Intercom

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. 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. 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. 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.