Intercom
From Wikipedia, the free encyclopedia
An intercom 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.
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[edit] 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 initating 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).
[edit] 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 very 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.
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.
[edit] Broadcast Intercom
 A Four-Wire intercom capable of 272 sources and destinations |
[edit] TWO-WIRE
Intercom systems are widely used in TV stations and outside broadcast vehicles such as those used at sporting events or entertainment venues. There are essentially two different types of intercoms used in the television world, Two-Wire party lines or Four-Wire matrix systems. In the beginning of TV broadcasting stations would simply build their own communication systems using old phone equipment, however, today there are several manufacturers of "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 technolgy that was available at the time. They used a central power supply to drive external stations or beltpacks. These systems were very robust and simple to design, maintain and operate but had limited capacity and flexibility as they were usually hardwired. This meant that a user on the system could not choose who to talk to, rather, they were always talking to the same person or group of people until the system was reconfigured to allow communication with a different group of people. Two-Wire "routers" or "SAPS" were then implemented in these systems to allow quick re-routing of a two-wire circuit, however, this was usually handled at a central location, and because voltage is used on the circuit to power the external user stations, there would usually be a loud "pop" sound when the channels were switched. So while you 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 be distracting to the rest of the production crew.
[edit] FOUR-WIRE
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 and 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; four wires. That said, in a modern four-wire system there are actually six wires; two for data or a "data pair" that make 5 & 6, nevertheless the phrase has stuck and this is the accepted term for these systems today. One major advantage of these systems vs. the two-wire systems is the ability to do 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, which 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 do point to point but also point to multi-point, party-lines, interruptible fold back(IFB) and many other configurations that are useful to the production environment simultaneously. Because of this these systems are essentially audio routers which 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. Again, in the past, things like communication, IFB, audio monitoring and, in some cases, point to point all had to be separate systems whereas with the four-wire system it is 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.
