master station, but are not capable of initiating calls with any other stations. Substations are also known as slave units. Door station: Like substations, door stations are only capable of initiating a call to a master station. They are typically weather-proof. Intercom station: A full-featured remote unit that is capable of initiating and receiving party-line conversation, individual conversations, 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. Telephonic: Intercom systems that utilize a transmitter (talk) and receiver (listen) to communicate, similar to standard telephone systems. Amplified: Intercom systems that utilize a single speaker to transmit and receive communications. Amplifier: An intercom system's main or central component, responsible for switching or connecting communication paths between master and substations. In addition, amplifiers distribute or switch power to auxiliary connected devices, such as door strikes, to allow entry. 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