Patent GB-553225-A (GB)
Title: Power line telephone system
IPC: H04B3/54
Filed: 1942-05-01
Abstract: 553,225. Carrier telephony. STANDARD TELEPHONES & CABLES, Ltd. May 1, 1942, No. 5886. Convention date, April 19, 1941. [Class 40 (iv)] [Also in Group XL] In a power-line carrier telephone. system, each substation has a gas-filled tube which is ionized by a carrier current of a characteristic calling frequency and passes on pulsating current to a ringer and to a relay which switches on the power supply to the carrier telephone set. The calling signal is sent under control of an exchange operator who can connect the subscriber to another of the same kind or to a normal network. The substations transmit a different calling frequency, the reception of which at a central relay point causes a lowfrequency signal to operate a drop indicator at an operator's position. Calling exchange. On the removal of the handset 51, Fig. 2, the carrier set 50 is energized by connection to the power-transformer 58 and its output circuit is connected to the line via condenser 56. Operation of the button 84 causes the set to generate carrier current of a calling frequency F3 which is received by the equipment shown in Fig. 3 and passed by a filter 141 and amplifier VT1 to a gas discharge tube GT2 which breaks down and causes the operation in succession of relays R1, R2, R5. This results in the energization of the carrier set 120 and of an oscillator 150 and when the carrier set valves have warmed up, R4 pulls up and completes a circuit from the power source to the uppermost pair of windings of a hybrid transformer 160. Current of.powerfrequency induced in line 110 operates a dropindicator at an operator's position, Fig. 4 (not shown). On the operation of R4, R2 is released and the calling signal is cut off but R5 remains locked up. In answering the call, the operator bridges the line 110, thus providing a circuit for R3 which unlocks R5 but itself keeps the carrier set energized. Completion of connection and release. The operator and subscriber are now in carrier telephonic communication, the subscriber's set now generating a frequency F1, and the call can be extended over a normal network or to another carrier subscriber. At the conclusion of the conversation, the calling party reoperates button 84 to change the oscillator frequency. Relays R1, R2 respond as before to operate the drop indicator but R2 is unable to lock up. When the operator clears, R3 falls, de-energizing the carrier set, whereupon R4 falls. Calling substation. The operator bridges line 110 to operate R5 which energizes the carrier set 120 and oscillator 150. After a brief interval to allow the oscillator to come into action, the operator projects a lowfrequency signalling current over the line 110. Relay R6 responds and completes the anode circuit of the oscillator to cause current of a frequency F4 to be sent over the power circuit. At each substation, this current is selected by a network 92 and passed to a gas tube GT 1  which breaks down and feeds pulsating current to a ringer 90 and a relay R10 which energizes the carrier set and relay R20 which locks and provides an alternative energizing circuit for the carrier set. If the subscriber replies, R20 is released at the cradle switch over which the energization of the carrier set is maintained. If he does not reply, the resistance of a thermally sensitive resistance T in shunt to R20 becomes so low that the relay falls and unlocks itself. The desired subscriber on the power network may be selected by code ringing. Communication between parties on the same power line. In this case the calling party transposes the frequencies F1, F2 normally used for transmitting and receiving respectively. Valve circuits. The argonfilled tube GT 1 , Fig. 2, has its grid biassed from transformer 99 and its anode supplied via choke 106 from the mains, the grid and anode supplies -being 180 degrees out of phase. A network 93 is connected between the grid and anode to reduce the effects of outer-electrode capacity. When a carrier current of the frequency to which the input circuit 92 is tuned is received, ionization takes place and during each positive half-cycle of the anode supply current flows to the ringer 90 and relay R20. Specification 549,623 is referred to.