An intercom, talkback or doorphone is a stand-alone voice communications system
for use within a building or small collection of buildings, functioning
independently of the public telephone network. Intercoms are generally mounted
permanently in buildings and vehicles. Intercoms can incorporate connections to
public address loudspeaker systems, walkie talkies, telephones, and to other
intercom systems. Some intercom systems incorporate control of devices such as
signal lights and door latches. There are simple house intercoms and
intercoms developed for collective apartments. Some are equipped with
video, and its wiring, can be connected to the outside with few pairs while
controlling an electric strike. The last generations are even compatible with
computers and some models include TCP/IP.
Permanent systems Traditional intercoms and public address
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 carried as well as
voice. Digital intercom stations can be connected using Cat 5 cable and can even
use existing computer networks as a means of interfacing distant parties.
Many schools and office buildings now use audio / video systems to identify
visitors trying to gain access to a locked building, and 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 systems 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. 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. 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 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. 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 stop 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.
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. 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, as well as structural features in your building,
can all 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, 494–608 MHz, 900 MHz, 2.4 GHz, 5.8
GHz, and MURS. IP Intercoms are now appearing that connect a Master to an IP
Substation elsewhere on the Internet, via an Ethernet port. Wireless intercoms
can also run over a mesh network that allows near-instant communication
throughout a house. 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.
Telephone Intercoms Some telephones include intercom
functions that enable paging and conversation between instruments of
similar make and model. Examples include Panasonic model KX-TS3282W(/B), AT&T
models 945 and 974, and TMC model ET4300.
A single device can add intercom functionality to multiple standard
telephones on a common phone line, even of different makes and models.
Installation effort is minimal, and is not vulnerable to the radio interference
and security issues of wireless systems. The Add-A-Com Whole House Intercom for
Standard Telephone Systems is such a device. Intercom paging is accomplished
by sounding a distinctive ring from all telephones after any phone is taken
briefly off hook. After paging, any number of phones may converse. The
device temporarily disconnects the external phone line during intercom use,
and reconnects when all phones are again on hook. During intercom use, an
external call’s ringing signal can be heard in the earpiece.
Many key telephone systems for office use provide access to multiple outside
lines plus an intercom, where the latter appeared as if it were one of the
individually-selectable lines. The Western Electric Model 207 and Model 6A
intercoms were designed for integration into such key system multiline
installations, providing one or two simultaneous intercom calls
respectively. See also
Courtesy phone Speaking tube
Bticino Fermax
Video door phone Door phone
References ^ "Intercom Systems" page of
GadgetShack.com. ^ "MURS Wireless Intercom Technology
Guide" page of IntercomsOnline.com.