Information about Intercom

An intercom (intercommunication device) is an electronic communications system within a building or group of buildings. Intercoms are generally composed of fixed microphone/speaker units which connect to a central control panel. 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, capable of broadcasting announcements.

In many schools, tones signaling the change of classes are sounded over the intercom, taking the place of the electromechanical bells used in older schools.

Intercom systems can also be found on passenger and rapid transit trains.

Basic terms

• Master Station - These are units that can control the system, i.e., initiate a call with any of the stations .
• Sub-stations - 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 Stations - Like sub-stations, these units are only capable of initiating a call to a Master Station. They are typically weather-proof.
• Power Supply - Used to feed power to all units.

An intercom may be connected to a door buzzer (see access control).

Wiring intercoms

While every system is different, most systems have much in common. The following is based on wiring for the LEF system from Aiphone, which is a popular brand in the Northeastern United States for residential use.

These must be connected to a 12v DC power supply:

• + Positive
• - Negative

Other wires include:

• E, or common wire
• C, or unit identifier / call terminal.
• R, or occupied terminal.
• number terminals (1 to 10 ) depending on master unit.

If one connects a wire color to the C screw terminal in a unit (called the 1 screw terminal in door stations), energizing that wire (by pressing the button associated with the screw terminal the wire is connected to) will initialize a call. In other words, if one attaches the pink wire to the LEF5C in the hallway, and attach the pink wire to screw terminal 4 in the LEF5C in the study, pressing button 4 in the study will energize the pink wire and the common (E) wire. This will initiate a call between the study and the hallway units, with the study unit monitoring the hallway unit.

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.^[1] 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 70's until the mid 90's 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 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 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 to the rest of the production crew.

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 circuit 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. 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 similar to how someone would call another person directly using a phone. This ability is extremely useful in today's complex production environments. The difference between a phone system, however, and a four-wire intercom is the ability to not only perform point to point but also point to multi-point, party-lines, interrupt fold back (IFB) and many other configurations, which are useful to the production environment. It should be noted that four-wire systems are essentially audio routers. This makes them very useful not only for the communications aspect of a production, but also routing of audio for confidence monitoring or actual on-air use. In the past, forms of communications such as IFB, audio monitoring and, point to point all had to be separate systems. In the modern four-wire intercom system, these forms of communications are typically all in one compact package. Today the tables have turned, making a four-wire system cheaper and easier to implement than a two-wire system.

Wireless intercoms

For installations where it is not desired to run wires to support an intercom system, there are also wireless intercom systems 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. But with that ease of installation and convenience there is risk of interference from other wireless and electrical devices. Other wireless devices near the intercom such as cordless telephones, wireless data networks, and remote audio speakers can interfere. Electrical devices such as motors can also cause noise. There may also be less privacy since conversations may be picked up on a scanner, baby monitor, cordless phone, or a similar device on the same frequency. However, there are wireless intercoms that reduce or eliminate both of these risks. Wireless Systems in the United States and Canada have several frequency ranges for wireless intercom systems and other wireless products. They are 49MHz, FM band (200KH - 270KHz), 900MHz, 2.4GHz, 5.8GHz, and MURS (150 MHz). There are also Power Line Carrier units that communicate over house wiring that are referred to as wireless intercoms ^[2].

See also

• Courtesy phone

References