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Intercom system in the Pittock MansionAn intercom
(intercommunication device), talkback or doorphone is an
electronic communications system intended for limited or
private dialogue, direction, collaboration or
announcements. Intercoms can be portable or mounted
permanently in buildings and vehicles. Intercoms can
incorporate connections to walkie talkies, telephones, cell
phones and to other intercom systems over phone or data
lines and to electronic or electro-mechanical devices such
as signal lights and door latches.
Permanent intercoms installed in buildings are generally
composed of fixed microphone/speaker units which connect 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, 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. Additionally
many schools now use audio / video 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 armoured
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
analogue electronics components but many new features and
interfacing options can be accomplished with new 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.
Contents [hide]
1 Basic terms
2 Wiring intercoms
3 Two-wire broadcast intercoms
4 Four-wire broadcast intercoms
5 Wireless intercoms
6 See also
7 References
[edit] Basic 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.
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. Connects to a belt pack.
Power Supply - Used to feed power to all units. Often
incorporated into the design of the base station.
[edit] Wiring intercoms
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.
Digital intercoms use Category 5 cable and relay
information back and forth in data packets using the
Internet protocol architecture.
[edit] 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.
[edit] Four-wire broadcast intercoms
A modern four-wire intercom system capable of 272 sources
and destinations manufactured by Telex Communications
Inc.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
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.
[edit] 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 a 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. The United States and Canada have several
frequency ranges for wireless intercom systems and other
wireless products. They are 49 MHz, FM band (200KH - 270
kHz), 494-608 MHz, 900 MHz, 2.4 GHz, 5.8 GHz, and MURS (150
MHz).
Power line communication units that send signal 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.[2].
Loudaphone brand intercom station aboard RMS Queen Mary.
The Loudaphone was specified for noisy environments such as
aboard trains and trams and within a ship's engine room
[edit] See also
Courtesy phone
[edit] References
^ "Intercom Systems" page of GadgetShack.com.
^ "Wireless Intercom System" page of IntercomsOnline.com.
Retrieved from "http://en.wikipedia.org/wiki/Intercom"
Category: Telecommunications equipment
Hidden categories: Articles with unsourced statements since
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