Patent GB-1306700-A (GB)
Title: Communication switching systems
IPC: H04M3/40
Filed: 1970-10-30
Abstract: 1306700 Transistor switching circuits; exchange systems PYE Ltd 27 Sept 1971 [30 Oct 1970] 51714/70 Headings H3T and H4K In a four wire crosspoint switching system in which two or more operators 1, 2 and remote stations, channels 1-3, may be selectively interconnected, the crosspoints comprise amplifier units which each have input and output impedances which are high when compared with the impedances of the switched lines, thereby allowing several lines e.g. 13, 15, 17 to be connected to a single line without mismatching difficulties. Each amplifier unit (Fig. 2) comprises a balanced amplifier 43 e.g. a monolithic unit, to the input of which a line 5a, 5b is connected via a relatively high input impedance circuit comprising emitter followers and the amplifier output is connected to a second line 14a. 14b, via a high impedance balancing circuit including potentiometers 54, 56. The amplifier is held off by applying a squelch bias voltage to line 24 and is held on by earthing line 24. The amplifier gain is chosen so as to compensate for the signal loss due to the impedance mismatching, thereby resulting in the signal applied to line 14a, 14b equalling that on line 5a, 5b. In the system described a number of remote stations are connected e.g. by voice frequency telephone lines, carrier telephony lines or cables or radio links, to terminal equipment (not shown) which is coupled by two wire connections to hybrid transformer inputs 12, 19, 20 of the Y-buses of the switch. One or more operators 1, 2 connected to respective X-buses are each interconnected with the Y-buses at crosspoints employing the amplifier units described. The Y-buses of the remote station channels 1-3 are interconnectable by amplifier units 65-70. The X-buses may also be likewise interconnected by amplifier units (not shown) to enable direct connections to be made between operators. Crosspoint selection is effected by switches e.g. 27, 33, 39 controlled by the operators. When no calls are in progress the amplifier units in the transmit speech paths from each remote station to the operators are switched on, e.g. units 23, 29, 35 and the corresponding units for the other operators, and all other amplifiers are switched off. Call initiated by operator.-To make a connection with a remote station operator 1 depresses switch 27, 33 or 39 switching on the appropriate amplifier unit 22, 28 or 34 thereby establishing a connection with the corresponding station. All three stations may be simultaneously called by the operator depressing all three switches. Each of the other operators may be connected with the stations in a similar manner and more than one operator may be connected to the remote stations at the same time. By switching on the amplifiers connecting the X-buses (not shown) an operator can communicate with one or more of the other operators without their signals being applied to the outgoing channels. Hence by switching appropriate amplifier units a conference circuit can be established enabling two way communication between all stations and operators. Call initiated by remote station.-A calling remote station lights up a corresponding indicator light at each operator's set by means not described. When one operator answers by switching on the corresponding amplifier unit e.g. 22, 28 or 24, the lights at the other operator's positions corresponding to the calling station dim to half brilliance. For a call to a second remote station the operator switches on the corresponding pair of amplifier units e.g. 65, 66 between the Y-buses of the two stations. The operator can communicate with the remote stations while talk through is in operation. In the above arrangement the establishment of a connection between an operator and a remote station does not prevent further stations speaking to the same operator. There is described with reference to Fig. 3 (not shown), a switching arrangement whereby the switching on of an amplifier unit e.g. 22 to make a connection between an operator and a remote station, switches off the amplifier units e.g. 29, 35 in the unselected remote station to operator speech paths. This action may also switch off the amplifier units connecting the selected station to the other operators. An operator's unit incorporating the unit 1, X-buses 5, 8 and associated amplifier units 22, 23 &c. may be made portable and a plug provided to mate with a corresponding socket connected to the Y- buses. Several such sockets may be provided throughout a building.