Patent GB-377407-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04M1/58
Filed: 1931-07-09
Abstract: 377,407. Anti-side-tone circuits: party-line systems. ERICSSON TELEPHONES, Ltd., 67, Kingsway, DOIG, P., 12, Carlton Vale, Kilburn Park, both in London, ROBINSON, E., 46, Ella Road, West Bridgford, and GREEN, B. A., 3, Bramcote Road, Beeston, both in Notting- hamshire. July 9, 1931, No. 19718. [Class 40 (iv).] In an anti-side-tone arrangement, speech currents incoming to and outgoing from a station traverse different circuits over the same line conductors. In one arrangement, Fig. 1, a centre tapped receiver RX is used at station X and speech from X to Y passes from transmitter TX over a simplex circuit to receiver RY using an earth return or a metallic return which may be a simplex circuit over another conductor pair. Battery is fed over RBY and condenser QY prevents a short-circuit. Speech from Y to X passes over coil ICY and the usual loop circuit. In the modification shown in Fig. 2, the simplex circuit is obtained by the use of centre-tapped retards RTX, RAY, and as a loud speaker LSX is used at X, the transmittercircuit at Y includes an amplifier AMY. A party-line system is described comprising a central station, Figs. 6 and 7, and street callstations, Figs. 3 to 5, having an ordinary telephone set such as R1, T1, Fig. 3, for use by police and an anti-side-tone loud-speaking set. LS, T2 for use by the public. The street station (police sets) may be selectively called from the main station and for this purpose one station, Fig. 3, has a polarized calling relay P and the other two stations, Figs. 4 and 5, each have an A.C. relay PI connected respectively to the upper and lower line wires. The street stations have contacts DSI-DS4 controlled by the door of the public callbox, and terminals with straps are provided to enable the station apparatus to be readily connected up as required. At the main station, the line may be connected to a private or exchange line or to another party line, and the arrangements are such that, under the control of the operator, anti-side-tone circuits with amplifiers are used for public calls and ordinary circuits without amplifiers are used for police calls. There may be more than one operator. Fault indicating arrangements are provided at the main station. Calls from street stations. Switch hook contacts SH1-SH3 of the police set at the terminal station, Fig. 3, disconnect calling relay P and close a low resistance loop which fully operates relay N at the central station, differential relay V remaining inert. (Normally, a small current flowing over this circuit causes N to pull up only its light contacts n1 for fault detection purposes to be described later.) Calling lamp L5 and relay PC controlling a pilot lamp are energized, and the operator throws K2 to the left to connect up the speaking set, Fig. 7, battery being fed to the calling station over retard RC, and lamp L5 extinguished. When the door of the public call box is opened, mercury contact DS4 closes a circuit from the supply mains to lamp L7 on the top of the call pillar to indicate to a police officer in the vicinity that a call to the central (police) station is being made. Contacts DS1, DS2 are definitely made by opening the door a temporary ground contact being previously made at DS3. These contacts may be controlled by a device similar to automatic dial transmitters which is released by opening the door and re-set by closing it. The temporary ground at DS3 over conductor 12 operates V, VA and A, Fig. 6, N being short-circuited. The street station calling relay P is also disconnected at this time. When contact is made at DS1, DS2, ground over transmitter T2 is connected to both conductors in parallel so that B, Fig. 6, pulls up and V, VA fall back, VA being slow releasing to ensure the operation of B before the short-circuit on N (lower) is removed. N remains short-circuited by A and B which light the public call lamp L1 and may energize a pilot lamp relay MP which may light a further supervisory lamp in police .superintendent's room. The operator moves K2 to the left as before but in addition throws key K1 which lights lamp L8 and operates relays C, D, H. Relay H disconnects feeding battery over RC used for police calls while relays C, D change over the connections of the speaking set so that it includes amplifier AM and a battery feed over RB and a centre-tapped retard RA. These connections are now similar to those of station Y in the anti-side-tone arrangement of Fig. 2 except for additional condensers QB, QC which prevent anode current from AM reaching the line over a leaky transformer OP. The operation of K2 also disconnects relays A, B so that calling lamp L1 is extinguished. The loudspeaking set in Fig. 3 is similar to that at X, Fig. 2, except that a step-up transformer IC2 is used for the transmitter. A lamp L8 is provided at each operator's position to indicate when the amplifier, common to all positions, is taken into use. There may be more than one common amplifier. Calls from central station. To call the terminal station, Fig. 3, operator throws K2 to the calling (right-hand) position. and CO connects the line conductors to battery over relav F and resistance RW, so that the normal polarity is reversed and P, Fig. 3, pulls up, relay F remaining inert. Relay S in the common equipment, Fig. 6, connects interrupter I to relay IP which breaks and makes the circuit of P so that mercury tube p1 flashes the lamp L7 to call a police officer in the vicinity. When the switch-hook contacts SH1-SH3 are operated, P is disconnected, L7 is extinguished, and F pulls up during a release of IP. Relay FF operates, locks and disconnects calling current, and supervisory lamp L6 lights up. The operator throws K2 into the speaking position transmission being over the normal loop circuit. To call the intermediate station, Fig. 4, key K3 is thrown. As there is no current reversal, P, Fig. 3, does not operate, but the previously described intermittent operation of IP connects ringing current from G over the upper conductor 12 to operate relay P1, Fig. 4, and further operations ensue as in a call to the terminal station. The intermediate station, Fig. 5, is called in similar manner over the lower conductor 13 by throwing key K4. Whichever intermediate station is called, there will be a circuit over P, Fig. 3, for the calling relay of the other, but this circuit is ineffective owing to the resistance of P. Two or more stations may be called simultaneously by throwing the requisite keys but calling current is cut off by F on the first reply. Reverting police-calls. The operator requests the calling police officer to hang up and throws the calling keys for both stations. When the calling officer notes that his lamp L7 ceases to flash (wanted station has answered) he removes. the receiver again. The operator restoresthe calling keys and speaking battery is fed over relay V in series with N which lights lamp L5. Exchange and other facilities. An exchange or private line or another party line may be plugged into jack J to extend a call from an officer at a street station. Faults. Normally, there is sufficient current in the loop over P, Fig. 3, for N to operate its light springs n1 so that if a disconnection occurs N falls back and lights lamp L4. If the upper conductor 12 has a earth fault, V (upper), VA and A energize and light lamp L2, and similarly an earth fault on the lower conductor lights lamp L3. A short circuit lights lamp L5 as in a call from the terminal station but the operator receives no reply on, answering. An earth fault on earth conductor when K3 or K4 is thrown immediately operates FF to disconnect generator G from the line.