Talkback originated in radio studios, where staff in the control cubicle who could hear the presenters and performers on their monitor loudspeaker were able to communicate with them using a dedicated microphone/loudspeaker system, “talking back” to the studio. This system was extended by the introduction of “reverse talkback” which provided the presenter with an additional microphone which communicated with a dedicated loudspeaker in the cubicle, usually fitted within the mixer desk. This enabled the presenter to talk to the staff in the control cubicle when the studio microphone was not faded up.
Extending this principle to inter-area communications led initially to wired Matrix systems and eventually to today’s digital matrix intercom systems.
For duplex two independent channels of communication are required. These may be two physical wired circuits, also known as a 4-Wire circuit, or two radio frequencies. These two channels are designated “Send” and “Receive” relative to the main site, and can be configured in various ways:
Both ends of the connection can hear and talk at the same time, like a normal telephone. If the “Receive” circuit is looped back to the “Send” circuit at the main site all others will also hear what is on the receive circuit.
The person talking on a circuit cannot hear when they talk. If the “Receive” circuit is looped back to the “Send” circuit at the main site all others will also hear what is on the receive circuit.
Only one person can talk at a time. Ordinary, single frequency, walkie-talkies are simplex in operation.