Intercoms 1561Intercoms, short for intercommunication systems, are
often more sophisticated than sound reinforcement
systems because sound systems are unidirectional while
intercoms are bidirectional. Sophistication varies from
the simple home intercom to hotel and apartment inter-
coms, to hospital nurse call and emergency systems, to
party line systems, to the multiprocessor controlled,
multimemory, analog and digital intercommunication
systems.43.1 General Purpose Intercoms
Intercom systems fall into three categories:
point-to-point, party line, and matrix, Fig. 43-1. The
point-to-point or dedicated line is a private line between
two stations. Normally, no other stations can hear your
conversation.
The party line, conference line, or distributed line is
a shared line where a number of people, usually sharing
a common task, are all talking to each other. This is like
being at a conference table where everyone can talk and
listen to one another at the same time and there is no
privacy capability. By having multiple circuits and
multiple wires, this system can communicate to
different parties at the same time. Most often this type is
found in broadcast intercoms.
Most major intercom manufacturing companies are
offering digital matrix systems. These systems are multi-
processor controlled, multimemory, and have analog and
digital audio and can combine the point-to-point and
party line communications together. All have advantages
and disadvantages and they can be either hard-wired or
wireless or a combination of the two.
Intercom systems can be simple, a two unit system
with only a call button and a microphone/loudspeaker
combination in each unit, to a complicated multichannel
multistation system with a separate microphone and
loudspeaker, display window, keypad, TV entrance
monitor, auxiliary inputs, can be programmable, plus
have a multitude of special features.
43.1.1 Point-to-Point Systems
Point-to-point systems are the simplest type of intercom
and are mostly used in residential and office applica-
tions, schools, apartments, and nurse call and emer-
gency call systems. With point-to-point, a caller or
originator makes contact with the desired receiver or
receivers and communicates only with them, Fig. 43-1.
All other stations are isolated so they cannot be part of
the one- or two-way conversation. This is like the tele-
phone system, you dial a party or parties through a
conference call, and the conversation can only be heard
by those parties that were called up even though the
telephone system has hundreds of thousands of other
phones on the system.
Systems are normally made up of master stations,
slave stations, door monitoring and opening stations,
and input devices such as AM/FM receivers.
These systems have a variety of features and are
used for intercommunications between rooms or desks,
intercommunications between an indoor station and an
outside door including door release, and provide some
form of playback signal at all or selected stations in the
system. The system can be one master to one or more
slaves, all master, or a combination of the two. Masters
have the ability to originate calls to any or all stations
while slaves can only call the master station they are
connected to, Fig. 43-2. Stations can be in-the-wall units
or selfcontained desktop units, hardwired or wireless.
Normally hardwired systems are the best for
point-to-point systems as they are less expensive and
less likely to have interference. The disadvantage is
they are more difficult to move or reposition.
The Bogen Model PI35A, Fig. 43-3 is a twenty five-
station intercom. It provides facilities with two-way
communication, emergency paging, time tones, andFigure 43-1. The three basic types of intercoms.Point-point systemParty-line systemMatrix systemIntercom
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stationComputer or
master module