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 (Azori 2016). 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 (electrical installation), can be connected to the outside
with few pairs (4-6 pairs) while controlling an electric strike.
The last generations are even compatible with computers and some models include TCP/IP.
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.
Besides fixed locations, intercom systems are used on many types of vehicles including trains,
watercraft, aircraft and armoured fighting vehicles.