Selective Telephone Communications

a service system of telephone communications between individual points (or subscribers) and a central point and among the individual points themselves. In such a system a large number of telephone stations are connected in parallel to one common electrical circuit. Each point can be called individually, or a certain group of points (or all points) can be called simultaneously.

Selective telephone communication systems are used in providing operational-technological service communications, characteristically in situations where the points are located far apart and along some kind of a line, such as a railroad, a gas pipeline, or an electrical supply system. Selective telephone communication systems are of two types. The first type provides for one-way selective calling only, that is, from the master office to the other stations only. (Such selective calling can be by voice or by a call signal). In the other type of system each point can selectively call any other point (selective reversion). Calling a subscriber is done by sending a certain pulse combination along the line. This combination is received by the call receivers at all points connected to a given line, but the bell-ringing circuit closes only in the receiver tuned to that pulse combination.

Selective telephone communication systems commonly used in the USSR have either a selector-operated or an improved, harmonic selective ringing. In the first case, a subscriber is called by sending along the line a combination of direct current pulses consisting of three trains. The total number of pulses in the combination is 17. Such a selective telephone communication system can provide service for 78 points. Each of these points utilizes a selector as a receiver. In the second case, selective ringing is done by sending along the line two pulses of differing frequencies, one following the other without any interval. The duration of the first pulse is 0.8 seconds; that of the second pulse, 1.6 seconds. Seven frequencies in the audio range are used (0.3 to 3.4 kilohertz), which make for 42 calling combinations. Seven of these combinations are used for group calls and the balance for individual calls. A harmonic selective ringing receiver is installed at each point. Each receiver is tuned to one group combination and one individual combination. It is possible to call all the subscribers on the line simultaneously.