Intercom Wiring

An intercom (intercommunication device) is an electronic communications system intended for limited or private dialogue, direction, collaboration, or announcements. Intercoms can be portable or can be mounted permanently in buildings and vehicles. Intercoms can incorporate connections to walkie talkies, telephones, cell phones, and other intercom systems over phone or data lines, as well as to electronic or electromechanical devices such as signal lights and door latches.

Permanent intercoms installed in buildings are generally composed of fixed microphone–speaker units that are connected to a central control panel by wires. A small home intercom might connect a few rooms in a house. Larger systems might connect all of the rooms in a school or hospital to a central office. Intercoms in larger buildings often function as public address systems. In many schools, tones signaling the change of classes are sounded over the intercom; these have taken the place of the electromechanical bells used in older schools. Additionally, many schools now use audiovisual intercoms to identify visitors trying to gain access to a locked school building. Many intercom systems can be interfaced with the building's access control system.

Intercom systems can be found on many types of vehicles, including trains, watercraft, aircraft, and armored fighting vehicles. Portable intercoms are commonly used by special event production crews and professional sports teams. Performing arts venues such as theaters and concert halls often have a combination of permanently mounted and portable intercom elements. Motorsports race tracks often have both portable and permanent intercom stations mounted at critical points around the racecourse for use by race officials and emergency medical technicians.

Traditional intercom systems are composed entirely of analog electronics components, but many new features and interfacing options are available with newer intercom systems based on digital connections. Video signals can be interlaced with the more familiar audio signals. Digital intercom stations can be connected using Cat 5 cable and can even use existing computer networks as a means of interfacing distant parties.

Intercom system in the Pittock Mansion

Basic Terms

• Master Station or Base Station—Units that can control the system, i.e., initiate a call with any of the stations and make announcements over the whole system.
• Substation—Units that are capable of initiating a call with a Master Station but are not capable of initiating calls with any other stations (these are sometimes called "slave units").
• Door Station—Like substations, these units are only capable of initiating a call to a Master Station. They are typically weatherproof.
• Intercom Station—Full-featured remote unit that is capable of initiating and receiving party-line conversation, individual conversation, and signaling. These may be rack-mounted, wall-mounted, or portable.
• Wall-Mount Station—Fixed-position intercom station with built-in loudspeaker. A wall-mount stations may have a flush-mounted microphone, a hand-held push-to-talk microphone, or a telephone-style handset.
• Belt Pack—Portable intercom station worn on the belt. Requires a headset or handset.
• 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.
• Power Supply—Used to feed power to all units. Often incorporated into the design of the base station.

Wiring Intercoms

While every intercom product line is different, most analog 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 that uses a pair of conductors. Signal light indications between stations can be established 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 analog 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. Digital intercoms use Category 5 cable and relay information back and forth in data packets using the Internet protocol architecture.

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 and 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 1990s, the two-wire party line systems were the most popular, primarily due to the technology that was available at the time. The two-channel variety used a 32 volt impedence-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 were simple to design, maintain, and operate, but they had limited capacity and flexibility since 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 panelswere later implemented to allow quick rerouting of a two-wire circuit. 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 to communicate, there would usually be a pop when the channels were switched. therefore, 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 production crew.

Four-wire Broadcast Intercoms

A modern four-wire intercom system capable of 272 sources and destinations manufactured by Telex Communications, Inc.

In the mid-1990s four-wire technology started gaining more prominence because the technology was getting cheaper and smaller. Four-wire circuit technology had been around for quite some time but had been very expensive to implement. It usually required a large footprint in the physical television studio, and thus was used only 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 term four-wire has stuck, and it is the accepted term for this kind of system today. One major advantage of four-wire vs. the two-wire systems is the ability to perform point-to-point communication at will. Point-to-point communication allows a user to speak directly to another user in much the same way as someone would speak to another person directly using a telephone. This ability is extremely useful in today's complex production environments. The difference between a phone system and a four-wire intercom is that the four-wire intercom also permits point-to-multipoint communication, party lines, interrupt fold back (IFB), and many other configurations that are useful in the production environment.

Wireless Intercoms

For installations where it is not desirable or possible to run wires to support an intercom system, wireless intercom systems are available. There are two major benefits of a wireless intercom system over the traditional wired intercom. The first is that installation is much easier since no wires have to be run between intercom units. The second is that you can easily move the units at any time. With that convenience and ease of installation comes a risk of interference from other wireless and electrical devices. Nearby wireless devices such as cordless telephones, wireless data networks, and remote audio speakers can interfere. Electrical devices such as motors, lighting fixtures, and transformers can cause noise. There may be concerns about privacy since conversations may be picked up on a scanner, baby monitor, cordless phone, or similar device on the same frequency. Encrypted wireless intercoms can reduce or eliminate privacy risks, while placement, installation, construction, grounding, and shielding methods can reduce or eliminate the detrimental effects of external interference.

Power line communication units that send signals over house wiring have been referred to as "wireless" intercoms. Though they are technically wired intercoms, they are based on existing wiring and thus require no additional wires.

Loudaphone brand intercom station aboard RMS Queen Mary. The Loudaphone was specified for noisy environments such as aboard trains and trams and within the engine rooms of ships.