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 armored
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 track soften 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, 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 with an ear piece 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 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.