An
intercom (intercommunication device),
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.
[edit] Permanent systems
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 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.
[edit] 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.
[edit] 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 (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 such as an interruptible feedback (IFB) 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.
[edit] Wiring intercoms
1980s MirTone intercom system
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 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 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. 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
television 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
television stations or
television networks.
[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, 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
(200KH – 270 kHz), 494–608 MHz, 900 MHz, 2.4 GHz, 5.8 GHz, and MURS
(150 MHz). IP Intercoms are now appearing that connect a Master to an IP
Substation elsewhere on the Internet, via an Ethernet port.
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] 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.
[edit] See also
[edit] References
- ^ "Intercom Systems" page of GadgetShack.com.
- ^ "Wireless Intercom" page of IntercomsOnline.com.
