Two-Wire Vs. Four-Wire Broadcast IntercomsTwo-Wire Broadcast Intercoms:
are commonly used in television stations and outside broadcast vehicles such as those seen at sporting or entertainment events. Two wire phone exchange systems provide signaling, intercom and paging functions. The system monitors and detects either a dual tone multi-frequency tone or a single tone generated on the line at the local telephone, activates a relay and places a calling party on hold and connects music to the incoming line. A party on hold is reconnected to the answering telephone by depressing a tone signal generating key button. There is an audible warning tone reminder for calls placed on hold which are not picked up after some desired time. The system also provides in-house intercom and paging functions and a release from these functions. When several local telephone instruments are connected on the same line, this system allows the recipient of a call to receive the call at any of the telephones by depressing a tone generating key button.
Four-Wire Broadcast Intercoms:
In the mid-90's four-wire technology started gaining more prominence due to the technology getting cheaper and smaller. Four-wire technology had been around for quite some time but was very expensive to implement. It usually required a large footprint in the physical TV Plant, thus was only used at very large stations or TV networks. Also, the large physical size made it virtually impossible to use on a mobile platform such as an outside broadcast vehicle. The term four-wire comes from the fact that the system uses a transmit pair and a receive pair for the audio to and from the intercom, i.e. four wires. That said, in a modern four-wire system there are actually six to eight wires: two (or four) for data and the remaining four for audio. There are also a few manufacturers that use digital audio techniques in the form of fiber or coax cable. Nevertheless, the four wire phrase has stuck, and it is the accepted term for this kind of system today.