While every intercom product line is different, most analogue intercom systems have much in common. Voice signals of about a volt or two are carried atop a direct current power rail of 12, 30 or 48 volts which uses a pair of conductors. Signal light indications between stations can be accomplished through the use of additional conductors or can be carried on the main voice pair via tone frequencies sent above or below the speech frequency range. Multiple channels of simultaneous conversations can be carried over additional conductors within a cable or by frequency- or time-division multiplexing in the analogue domain. Multiple channels can easily be carried by packet-switched digital intercom signals.
Portable intercoms are connected primarily using common shielded, twisted pair microphone cabling terminated with 3-pin XLR connectors. Building and vehicle intercoms are connected in a similar manner with shielded cabling often containing more than one twisted pair.
Two-wire broadcast intercoms
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 whom 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.
Four-wire broadcast intercoms
In the mid-90s four-wire technology started gaining more prominence due to the technology getting cheaper and smaller. Four-wire circuit technology had been around for quite some time but was very expensive to implement. It usually required a large footprint in the physical television studio, thus was only used at very large television stations or television networks.
Basic intercom system terms
- Master Station or Base Station – These are units that can control the system, i.e., initiate a call with any of the stations and make announcements over the whole system.
- Sub-station - Units that are capable of only initiating a call with a Master Station but not capable of initiating calls with any other stations (sometimes called slave units).
- Door Station - Like sub-stations, these units are only capable of initiating a call to a Master Station. They are typically weather-proof.
- Intercom Station - Full-featured remote unit that is capable of initiating and receiving party-line conversation, individual conversation and signalling. May be rack-mounted, wall-mounted or portable.
- Wall Mount Station – fixed-position intercom station with built-in loudspeaker. May have flush-mounted microphone, hand-held push to talk microphone or telephone-style handset.
- Belt Pack - portable intercom station worn on the belt such as an interruptible feedback (IFB) with an earpiece worn by talent.
- Handset - permanent or portable telephone-style connection to an intercom station. Holds both an earpiece and a push to talk microphone.
- Headset - portable intercom connection from a belt pack to one or both ears via headphones with integrated microphone on a boom arm. Connects to a belt pack.
- Paging Signal - An audible and/or visual alert at an intercom station, indicating that someone at another station wants to initiate a conversation.
- Power Supply - Used to feed power to all units. Often incorporated into the design of the base station.
- Telephonic - Intercom systems that utilize a transmitter (talk) and receiver (listen) to communicate, similar to standard telephone systems.
- Amplified - Intercom systems that utilize a single speaker to transmit and receive communications.
- Amplifier - An intercom systems main or central component that is responsible for switching or connecting communication paths between master and substations in addition to distributing or switching power to auxiliary connected devices i.e. door strikes to allow entry.