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Intercom systems are widely used in TV stations and outside broadcast vehicles such as those seen at sporting events or entertainment venues.

There are essentially two different types of intercoms used in the television world: two-wire party line or four-wire matrix systems.

In the beginning, TV stations would simply build their own communication systems using old phone equipment.

However, today there are several manufacturers offering off-the-shelf systems.

From the late 1970s until the mid-90s the two-wire party line type systems were the most popular, primarily due to the technology that was available at the time.

The two channel variety used a 32 Volt impedance generating central power supply to drive external stations or belt packs.

This type of format allowed the two channels to operate in standard microphone cable, a feature highly desired by the broadcasters.

These systems were very robust and simple to design, maintain and operate but had limited capacity and flexibility as they were usually hardwired.

A typical user on the system could not choose who to talk to.

He would communicate with the same person or group of people until the system was manually reconfigured to allow communication with a different group of people.

Two-wire routers or source assignment panels were then implemented to allow quick re-routing.

This reconfiguration was usually handled at a central location, but because voltage is used on the circuit to power the external user stations as well as communicate, there would usually be a pop when the channels were switched.

So while one could change the system on-the-fly, it was usually not desirable to do so in the middle of a production, as the popping noise would distract the rest of the television crew.