Versatile Single Channel Four Wire Desktop Unit

Intercoms using 4-wire topography are often used in broadcast environments, particularly in outside broadcasting, where the simplicity of the wiring simplifies the connection of an ad-hoc or semi-permanent system to communicate between personnel and, with IFB, provide cue to operators and presenters. The models in this selection are all designed to inter operate with equipment using standard broadcast operating levels and connectors. They range from a single-circuit belt-pack for use with headphones to six circuit, rack-mounting intercoms, with integrated loudspeaker and talkback microphone.

Terminology

4 Wire Circuit – 4W circuit

This is a term used here to describe communication between units using two separate balanced audio circuits; one to each unit, one from each unit.
Unless otherwise described, a ‘4-wire circuit’ means a balanced input using a 3-pole female XLR connector and a balanced output using a 3-pole male XLR, both at nominally 0dB.

2-Wire

A balanced audio circuit, containing a mix of both send and receive signals.

IFB

An acronym for interruptible or interrupted feedback or foldback, sometimes also known as ‘programme cue interrupt’, ‘switched talkback’ and ‘interrupt for broadcast’.

Unless otherwise described, it is used here to mean a facility to receive an audio signal – commonly a feed of the programme, possibly interrupted by an upstream talkback unit to inject voice communications – and to send that signal out on the 4-wire output, interrupted by a local talkback microphone when a talkback button is operated.

Headphone socket

Unless otherwise described, this is used here to mean a 6.35mm, 3-pole jack socket providing an output to headphones with an impedance of 200 ohms or higher. Unless otherwise stated there is an associated volume control, which, in desktop and rack mounting designs will be shared with a panel loudspeaker.

Microphone input

Here this means a female 3-pole XLR connector feeding an input suitable for a dynamic microphone of 200 ohms or greater impedance. There is an associated gain control. If phantom power is available, it may be used with condenser or electret types with appropriate powering requirements.

Panel microphone

Rack mounting and desktop designs have a stub microphone, unless otherwise described, mounted on the panel facing the user. This is accompanied by a 3-pole female XLR socket; an alternative microphone input selected by a switch.

Panel loudspeaker

Rack mounting and desktop designs have a loudspeaker, unless otherwise described, mounted on the panel facing the user, with a volume control shared by a headphone socket, which deactivates the speaker when a jack is inserted. The speaker is dimmed when a talk key is operated.

‘Talk’ switch

This used here to mean a switch which is operated when the user wishes to talk to units fed from the 4-wire output. Several types are used by these designs.

Three position talk switch – ‘3-way talk switch’

This locks in the ‘up’ and centre positions. Pressure must be maintained on the switch to hold it in the ‘down’ position. In ‘up’ and ‘down’ positions, the local talkback microphone is sent to the 4-wire output. In the centre position nothing is sent to the 4-wire output unless the unit has an IFB facility.

Momentary talk button – ‘PTT switch’

Pressure must be maintained on the switch to hold it in the ‘on’ position, sending local talkback to the 4-wire output. When released, no talkback is sent to the 4-wire output unless the unit has an IFB node, in which case the incoming 4-wire signal can be sent to the 4-wire output.

XLR input

This is used here to mean a balanced, line-level, input using a female 3-pole XLR socket.

XLR output

This is used here to mean a balanced, line-level, output using a male 3-pole XLR socket.