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  Glossary of PA Terms - J

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The glossary pages provide definitions for over 2680 PA-related terms. If you can't find the term you are looking for, or would like any of the existing definitions to be expanded, please email me − likewise of course if you find any errors in the links etc. Use of this information is conditional upon acceptance of the Disclaimer on the PAforMusic home page.

J-curve * Jack * Jack-to-jack * Jack field * Jacket * Jacketed multicore * Jackfield * JFET * JFMG * Jitter * Johnson's noise * Joule * Joules * Jumper

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.

J-curve
See Line array.

Jack
A type of connector commonly used for audio interconnections. In PA work, the term usually refers to the variety whose male connector has a barrel (or 'sleeve') of ¼″ (6.35 mm) diameter, which fits the style of socket that is commonly found on guitar and keyboard outputs, instrument amplifier inputs, speakers and the line-level inputs and insert connections of mixers. These jacks come in two types: the 2-pole 'mono' version is used for unbalanced mono connections, whilst the 3-pole 'stereo' version (also called a TRS connector) may be used for unbalanced stereo connections, for balanced mono connections, or for mixer insert connections. Both types of this style are properly known as 'A-type' ('A-gauge', or 'type A') connectors.

Two miniature sizes of this style of jack are available, having sleeve diameters of 3.5 mm and 2.5 mm respectively (known respectively as 18″ and 332″ in the USA, though these dimensions are only approximate). Both of these sizes are available in mono (2-pole) and stereo (3-pole) versions and are most commonly used for connections to personal music players and to computer sound cards and laptops. The 3.5 mm size is frequently referred to as a 'mini-jack'.

A more recent innovation is a 4-pole version of the 3.5 mm jack. This is used as a headset (or earphone plus microphone) connector on mobile devices such as mobile phones and tablets and as a space-saving device on some computer sound cards, where many output channels are provided (e.g. for 7.1 analogue audio connections). Note that where such sockets are designed to accept 3-pole as well as 4-pole jacks, it is common practice for the signal earth to be carried on the 'ring 2' pole (the furthest ring from the 'tip' pole) rather than on the sleeve pole (for details see TRRS). However, check the card manufacturer's specifications. Another application of the 4-pole 3.5 mm jack is for 5.1 digital audio connections, where usually the tip carries the Front Left and Front Right signal, ring 1 carries the Rear Left and Rear Right signal and ring 2 carries the Centre and Sub-bass signal.

The 'B-type' ('B-gauge', or 'type B') jack has the same diameter sleeve as the A-type ones, but has a rounded and narrower tip and a narrower ring (it is generally available only as a 3-pole version). It is used almost exclusively in standard-sized patch bays, and is also known as a 'longframe jack' or as a 'Post Office jack', 'GPO jack' or 'phone jack' because of its original use in the manual switchboards of the British telephone network − however if the term 'phone jack' is used beware possible confusion with the 'phono' audio connector or with the modern style of telephone connectors (e.g. BT431a or RJ11), both of which are entirely different types of connector.

Some patch bays use a miniature version of the B-type jack, to enable more connectors to be accommodated per row. These have a sleeve diameter of 4.4 mm and are called Bantam jacks or TT (Tiny Telephone) jacks; both names are trademarked by Switchcraft.

Note that the ¼″ A-type and B-type connectors are not compatible. At the very least, connections between them would be unreliable, and there is a real danger of permanent damage being caused to the sockets if mating is attempted.

For information on the connections of all the 3-pole versions of jacks, see TRS.

[Note on history of usage: The term 'jack' historically relates to any mechanical device, especially one in which a lifting action is involved. In an electrical context, the term originally referred to a fixed female socket connector in which insertion of the plug operates one or more sets of make or break switching contacts (which may or may not be electrically independent of the connector contacts). Such switching contacts were originally required in the sockets of manual telephony switchboards, where the orientation of the sockets was usually such that the insertion of a plug operated the contacts via a lifting mechanism − hence the the adoption of the term 'jack' for these sockets. However, in time the term 'jack' was also applied to similar styles of socket, such as those used in PA applications, regardless of whether or not they incorporated switching contacts. The male plug which mates with these styles of socket was therefore referred to as a 'jack plug'. More recently, the word 'plug' was dropped (as superfluous) from that term. So, in modern usage, the term 'jack' may refer either to a female socket of any of the types described above (with or without switching contacts), or to its mating plug.]

View jack image (A-type 6.35mm)

Jack-to-jack
Describes, or refers to, a cable that is fitted with jack plugs at both ends. The term is nearly always used to refer to an unbalanced screened cable, such as an instrument cable fitted with 2-pole plugs. Short screened jack-to-jack cables are frequently used as patch cables to interconnect musicians' effects pedals. Unscreened jack-to-jack speaker cables constructed from heavy-gauge cable are used to connect speakers to amplifiers in some low-power applications such as to connect a backline speaker to its head. These speaker cables are not interchangeable with screened jack-to-jack types intended for small-signal applications, such as for connections to instruments or effects.

Jack field or Jackfield
A substantial number of jack sockets located close together, especially on a patch bay.

Jacket
The outer protective covering of a cable − see Sheath. Or, the insulating covering of an individual screened pair within a multicore cable.

Jacketed multicore
A multicore cable whose pairs have jackets.

Jackfield
See Jack field.

JFET
An abbreviation for 'junction field effect transistor', a specific type of FET. The term is often used to describe operational amplifiers ('op-amps') that have input transistors of this type.

JFMG
An abbreviation for 'Joint Frequency Management Group', the organisation previously responsible for the issuing of licences for radio microphones, in-ear monitoring systems, etc. in the UK. This role is now undertaken by PMSE, operating under the control of Ofcom − see PMSE.

Jitter
Unwanted variations in the timing of the voltage transitions in a digital signal. A small amount of jitter is inevitable, but if excessive amounts are present then the destination of the signal may have difficulty in synchronising with it, and as a result data errors may occur. See also Bit error rate.

Johnson's noise
See Thermal noise.

Joule, Joules
The unit of energy. If energy is being transferred at a constant rate then the amount of energy transferred, in joules, is given by the power level (in watts) multiplied by the duration of the transfer (in seconds).

Jumper
A device, located on a circuit board inside an item of equipment, that allows rarely-changed operating parameters to be adjusted. For example, to configure whether an insert point is pre- or post-EQ. Used by manufacturers to reduce cost, save front panel space, and restrict access to skilled persons. To change the setting once access has been gained, it is necessary to remove the jumper from its present position on the pins, and then replace it in a different position − always consult the handbook first. Effectively, a switch that is inaccessible to operators during equipment use. Also called a 'link'.

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This page last updated 07-May-2017.