Patent GB-318946-A (GB)
Title: Improvements in signalling systems
IPC: H04J1/14
Filed: 1928-06-13
Abstract: 318,946. Standard Telephones & Cables, Ltd., (Western Electric Co., Inc.). June 13, 1928. Two-way repeating systems.-In a system in which a two-way line is connected to outgoing and incoming circuits by a hybrid coil, a calling signal circuit is connected to the two-way line in conjugate relationship thereto. The calling circuit may be arranged as an extension of the usual balancing network of the line or its impedance may be such that it constitutes the balancing network. In the arrangement described for working over a carrier channel, the two-way line L is connected by the coil H to a transmitting channel TC signals from which are impressed on a balanced modulator M associated with an oscillator TO and of the kind in which the unmodulated carrier wave is suppressed. The output of the modulator is passed through a band-filter TBF to remove one side band, and thence to the carrier line CL. The receiving channel comprises a band filter RBF, a de-modulator DA associated with an oscillator RO to supply a carrier wave of substantially the same frequency as the carrier suppressed at the distant transmitter, and a lowpass filter LPF having a cut-off at the upper limit of the frequencies to be received. The tunedinput transformer 45 of the signal circuit is connected across the balancing network N and consequently is substantially free from interference by currents over line L, and owing to attenuation of incoming distant transmissions of speech frequency, is not liable to false operation on this account. The carrier line CL may be replaced by a wireless link. Calling signals, of a frequency of 20 cycles for example, transmitted east over line L operate relays 10 and 11 tuned to this frequency by a condenser 12 which also prevents direct current flow through the relays. Relay 10 remains operated during the passage of the calling current but relay 11 is polarized and operates intermittently. Relay 15 energizes, disconnects line L and relays 10, 11 from the hybrid coil. and short circuits the pairs of terminals 2, 4 of the hybrid coil so that harmonics of the 20- cycle signal and the signals about to be generated in the circuit TC are prevented from passing into the transmitting and receiving channels respectively. Relay 20, which is operated by relay 10, decreases the tuning capacity of oscillator TO to shift the carrier frequency, say from 10,000 to 11,000 cycles, and opens one of the grid circuits of modulator M so that this carrier is sent out over line CL. A resistance 35 is shunted across the output to attenuate the carrier wave and is intermittently short-circuited 20 times per second by relay 11. The operation of the circuit on receipt of calling signals from the distant transmitter will now be described. The carrier frequency for transmission west is different from that for transmission east, and may be for example 6000 cycles which is shifted to 7000 cycles in response to 20- cycle signal currents. Consequently, signals comprising a 7000-cycle carrier interrupted 20 times per second arrive over line CL and are combined in the demodulator DA with a 6000-cycle wave to produce an interrupted 1000-cycle wave which is put into the hybrid coil at terminals 4. A portion of the current is shunted by the balancing network N, and a portion passes through condensers 30 and resistances 40 to the tuned input of amplifier A.50, the output of which operates relay 55 having an armature tuned to 1000 cycles. This armature thus operates 20 times per second. At each operation condenser 56 charges up over resistance 61 and polarized relay 60 operates its armature at the same rate, the contacts of relay 55 being protected by resistance 57 during the discharging of condenser 56. Relay 65, which is slightly slow-to-release, pulls up owing to discharges and charges of condenser 66 over its lower and upper windings alternately, the choke 67 rendering the circuits resonant at 20 cycles so that relay 65 operates only on signalling current of this rate. The normally operated relay 75 now falls back and completes a circuit for relay 80 which connects 20-cycle ringing current from 85 to line L and disconnects the line from relays 10, 11 and the hybrid coil. The relay 75 remains operated, due to charging up of condenser 70, for a short period after its circuit has been opened by relay 65 so that if relay 65 has been operated by waves other than call signal waves, for example speech waves, 65 would fall back before 75 had released. Resistance 72 is large to make the delay period sufficiently long and resistance 71 is large enough to protect the contacts of relay 65 but not so large as to prevent rapid discharge of condenser 70, a slow discharge of which would allow false operation of relay 80 due to the holding of 75 if 65 receives a rapid succession of waves of short duration. In the arrangement shown, the combined impedance of the network N and input of the signal circuit may match that of the line L for speech frequen cies, the impedance of the input being low and the resistances 40 having an aggregate value of 40,000 ohms. If the signal circuit is used as the balancing network, the network N and all apparatus between it and the. relay 55 are dispensed with, the relay being of proper impedance to balance the line. A speech frequency channel may be operated over the line CL simultaneously with the carrier working, and in order that the transmission equivalent of the carrier channel from the distant terminal to the hybrid coil H may bear a given relation to the speech-frequency transmission equivalent of the line CL whether this line is long or short, a network 90 May be cut in or out of the carrier channel by switches 91-93, and a corresponding network 95 is cut in or out of the call-signalling circuit to maintain the transmission level of the signal current,