates the tolerance.

Of the first three bands, the first two indicate the initial two digits of the value according to the following code:

• 0 Black
• 1 Brown
• 2 Red
• 3 Orange
• 4 Yellow
• 5 Green
• 6 Blue
• 7 Violet
• 8 Grey
• 9 White

The fourth band on a resistor that carries the most common 4-band code indicates the tolerance, as follows:

• 20% No band
• 10% Silver
• 5% Gold
• 2% Red
• 1% Brown

Some resistors employ a 5-band code, in which the first three bands indicate the initial three digits of the value, the fourth band indicates the number of zeros to be appended to that 3-digit value, and the fifth band indicates the tolerance. In this scheme, a Gold fourth band is used for values from 10.0 to 99.9 and a Silver band for values from 1.00 to 9.99.

There are many different colour coding schemes in use for audio interconnections.

A fairly standard colour coding used in analogue stereo interconnections is for Red to identify the Right channel and White to identify the Left channel. This coding is most often used for connectors.

See Speakon for a common coding system for the allocation of multiway speaker cable cores to the Speakon terminals.

The tails of multicores are sometimes identified using the resistor colour code for circuit numbers 1 to 9, continuing as follows for numbers up to 12:

• 10  Black
• 11  Tan
• 12  Pink

• 13-24  Light grey
• 25-36  Light blue
• 37-48  Lime
• 49-60  Aqua

		IEC & Van Damme®		¦	Belden®		¦	Klotz®
Pair number		Hot	Cold	¦	Hot	Cold	¦	Hot	Cold	Pair number
1		White	Blue	¦	Black	Red	¦	Black	Red	1
2		White	Orange	¦	Black	White	¦	Black	White	2
3		White	Green	¦	Black	Green	¦	Black	Green	3
4		White	Brown	¦	Black	Blue	¦	Black	Blue	4
5		White	Grey	¦	Black	Yellow	¦	Black	Yellow	5
6		Red	Blue	¦	Black	Brown	¦	Black	Brown	6
7		Red	Orange	¦	Black	Orange	¦	Black	Orange	7
8		Red	Green	¦	Red	White	¦	Red	White	8
9		Red	Brown	¦	Red	Green	¦	Red	Green	9
10		Red	Grey	¦	Red	Blue	¦	Red	Blue	10
11		Black	Blue	¦	Red	Yellow	¦	Red	Yellow	11
12		Black	Orange	¦	Red	Brown	¦	Red	Brown	12
13		Black	Green	¦	Red	Orange	¦	Red	Orange	13
14		Black	Brown	¦	Green	White	¦	Green	White	14
15		Black	Grey	¦	Green	Blue	¦	Green	Blue	15
16		Yellow	Blue	¦	Green	Yellow	¦	Green	Yellow	16
17		Yellow	Orange	¦	Green	Brown	¦	Green	Brown	17
18		Yellow	Green	¦	Green	Orange	¦	Green	Orange	18
19		Yellow	Brown	¦	White	Blue	¦	White	Blue	19
20		Yellow	Grey	¦	White	Yellow	¦	White	Yellow	20
21		White/Blue	Blue	¦	White	Brown	¦	White	Brown	21
22		White/Blue	Orange	¦	White	Orange	¦	White	Orange	22
23		White/Blue	Green	¦	Blue	Yellow	¦	Blue	Yellow	23
24		White/Blue	Brown	¦	Blue	Brown	¦	Blue	Brown	24
25		White/Blue	Grey	¦	Blue	Orange	¦	Blue	Orange	25
26		Red/Blue	Blue	¦	Brown	Yellow	¦	Brown	Yellow	26
27		Red/Blue	Orange	¦	Brown	Orange	¦	Brown	Orange	27
28		Red/Blue	Green	¦	Orange	Yellow	¦	Grey	Black	28
29		Red/Blue	Brown	¦	Violet	Orange	¦	Grey	White	29
30		Red/Blue	Grey	¦	Violet	Red	¦	Grey	Red	30
31		Black/Blue	Blue	¦	Violet	White	¦	Grey	Green	31
32		Black/Blue	Orange	¦	Violet	Dark Green	¦	Grey	Blue	32
33		Black/Blue	Green	¦	Violet	Light Blue	¦	Grey	Yellow	33
34		Black/Blue	Brown	¦	Violet	Yellow	¦	Grey	Brown	34
35		Black/Blue	Grey	¦	Violet	Brown	¦	Grey	Orange	35
36		Yellow/Blue	Blue	¦	Violet	Black	¦	Violet	Black	36
37		Yellow/Blue	Orange	¦	Grey	White	¦	Violet	White	37
38		Yellow/Blue	Green	¦			¦	Violet	Red	38
39		Yellow/Blue	Brown	¦			¦	Violet	Green	39
40		Yellow/Blue	Grey	¦			¦	Violet	Blue	40
41		White/Orange	Blue	¦			¦	Violet	Yellow	41
42		White/Orange	Orange	¦			¦	Violet	Brown	42
43		White/Orange	Green	¦			¦	Violet	Orange	43
44		White/Orange	Brown	¦			¦	Violet	Grey	44
45		White/Orange	Grey	¦			¦	Pink	Black	45
46		Red/Orange	Blue	¦			¦	Pink	White	46
47		Red/Orange	Orange	¦			¦	Pink	Red	47
48		Red/Orange	Green	¦			¦	Pink	Green	48
49				¦			¦	Pink	Blue	49
50				¦			¦	Pink	Yellow	50
51				¦			¦	Pink	Brown	51
52				¦			¦	Pink	Orange	52
53				¦			¦	Pink	Grey	53
54				¦			¦	Pink	Violet	54
55				¦			¦	Turquoise	Black	55
56				¦			¦	Turquoise	White	56

Colouration (Coloration)
The degree to which one or more ranges of frequency are over- or under-emphasised in the sound. So, the term is usually used in a derogatory sense. ('Coloration' is the US spelling.) See also Off-axis colouration and Tone (1). Compare Flat (1).

Column speaker
A speaker containing several drivers arranged as a vertical line array, so as to reduce vertical dispersion. In many types all the drivers are identical, but some full range types are of 2-way design, the high frequency driver(s) often being placed centrally in the array.

Comb filter
A filter that provides many notches of attenuation, each centred on a different frequency, with the ranges of frequency between them being essentially unaffected. The name arises because a graph of the frequency response of such a filter has a comb-like appearance.

This kind of filtering, which usually produces a 'hollow' effect in the sound, may be unintentionally introduced by two or more microphones picking up the same sound source at similar levels but from different distances. This is caused by the difference in phase due to the additional time that the sound takes to reach the further microphone, and results in cancellation effects occurring when the signals from the microphones are mixed. (For a way to help avoid this effect, see the 3-to-1 rule.) If the difference in distance between the microphones and the source varies, the comb of filtered frequencies sweeps through the frequency range, producing an effect very similar to flanging.

Assuming that the microphones are picking up exactly the same sound, are connected with the same polarity and have the same equalisation settings, then the notches in response will occur at those frequencies at which an exact odd number of half-wavelengths fit into the difference in distance between each microphone and the source of sound. By applying the equation to derive wavelength from frequency using the speed of sound, it can be shown that the first notch will be at a frequency (in Hz) given by 172 divided by the difference in distance (in metres), and subsequent notches will be at intervals of 343 divided by the difference in distance. For example, a difference in distance of 30 cm gives notches at 572 Hz, 1.72 kHz, 2.86 kHz, 4.00 kHz, 5.15 kHz, etc.

Combo
A single item of equipment which combines the functions of a head and its associated speaker, usually intended for use as backline for guitar, bass or keyboards. Often referred to by musicians (and other non-technical personnel) as an 'amplifier' (or 'amp').

Common impedance coupling
The phenomenon whereby a signal current or noise current that is flowing on one path impresses itself to some degree onto the signal flowing on another path because the two paths share the same conductor at some point, usually a signal earth conductor. Such path-sharing is only problematic when the impedance of the shared conductor is sufficient to cause a significant voltage drop across it (relative to the signal or noise levels in question).

For example, any small mains-frequency current flowing in the signal earth conductor of an unbalanced interconnection (typically as a result of safety earth voltage differences) will cause an interferring mains-frequency voltage to be added to the signal voltage being carried. The magnitude of this interferring voltage will depend on the amount of mains-frequency current flowing in the signal earth conductor and on the impedance of the part of the signal earth path that it flows through.

Apart from ensuring that any shared paths are of sufficiently low impedance, a common strategy for avoiding common impedance coupling is to eliminate (as far as possible) such shared paths completely, for example by use of star point earthing. The use of balanced interconnections can also be of very significant benefit. See also Crosstalk, Earth loop and Pin 1 problem. Compare Capacitive coupling and Inductive coupling.

Common mode choke
See Longitudinal choke.

Common mode interference
Interfering signals that arrive identically on the two signal connections ('hot' and 'cold', or '+' and '−') of a balanced input. Typically such interference would be hum originating from earth loops or induced by stray mains magnetic fields, and radio-frequency interference. Balanced interconnections are arranged so that, as far as possible, interference from such sources affects the hot and cold conductors equally. So, the interference appears at the hot and cold connections of the balanced input with in-phase waveforms of the same amplitude, and is therefore substantially rejected by the input circuitry − see the next definition.

Common mode rejection ratio (CMRR)
A measure of the ability of a balanced input to reject (i.e. ignore) signals that arrive identically on its two signal connections ('hot' and 'cold', or '+' and '−'), in comparison with signals that differ between those two connections.

It is measured by comparing the response of the input to a signal applied between the hot and cold connections with its response when exactly the same signal is applied (relative to signal earth) to both of these two connections simultaneously. (The given source impedance conditions apply in both cases.) A larger figure indicates that the input is better at ignoring common mode interference. Usually measured in decibels. As the figure is a value of rejection, it is a positive quantity; unfortunately, however, it is sometimes erroneously indicated as a negative value.

Comms
An abbreviation for 'communications', a general term used to refer to facilities which enable staff (such as sound engineers, lighting engineers, production managers, stage managers, camera operators, etc.) to communicate with each other during an event. See also Talkback and Clearcom.

Compander
A compressor and an expander acting together as a pair. The intention is usually that there is no net effect on the signal − so what is the purpose? An example of where this is useful is in a radio microphone, where in order to achieve a satisfactory signal-to-noise ratio over the radio link, the dynamic range of the audio signal has to be reduced in the microphone and then restored to its original value in the receiver. Rarely, the term relates to the data compression of a digital signal and its subsequent de-compression − see for example NICAM.

Competent person
A person whose knowledge, experience and skill enables them to carry out a specific kind of task (whether of a practical, design, or management nature) to the required standard and without danger to themselves or others. Such a person will usually have received formal or informal training in the relevant area(s) of work, but will not necessarily hold formal qualifications. Note that different disciplines may use their own specific definitions of this term, especially in relation to matters of health and safety.

Component video
A particular type of analogue video interconnection, using three signals. The luminance and synchronisation information (both line and field) are carried by one of the signals, referred to as the Y signal. The the chrominance information is carried in baseband form by the other two signals; these two chrominance signals are referred to as C_R (or P_R) and C_B (or P_B) and carry the Red and Blue information respectively. (The Green information can be derived from this set of three signals, because luminance equals the sum of Red, Green and Blue.) Compare Composite video, S-Video and RGB.

Composite video
Originally, an analogue video interconnection in which the synchronisation information (both line and field) is combined with the picture content, so that the whole may be carried by a single-signal connection between items of equipment. However, since the advent of analogue colour video the term is has been used to refer to an interconnection in which the chrominance information (coded in some form and impressed on a subcarrier) is included in the same signal as the luminance information and syncs.

There are many different standards for composite video, which are not compatible for interconnection or for playback of pre-recorded material. Most standards fall into the category of PAL, SECAM or NTSC, but each of these have several different 'flavours' indicated by a suffix letter. The table below compares some basic parameters of a common flavour of each standard, and for completeness also shows the analogue sound subcarrier frequency used in broadcast TV (although that is not part of a composite video signal). See also NICAM. Compare Component video, S-Video and RGB.

Compression (1)
A process which reduces the dynamic range of a signal. The device which does this is called a compressor, and the extent of compression provided is called the compression ratio. Compressors are frequently set up to apply compression only to signals that exceed a particular level, called the threshold level.

Compression has four(!) basic uses:

• To create a particular effect (typically used with electric guitars and basses), both in sustaining long notes and in reducing initial attack.
• To reduce unwanted variations in level − such as in public address (speech) applications and in induction loop systems.
• For practical reasons such as to accommodate a signal to the restricted dynamic range of particular equipment, such as a recording device (usually analogue tape) or radio link. As compression reduces the difference between the signal's average level and its peak level, the required headroom is reduced.
• To increase the average level of the signal (relative to the peak level) and therefore make it appear to be louder and more 'intense' − this is extensively done in pop music recordings.

A form of compression is sometimes used internally within speakers in order to provide some protection of the drivers (often just the horn(s)) from overload. This is usually provided in the form of a series-wired filament lamp (often incorporated within a passive crossover unit) that may need to be replaced from time to time.

Compression can occur unintentionally, for example see Power compression. See also Signal processing, Speaker protection, Ride, Compander, De-esser and Dolby A, B and C. Compare Expansion.

Compression (2)
A process which reduces the quantity of data required to adequately represent some information, such as digital audio − usually according to some agreed specification as to exactly how it should be done (see Algorithm). In the case of stored information, compression allows less storage space to be required for it. In the case of information being conveyed from one point to another in real time, compression allows a lower data-rate (bit-rate) to be used on the link, or more information to be transferred in a given amount of time. The effectiveness of the compression is called the compression ratio.

In order to make use of the compressed information, it must be restored to its original form (or nearly so) by reversing the compression process. If the original data can be exactly restored, the compression is described as 'lossless' − no meaningful part of the information was discarded in the process. If the restored version of the information is not an exact copy of the original, the compression is described as 'lossy' − even if it is not possible for the end user of the information (e.g. a listener of the audio) to tell the difference. Examples of audio data compression schemes are MP3, AAC and ATRAC. See also Masking, Perceptual coding, Codec and Variable bit rate.

Compression (3)
An increase in pressure, in relation to the propagation of sound through a gaseous medium such as air; the opposite of rarefaction. Sound waves in air consist of alternate regions of compression and rarefaction along the length of the wave in its direction of travel, and are therefore described as 'longitudinal'.

Compression driver
The transducer that is used at the back of the throat of a high frequency horn driver. This is frequently a separately replaceable component, which bolts to the throat. Note that the meaning of the word 'compression' here is compression (3), and there is no direct relation to any power compression that may occur in the driver.

Compression ratio (1)
A measure of the amount of signal compression taking place. The ratio control of a compressor usually has settings labelled 2:1, 4:1, 6:1 etc, pronounced '2 to 1', '4 to 1', etc. For example 2:1 means that, for signals above the threshold level, 2 dB of change in level at the input produces only 1 dB of change in level at the output; 4:1 means that, for signals above the threshold level, 4 dB of change in level at the input produces only 1 dB of change in level at the output, etc. (For an explanation of decibels, see the Decibels page.) A setting of 1:1 means that the compressor is providing no compression (i.e. is inactive, or bypassed), whilst Infinity:1 means that it is acting as a limiter. Compare Expansion ratio.

Compression ratio (2)
A measure of the effectiveness of a data compression algorithm. The ratio is usually expressed as 2:1, 4:1, 10:1 etc, pronounced '2 to 1', '4 to 1', etc. For example 2:1 means that the data is compressed to half its size, 4:1 means that it is compressed to a quarter of its size, etc.

Compressor
A signal processing unit which reduces the dynamic range of a signal, usually taking effect only while the signal exceeds a pre-set threshold level. This reduction in dynamic range is called compression. The 'amount' of compression is determined by the compression ratio control.

Most compressors have the facility for adjustment of the speed at which compression is applied, after the threshold level is exceeded. The time taken to reach the set amount of compression is called the 'attack time', and its adjustment may be useful in avoiding the significant compression of transients and other short-duration peaks in level.

Two adjustments may be provided to control the behaviour when the level falls below the threshold level: 'hold time' and 'release time'. The hold time extends the period for which the set amount of compression is applied, and its adjustment may be useful in avoiding rapid switching in and out of compression (especially when short attack times are used). The release time controls the rate at which compression is removed, after the expiry of the hold time. See also Gain reduction, Side chain, Knee, Pumping, Breathing and Compander. Compare Expander.

Concentric
Describes two or more items that are located such that their centres are at the same point.

For example, in a concentric 2-way speaker design, the high frequency driver is mounted at the centre of the woofer, so providing a 'point source' of sound across the speaker's whole frequency range. In a conventional design, where the two drivers are laterally displaced, the phase difference in different directions of radiation causes directional interference effects to occur between the sound-waves from the two drivers around the crossover frequency, which can be a particular problem in stereo close-monitoring applications. The concentric arrangement has the advantage of eliminating the directional aspect of such interference effects, and also reduces the physical size of the speaker. Such speakers are sometimes referred to as dual concentric, coaxial or point source speakers. See also Sweet spot.

Another example of a concentric design is the arrangement of sweep EQ frequency and boost/cut controls in which these controls are located one inside the other rather than side by side. Compare Coaxial.

Concert pitch
A standard tuning pitch for musical instruments, in which the note referred to as 'A4' (the first 'A' above middle C) has a fundamental frequency of exactly 440 Hz. See also Cent.

Condenser microphone (Capacitor microphone)
A type of microphone in which sound is converted to an electrical signal by causing the vibration of one of the plates of a charged capacitor, whilst the other plate remains fixed. So, the spacing between the plates varies and this causes a variation in capacitance, which in turn causes tiny variations in the voltage between the plates. The vibrating plate is called the diaphragm and the fixed plate is called the backplate.

As this process generates a very small signal level, some initial amplification of the signal is required within the microphone itself. This internal amplifier may be powered either by an internal battery or by power supplied from the mixer (see phantom powering). The current demand of a condenser microphone is typically 2 to 4 mA. The charge on the capacitor may be provided either by the same power source that powers the internal amplifier (a so-called 'true' condenser microphone) or, more usually in stage mics, may be permanently created by use of special materials on one plate of the capacitor. In the latter arrangement, called an 'electret microphone', best results are obtained when the backplate (rather than the diaphragm), carries the charge, and this type is termed a 'back electret' microphone.

The name 'condenser microphone' arose because 'condenser' was the old word for a capacitor. So, this type of microphone may equally be called a 'capacitor microphone'. (Note that some types of condenser microphones are readily adversely affected by the condensation of moisture onto the capacitor plates − the name of this type of microphone is completely unrelated to this phenomenon.) The alternative name 'electrostatic microphone' is sometimes encountered; this indicates that sound causes hardly any movement of electrons through the microphone capsule.

This type of microphone is most useful for distant pickup applications such as overhead microphones (for choirs, drum kits, etc.) and when very precise reproduction is required, such as in studio applications. Some types are fitted with facilities such as attenuator switches, low cut switches and pick-up pattern selectors. (See the Microphones page for more information.) Compare Dynamic microphone.

Condition report
A written report on the condition of a fixed electrical installation, produced by a competent person in accordance with the relevant regulations and procedures. Its full name is an electrical installation condition report (EICR). In the UK, such a condition report (previously referred to as 'periodic inspection reports') must be produced in accordance with BS 7671 and is a record of the results of the prescribed inspections and tests performed on the installation; it should indicate any defects that were detected at that time. A copy of the ECIR for venues hired out for performers etc. will usually be made be available for viewing by the hirer on request. However, remember that defects may have arisen (or got worse) since the date of the report.

Condom
A slang term for a disposable protective cover used for protecting bodypacks from perspiration or from fluids used during a performance. Usually made from latex.

Conductor
That part of a cable, connector or equipment, which is intended to carry (or to be capable of carrying) an electric current. Or, any material which presents a low resistance to the flow of current. See also Wire, Pole, Core, Gauge, Oxidation, Oxygen-free copper, Tinned conductor and Galvanic connection. Compare Insulator and Semiconductor.

Conduit
A tubular means of containment and mechanical protection for cabling fixed within a building, typically made of steel or PVC. Cables must be drawn into conduit from the ends, having due regard to the maximum capacity of the size of conduit being used and to the possible increased heating effect obtained when many cables carrying a substantial current are enclosed together. Metallic conduit for mains cabling must be connected to safety earth. Compare Trunking.

Cone
The conically-shaped part of a woofer or a mid-range driver, which moves back and forth in sympathy with the signal applied to the driver. This causes displacement of the air in the vicinity of the cone and so creates sound waves. In most drivers, the cone's movement is produced by a magnetic force between the voice coil and a permanent magnet. See also Recone, Diaphragm, Excursion, Compression (3) and Rarefaction.

Conga
A type of tall narrow drum, often fixed in pairs on a metal stand (each drum of the pair having a different pitch).

Connector
Any device fixed at the end of a cable, or on equipment, to enable cables to be conveniently attached and detached from equipment, and from each other. Some common types of connector are listed below.

Link to definition	View image (in new window)
Banana plug	Banana plug image
BNC	BNC image
BS 1363A	BS1363A image
BT431a	BT431a image
Camlock	Camlock (400A) image
CEE-form	CEE-form (16A 230V) image
D-Type	D-Type (9 pole) image
DIN	DIN (5 pole 180º) image
DVI	DVI image
Ethernet	Ethernet (RJ45) image
Firewire	Firewire (4 pole) image  Firewire (6 pole) image  Firewire (9 pole) image
HDMI	HDMI (Type A) image
IEC	IEC (IEC320/60320 C13 mains) image
Jack	Jack (A-type 6.35mm) image
MUSA	MUSA image
Phono	Phono image
PowerCon	PowerCon image
Powerlock	Powerlock image
SCART	SCART image
Snaplock	Snaplock image
Socapex	Socapex (37 pole) image
Speakon	Speakon (2 pole) image
TOSLink	TOSLink image  3.5mm optical image
USB	USB A image  USB B image
VGA	VGA (HD15) image
XLR	XLR (3 pole) image

See also Input, Output, Interface, Binding post, Barrier strip, Spade terminal, Coaxial, Boot, TCMCC, QTBMC, Male, Female, Mate, Gender changer, Y-connector, Source, Drain, Termination (1) and Insulation displacement.

Console
Another name for a mixer or other control panel such as a lighting control desk. Considered to be an 'outdated' term by some people (in favour of 'mixer' or 'desk'), but widely used by many others.

Console tape
Self-adhesive tape which is fixed to a mixer, normally just below or above the faders, and written on to indicate the use to which each channel, group, etc. is allocated. Use of such tape avoids marking the mixer's scribble strip directly; enables rapid, convenient and complete removal of markings; and enables a 'permanent' marker pen such as a 'Sharpie' to be used, which smudges less easily than wipeable markers. (In the event that a marking needs to be changed, another short length of console tape can be stuck on top of the original, incorrectly marked, piece.) If purpose-made console tape is not available, white PVC insulating tape can usually be used instead.

Constant directivity
Describes a speaker whose directivity is relatively unaffected by the frequency of the sound being reproduced. May also be termed 'uniform directivity'. Generally applied only to horns and full range speakers.

Constant directivity horns exhibit a roll-off in response towards the upper end of their frequency range. In a full-range speaker this is compensated for by the internal crossover, but in multi-amped systems the compensation may need to be provided externally. Compare Exponential horn.

Constant Q
Describes a particular type of frequency response curve, provided by some peaking equaliser (EQ) controls − especially the controls of some graphic equalisers. This response maintains a near-constant value of Q (see Q (1)) as the amount of cut or boost is increased on the control. In the case of a graphic equaliser, this helps to reduce the interaction between adjacent frequency bands of the equaliser. However, as some interaction still occurs, the true overall response of the equaliser is not precisely that indicated by the position of the controls when two or more adjacent bands are adjusted from their 0 dB position. It is claimed by some that constant Q behaviour is the most suitable for correcting the response of microphones, speakers etc., but is not so suitable for creative, sound-shaping applications.

[It should be noted that, in the usual implementation of this type of response, the shape of the boost and cut curves are identical. In such designs the term 'constant Q' is strictly applicable only to boost settings, and not to cuts. This is because the strict definition of Q relates to the bandwidth measured at points 3 dB short of the maximum gain, which in the case of a boost is the uppermost point of the peaking boost curve but which in the case of a cut is the 0 dB ('flat') line of the response. In other words, a 'truly' constant Q control would provide a different response shape when cutting to its shape when boosting − but this is not the behaviour provided by most equalisers described as 'constant Q'. To avoid such ambiguity, some manufacturers use the term 'symmetrical Q' to describe a response that maintains a constant 'true Q' when boosting, but that when cutting maintains a constant Q determined relative to the point of minimum gain, i.e. relative to the lowermost point of the 'peaking' cut curve.] Compare Proportional Q.

Constant voltage
See 100 volt line.

Constructive interference
The phenomenon whereby certain sounds seem to increase in level at a particular position in a room (or indeed outdoors). This is caused by the sound having arrived at this position along two different paths or from two similar sources − e.g. from two speakers − such that the two versions of the sound are essentially in-phase. They therefore reinforce each other, and an increased sound level is heard.

Since, for a given amount of delay, the phase relationship varies with frequency, sometimes only certain parts of a complex sound will seem to increase at a given point in the room − but other parts may increase at other points. Compare Destructive interference.

Consumer
Describes equipment which is mass-produced for use in the home, such as common types of Hi-Fi equipment and audio-visual equipment, as compared to that which is designed for professional or semi-professional use. Also referred to as 'domestic' equipment. Compare Prosumer.

Contact microphone
A microphone that does not pick up sound waves carried through the air, but by direct contact with a solid material through which the sound waves are conducted. Popular as pick-ups for acoustic guitars, pianos, and certain other instruments. See also C-ducer and Sound board (2).

Control surface
The physical surface of a mixer on which its controls are mounted, usually its top surface. Or, an item of hardware that provides a means of control for a remotely located mixer (usually a digital mixer), or which provides physical controls for use in conjunction with a software mixing application, as an alternative to screen-based controls. In some software-based systems the control surface may be located remotely from the computer running the application. See also SAC and DAW.

Convection-cooled
Describes equipment that dissipates heat by natural air circulation, rather than with the assistance of an internal fan. This method of cooling has the advantage of silent operation. The flow of air must not be obstructed, or the equipment may overheat and be seriously damaged. Sometimes the cooling of convection-cooled equipment is assisted by the use of one or more fans installed in the same equipment rack. Convection-cooled equipment is sometimes described as using passive cooling. See also Heatsink. Compare Forced cooling.

Cord
Another name for a cable, especially one no longer than about 5 metres. (This term is used mostly in the USA and mainland Europe.)

Core (1)
An individual insulated wire within a cable. So, a '4-core' cable is one that incorporates four independent conductors, insulated from one another. In the case of a screened cable, the screen is not usually counted as one of the cores. The cores of cables expected to be flexed in use are generally stranded (i.e. have conductors that consist of several strands), whereas some cables intended for fixed installation purposes have 'solid' cores (i.e. conductors consisting of a single strand). See also Multicore, Tinned conductor, Terminal and Pole.

Core (2)
That part of an inductor or transformer whose purpose is to concentrate and contain the magnetic field in the vicinity of the winding(s). Cores are usually made from ferrous materials, and in the case of low-frequency components are constructed of many thin steel laminations. However, audio-frequency inductors are most usually air-cored.

Corner frequency
See Cut-off frequency.

Coulomb
The unit of electric charge. It takes the overall charge of 6.242 x 10¹⁸ electrons to make up one coulomb. (That's a lot!) A charge of one coulomb is transferred by a steady DC current flow of one ampere for one second. See also Voltage.

Coupler
A device that enables the connectors of two cables to be joined together (so linking the cables end-to-end), when the gender of the connectors, or their design, precludes the direct mating of the connectors. See also Male, Female and Gender changer.

Coupling
The transfer of a signal, of noise or of interference from one circuit path into another, often undesirably. For further information see Capacitive coupling, Inductive coupling and Common impedance coupling. Unwanted coupling of a signal (rather than of noise or interference) into the path of a different signal may be referred to as crosstalk.

CPC
An abbreviation for 'circuit protective conductor'.

CPU
An abbreviation for 'central processing unit', the part of a computer (or computer-controlled item of equipment) that performs calculations and makes decisions.

Crank
A slang term meaning to increase the level of a signal or sound. Often used with 'up', as in "Crank it up". The origin of the term is in the meaning of 'to crank' as 'to turn', i.e. to turn up a (rotary) level control. See also Loudness and Pump up.

Crash
A type of cymbal. See also Ride (2).

CRC
An abbreviation for 'cyclic redundancy check'.

Creative leakage
Leakage that is deliberately allowed for artistic effect. Usually relevant only to recording and broadcast situations, rather than to PA work.

Crest factor
The ratio between the highest instantaneous value, positive or negative (measured from zero), occurring in a waveform and the RMS value of that waveform.

Note that the crest factor is not usually of particular interest in PA work; it is not equivalent to the ratio between the peak level of an audio signal and its nominal level, which are both RMS values. (For example, the crest factor of a constant-level sine wave tone is approximately 1.41, but its peak level and nominal level are the same.) Unfortunately, however, the term 'crest factor' is sometimes used to refer to that ratio. See also Dynamic range and Headroom. Compare Form factor.

Crew
A collective term for the personnel involved in the setting up, production and dismantling of an event − other than the performers. Typically includes the sound engineer, lighting engineer, production manager and assistants, stage hands, roadies, drivers, etc. See also Blacks (1).

Crisp
See Clarity.

Critical band
In psychoacoustics, the bandwidth of the auditory filter having a particular centre frequency. It is about 130 Hz at 1 kHz and about 650 Hz at 5 kHz (representing a Q of about 7.7 in both cases).

Critical distance (1) − Microphones
The distance, from a sound source, of a point at which the level of natural reverberation is equal to the level of the sound that has travelled directly to that point from the source. This distance will be dependent upon the acoustics of the room and upon the directional characteristics of the microphone. It is of importance in the placement of microphones, especially in recording work where it is intended that some natural reverberation of the room is picked up.

In PA work, however, it is usual for each microphone to be placed as close as possible to the sound source that it is meant to pick up (subject to still obtaining a suitable picked-up sound quality), so as to increase the picked-up level of the wanted source relative to the pick-up of natural reverberation and of other sources (leakage). (Additionally, increased pick-up of the wanted source means that less gain is applied to the unwanted pick-up of PA speakers, which is helpful in reducing the likelihood of feedback). If reverberation is required, it is normally added artificially by the use of effects.

If a microphone is placed at greater than the critical distance from the sound source that it is meant to pick up, the resulting pick-up is likely to be indistinct because the natural reverberant sound will dominate. See also Radius of reverberation, Reverberation time, Polar response and Distance factor.

Critical distance (2) − Speakers
See Line array.

Critical frequency
In room acoustics, the frequency below which modal behaviour dominates over the reverberant field. Its value is affected by the size of the room and by the degree of absorption. In any particular room, the transition from modal to reverberant behaviour occurs over a range of frequencies rather than abruptly, and so no precise figure can be given for it. However, as a rough approximation, for a room with average absorption the figure can be calculated as 130 x the room surface area (in m²) / the room volume (in m³). See also Diffuse field, Cut-off frequency and Inverse square law.

CRO
An abbreviation for 'cathode ray oscilloscope' − see Oscilloscope.

Cross-fader
A control, the operation of which fades up one signal source whilst simultaneously fading down another signal source, in a single physical action. Not usually encountered on PA equipment, but common on DJ consoles. Usually a slider control. See also Active fader.

Cross-fill
See Side-fill.

Crossover
A filtering device for separating an audio signal into a number of frequency ranges or 'bands', usually two or three. There are two types:

• A passive crossover is an internal component of a passive full range speaker. Its purpose is to direct each frequency range in the output of a single amplifier to the appropriate driver of the speaker. It operates as a single-input, multi-output filter, connected so as to supply the HF part of the signal to the horn(s), the LF part to the woofer(s) and (when the crossover separates the signal into three parts) the mid-range part to the mid-range driver(s). It consists of a network of inductors, capacitors and resistors, and therefore requires no power source of its own. Passive crossover units also sometimes incorporate driver protection components, particularly to guard against overload of the horn.
• An active crossover may be encountered either as a separate item of equipment that allows a separate amplifier and speaker to be used for each frequency range, or as an internal component of a multi-amplifier powered speaker. Like the passive crossover it operates as a single-input, multi-output filter, but in this case it is connected between the output of the mixer and the inputs of the amplifiers. The amplifier that is connected to the HF output of the crossover drives the HF speakers (or horn(s)), the amplifier connected to the LF output of the crossover drives the bass bins (or woofer(s)) and (when the crossover separates the signal into three parts) the amplifier connected to the mid-range output of the crossover drives the mid-range speakers (or mid-range driver(s)). It requires its own power source (usually a mains supply).

'Crossover' may also be written 'cross-over' or 'cross over', and is sometimes abbreviated to 'X-over' or 'Xover'. See also Order, Linkwitz-Riley, Butterworth, Chebyshev and the next definition.

Crossover distortion
Absolutely nothing to do with a crossover device! Rather, it is the distortion that is introduced into a signal by an amplifier not operating linearly around the zero-crossing part of the signal waveform.

The particularly significant thing about this form of distortion is that, in contrast to that caused by overload, it is of a relatively constant magnitude and is therefore most noticeable at very low output levels.

It can occur as a result of poor amplifier design, as a result of a fault, or as a result of mis-adjustment of the output stage quiescent current. For further information see Amplifier Classes on the Amps and Speakers page.

Crossover frequency
The frequency at which a crossover splits the higher frequencies from the lower ones. Sometimes referred to as the 'turnover frequency'.

Crosstalk
The appearing of a signal from one circuit, in another circuit in which it does not belong. It typically occurs in cabling due to capacitive or inductive coupling between circuits in close proximity, but can also occur within equipment (for example, due to common impedance coupling). The use of balanced interconnections is usually of significant help in the avoidance of crosstalk problems. See also Separation.

CRT
An abbreviation for 'cathode ray tube' − the original device for creating the visible image in televisions and computer display monitors. Due to the large size and weight of this device (and the extremely high voltages it requires), it is gradually being superceded by other devices such as liquid crystal displays (LCDs) and plasma screens. The term 'CRT' is therefore now used to describe display equipment that uses the earlier technology. See also Raster.

Crystal-controlled
Describes equipment, or a reference signal of some kind, whose operating frequency is accurately controlled by means of a quartz crystal, or by locking the equipment or signal to a central crystal-based reference source. See also DARS.

CSA
An abbreviation for 'cross-sectional area', a measure of the gauge of an electrical conductor, especially of a cable conductor.

CTS
An abbreviation for 'Certfied Technology Specialist', a USA-based AV qualification provided by InfoComm International. For further information see the InfoComm website (opens in a new window).

Cue
An indication that something needs to happen imminently, or is about to happen imminently. For example, an indication to a sound engineer to start a music track, or to a lighting engineer to blackout the lights. Cues may be explicitly provided manually by a Director or other person (using visual or audible means, for example by saying the word "cue" via an earpiece), or may be electronically programmed. Alternatively, they may be implicit in the performance (e.g. "Start the music when the actor falls to the floor."). Sometimes abbreviated to 'Q'. See also LXQ and IFB.

Current
The rate of flow of electricity. Or, more precisely, the amount of electrical charge flowing per unit time. It is measured in amperes (amps for short − not to be confused with amplifiers!). A current of one amp means that one coulomb of charge is flowing per second.

A current will flow between two points whenever there is a voltage difference between those points and, at the same time, a conducting path exists between them.

If the flow of current rapidly exhausts the source of voltage difference, then the current will be short-lived − this is called a static electrical discharge. But if the source of voltage difference remains persistent then the current will be continuous, and such currents are much more useful.

For a continuous current to flow, two conditions must be satisfied simultaneously. Firstly, there must be a complete (i.e. unbroken) conducting path, or 'circuit', for the current to flow around, and secondly there must be a continuous applied voltage, to drive the current around that path. If the applied voltage varies, then the resulting current will usually vary in a similar manner.

If, regardless of any variations in the current, it always flows in the same direction, then it is described as a 'direct current' or 'DC'. This is the type of current obtained from a battery. If, however, the current repeatedly reverses its direction of flow then it is described as an 'alternating current' or 'AC'. All audio signals, and most mains supplies, are AC. In the case of AC having a repetitive waveform, one complete pattern of reversals is called a 'cycle', and the rate at which the cycles occur is called the 'frequency'.

[Academic note: Conventionally, current is assumed to flow from the positive terminal of a source of voltage, and back to its negative terminal. However, in reality a current consists of a flow of electrons, and these actually move in the opposite direction! The original reason for this inconsistency was just that the people who long ago decided upon the convention didn't know about electrons and how they move. It has been retained because it makes circuit analysis equations simpler.]

The amount of current that flows around the path will depend upon the applied voltage level and upon the overall amount of opposition to the flow of current that exists in the circuit path. This opposition is referred to as 'impedance' (or 'resistance' in the case of DC). The voltage (in volts) divided by the impedance or resistance (in ohms) gives the current in amps. For other calculations involving current, see the How do I calculate ...? question on the FAQ page.

Electric current can be dangerous, for two main reasons:

• If sufficient current flows through the body then serious injury or death can be caused by electric shock. Death is typically caused by disruption of the heartbeat, and the amount of current necessary for this to occur is quite small (for example around 40 milliamps (0.04 A), though this depends many factors such as the age and health of the person). Serious burns may also be caused, and/or other injuries such as falls.
• A flow of current always results in the generation of heat in the conductors, and if the resulting temperature rise is excessive then serious damage could be caused to cables, connectors or equipment, or a fire could be started. Therefore, it is essential that effective safety measures are put in place to reduce the risk from such hazards to acceptable levels. Such heating considerations are generally only of significance in cases where either the intended current flow is substantial (e.g. greater than 2 A), or where the current flowing in the event of a short circuit would cause damage, i.e. in power distribution arrangements. In such cases, all cables and other current-carrying equipment must be adequately protected against current overload and against short-circuit currents, typically by means of fuses and/or MCBs. These considerations are not generally significant in the case of signal interconnections, with the exception of power amplifier to speaker interconnections (see Gauge).

It is useful to know how much current will be taken from the mains supply by mains-powered equipment, especially current-hungry equipment such as power amplifiers and lighting, because:

• The current rating of mains cables, connectors and power distribution equipment must not be exceeded.
• The total current demand of all the current-using equipment in use must not exceed the capability of the supply.

The maximum current drawn by a particular item of powered equipment can be calculated by dividing its power input rating in watts by the supply voltage in volts (taking into account its power factor, where relevant), or by dividing its VA rating by the supply voltage. (Note that the maximum power drawn from the mains supply by a power amplifier will be somewhat greater than its audio power output rating − see Efficiency.)

The maximum current that may be safely carried by a conductor of a cable without overheating is governed by many factors, including the following. (If in doubt, refer to the cable manufacturer's information.)

• The gauge of the conductor − this determines the amount of heat produced per unit length, for a given current flow. (It also affects voltage drop − see below.)
• The type of insulation surrounding the conductor − some types can withstand higher temperatures than others.
• The maximum ambient (i.e. surrounding) temperature in which the cable will be used − this affects the ability of the cable to lose heat.
• The method of installation and type of materials in contact with the cable − these factors also affect the ability of the cable to lose heat.
• The close proximity of other cables or equipment producing heat.
• The duration of, and interval between, the maximum flows of current.

In addition to a heating effect, a flow of current also results in a magnetic field around the conductor (see Inductive coupling) and in a voltage drop along the length of the conductor. The overall voltage drop in a cable, for a given flow of current, will depend on the gauge of the conductors and on the cable's length. See also Insulator and RCD.

Current dumping
Describes a power amplifier that utilises a specific type of output stage. Popular in the 1980's among audiophiles and for studio monitoring applications. For more information on amplifiers see the Amplifiers and Speakers page.

Curve
A term often used to refer to a particular type of frequency response, particularly of an equaliser or a crossover. Or, a general term for the shape of a particular frequency response graph.

Cut
A common term for attenuation; a reduction in level, usually specified in decibels. The term may be refer to a reduction across the whole audio spectrum, or, more often, refers to a reduction at specific frequencies only (filtering) − e.g. by use of an equaliser. Or, as a verb, to make such a reduction in level. Compare Boost.

Cut-off frequency
The frequency at which the attenuation of a filter (or other frequency-limited device) is 3 dB greater than the attenuation at frequencies well within its passband. Also called the 'corner frequency' or the '3 dB point'.

For example, a low pass filter will allow frequencies below the cut-off frequency to pass relatively unaffected, while those increasingly above the cut-off frequency suffer progressively greater attenuation. Likewise, a high pass filter will allow frequencies above the cut-off frequency to pass relatively unaffected, while those increasingly below the cut-off frequency suffer progressively greater attenuation. A bandpass filter will have two cut-off frequencies, respectively called the 'lower' and 'upper' cut-off frequencies.

In room acoustics, the room cut-off frequency is the frequency of the lowest possible mode − that is, the frequency whose half-wavelength is equal to the largest dimension of the room. At frequencies below this value the room will have an increased loading on sound sources, which is likely to result in a reduction in their ability to radiate such sounds into the room. Its value (in Hz) can be calculated as 172 / the largest dimension (in metres). This shows that room cut-off is of significance to PA applications in very small venues only, as its value will be below 20 Hz if any dimension exceeds 8.6 m. See also Shelving response and Critical frequency.

CVS
An abbreviation for 'composite video and syncs'. (As the term 'composite video' is generally understood to refer to a signal which includes synchronisation information, the 'S' in this abbreviation could be considered to be redundant.)

Cyc
A slang abbreviation for 'cyclorama'. However, in a stage lighting context, it may also be used as a slang term for a specific type of lantern, used for lighting cycloramas. Pronounced 'sike'.

Cycle
One complete pattern of change of a repetitive waveform. The time taken for one cycle is called the period, and the number of cycles occurring per second is called the frequency. See also Wavelength and Phase (1).

Cyclic redundancy check
An extra value that is often incorporated into stored or communicated data at regular intervals (or just at the start or end of the data), in order to facilitate subsequent checking of the data’s integrity. Commonly abbreviated to 'CRC'.

Each CRC value is calculated from the preceding set of data values (since the previous CRC, if any) by the storing or sending equipment, according to a pre-defined method. The reading or receiving equipment then performs the same calculation on the data and compares the resulting CRC value with the read or received one; any discrepancy indicates an error in the data (or in the CRC). See also Checksum, Parity, Bit error rate and Error correction.

Cyclorama
A large cloth drop, forming a background for a theatre stage set. See also Cyc, Flat (2) and Scene (1).

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This page last updated 03-Feb-2012.