Patent GB-801269-A (GB)
Title: Carrier wave telephony
IPC: H04J1/08
Filed: 1955-02-11
Abstract: 801,269. Exchange systems. DILLON & CO., Inc., W. C. Feb. 11, 1955 [Feb. 12, 1954], No. 4164/55. Class 40 (4). [Also in Group XL (c)] In a party line system using carrier transmission, the carrier is rectified at a substation and provides both the signal and the power for a transistor amplifier feeding the receiver. Manual switching is used at the exchange described. The substation may have dialling facilities. Operation of the switch hook or of dialling contacts at a substation affects the load on the line and the carrier voltage across the line at the exchange. The change of voltage is detected by a thermionic valve which cuts off when the voltage is reduced. The two substations 22, 23, Fig. 1, on line 10, 11 operate on different carrier frequencies generated by oscillators 12a, 13a in substation units 12, 13 in the exchange. In the unit 12 allotted to substation 22, the carrier from oscillator 12a is rectified at 34, Fig. 2, and applied to the grid of a thermionic valve 38 which conducts in series with a relay 39. If the receiver is lifted at a substation, the exchange carrier voltage drops, the valve 38 cuts off, relay 39 falls back and lamp 44 indicates the calling condition of the station. The lamp is extinguished when the operator inserts plug 51 to answer and opens contact 52. The carrier is demodulated at 32 and is fed through transformer 33 to the operator's set. The substation is connected to line by a H.F. transformer 124, Fig. 5, with three secondary windings. When the switch hook 131 is up, rectifier 125 is connected across transformer winding 124c and increases the load on the line 10, 11. At the same time, the rectifier provides current for the telephone transmitter 120a. Dialling contacts 140 are included in the circuit of rectifier 125. The audio signal and a control potential are derived from the carrier by full wave rectifiers 132 and 127 and - are connected respectively to the base and the emitter of transistor 130. The collector is connected to transformer 126 which feeds the transistor output either to the receiver 120b or to the loudspeaker 122 in accordance with the position of the switch hook. The loud-speaker is tuned to the ringing frequency. When the switch hook is up, the audio-signal forms a component of the output of rectifier 125 which is added to the transistor output in transformer 126 through winding 126d. Operator's cord circuit. The operator plugs cord 80, Fig. 4, into a calling unit and relay 83 pulls up over the a, b wires and a contact of relay 39, Fig. 2, before lamp 85 can operate. The switch 82 is thrown to the left to connect the operator's head set 86 to the line. The cord 90 is then plugged in to the exchange unit of the wanted line and a circuit for indicator lamp 91 is completed over the c wire. The switch 82 is thrown to the right to connect audio-frequency ringing current from generator 100 to the line, the ringing current being interrupted by cams 103, 104. When the switch is returned to the central position, relay 93 pulls up and lamp 91 extinguishes as soon as the called station. replies and the corresponding relay 39 falls back. A sub-audio frequency generator 105 is used for calls to conventional substations.