27 Nov 2018

Technical Terms Used In Comms


Technical Terms Used In Comms

by Simon Byrne.

A mix-minus, or clean feed, is a particular way to setup a mixing console, such that a mix-minus output of the mixer contains everything except the associated input. For example, in a teleconference, the remote presenter’s audio is fed into the PA system, whereas the output fed back to the remote presenter is everything in the mix, minus themselves.

Get it? This is to prevent echoes or feedback in broadcast or sound reinforcement systems.


A true mix-minus is often very hard to achieve in a live situation because the incoming remote audio is fed through the PA, that sound is picked up on the local presenter’s microphone, and inadvertently fed back in the feed to the remote presenter.

Solutions to this include very close miking of the local presenters so less room audio gets in, automixing where the local microphones are turned down while the remote presenter talks, and vice versa (Dugan automixing is fantastic for this [CX June – Automixing – Your Extra Hand] and giving the local presenters earpieces so less room level is required.

Mix-minus is also often used together with IFB systems for television news reporters and interview subjects speaking to a host from a remote outside broadcast (OB) location. Because of the delay that is introduced in most means of transmission (including satellite feeds and audio over IP connections), the remote subject’s voice has to be removed from his earpiece. Otherwise, the subject would hear himself with a distracting delay.

Full Duplex
Refers to the transmission of audio in two directions simultaneously. For example, a two wire talkback circuit like an analogue Clear-Com party line is a full-duplex circuit because both parties can talk at once.

Half Duplex
In contrast, a two-way radio is a half-duplex device because only one party can transmit at a time.

Two Wire, and Four Wire Circuits
A two wire circuit is transmission of audio in two directions simultaneously on a single pair, for example, party line talkback systems. Whereas four-wire circuits have separate pairs for transmit and receive.

Hybrid Transformer Arrangement
A Hybrid Transformer is a very clever design of the transformer coils that converts a two wire circuit into a four wire, which is needed when interfacing to broadcast and PA systems (otherwise you would have a feedback loop).
The principle is that of impedance matching. The incoming signal is applied to both the two wire line and a “balancing network” that is designed to have the same impedance as the line. The outgoing signal is derived by subtracting the two, thus cancelling the incoming signal from the outgoing signal.


From the November 2018 edition of CX Magazine. CX Magazine is Australia and New Zealand’s only publication dedicated to entertainment technology news and issues – available in print and online. Read all editions for free or search our archive
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Further reading on Comms:
Technical Terms Used in Comms
– Simon Byrne (CX Nov 2018)
Communications – A ‘How To’ – Jimmy Den Ouden (CX June 2009)
Talkback Systems Compared – Andy Ciddor (Connections July 1999)




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