internal communications systems used at enterprises or institutions to summon employees or to transmit business information to them. There are two types of systems: wired and wireless.
Wired paging systems may transmit visual, acoustic, or voice signals. In a system that transmits visual signals, the signal is displayed on an indicator board or by means of signaling lamps of a specific color; such a system is used chiefly in manufacturing areas where the noise level is high. In systems using acoustic signals, employees are usually summoned by means of a bell, chiefly in “supervisor-to-secretary” systems. In a voice system, the message may be transmitted on a public-address system with loudspeakers located on the premises of an enterprise and in the work areas. A variant of this system uses a loudspeaker attachment on a telephone apparatus that makes it possible to call an employee without his having to lift the receiver. Voice systems are used for dispatcher communications in industrial enterprises and institutions and at construction sites.
The principle of operation and the facilities of a wireless paging system are similar to those of a radio communications system. Such a system permits employees to be found quickly on the premises of an enterprise without disturbing other people, and it can also be used to exchange brief messages while the two speakers remain at their work positions. Wireless paging systems may be either of the very high frequency (VHF) or the inductive type. The VHF systems consist of a central transmitter and small transceivers, each of which is tuned to a specific frequency in the VHF range. Inductive systems transmit signals with a low-frequency power amplifier that is connected to a closed wire loop laid out around the perimeter of an enterprise. When an employee is called, the electromagnetic field created by the loop induces an electromotive force in the antenna of the individual receiver. For selective calling, each receiver is tuned to a specific frequency. Wireless paging systems are used in institutions and at industrial enterprises; they also have applications in transportation.
I. S. DEMIDOV