Patent GB-711514-A (GB)
Title: Carrier wave telephone systems
IPC: H04M7/16
Filed: 1952-01-11
Abstract: 711,514. Exchange systems. WESTERN ELECTRIC CO., Inc. Jan. 11, 1952 [Jan. 13, 1951], No. 893/52. Class 40 (4) Party line subscribers are connected to an automatic exchange by high frequency on a power line L. The central exchange is linked to the power line by a common carrier terminal CCT and a coupling unit CU. Groups of subscribers SSA1 &c., SSB1 &c. are connected to the line by coupling units and subscriber carrier terminals SCTA, SCTB. The exchange terminal CCT always transmits on a frequency F2 and the subscriber terminals normally transmit on frequency F1. Code ringing is employed, 20 cycle ringing current being transmitted as a modulation on the carrier F2. If both frequencies F1, F2 are already on the line L a calling subscriber can neither gain access to the line L nor listen in. In a reverting call between subscribers on different terminals the calling subscriber dials and hangs up.. The exchange rings on carrier F2, the called subscriber answers and transmits and receives normally on frequencies F1, F2. The frequency F2 is stopped in the exchange terminal and the remaining frequency F1 cause the transmitting and receiving frequencies to be interchanged at the calling station when the receiver is lifted to complete the connection. If both subscribers are on the same terminal the two subscribers are directly connected as soon as the called subscriber lifts the receiver on cessation of ringing. Frequency F1 is transmitted to the exchange to busy the line. Common carrier terminal CCT, Fig. 2.- Code ringing from the exchange on the tip conductor T operates an A.C. relay 20 which starts the oscillator 12 to generate frequency F2. The ringing current is also fed over conductor 45 to a modulator 9. The modulator frequency F2 is amplified, filtered and passed to line C through a coupler unit. The incoming frequency F1 is filtered, demodulated and amplified to operate a relay 17. Relay 18 follows to complete a two-way connection between the line L and the exchange line through a hybrid coil H1. If a party-line subscriber rings, the incoming frequency F1 operates relay 17 which is followed by relay 18. Relay 17  repeats interruptions in the carrier F1 as d.c. impulses on the exchange line, relay 18 being slow to release and remaining operated. If the call is a revertive one, code ringing from the exchange is on both T and R conductors and relays 19, 20 both operate and start oscillator 12. The frequency F2 is modulated with ringing current at 9. A memory relay 21 indicates that revertive ringing has been in progress. When the called subscriber replies, frequency F1 is returned to trip the ringing at the exchange and to operate relay 22 over a contact 21 to stop oscillator 12 at CCT. The exchange line is held by frequency F1 which is still transmitted from the subscriber terminals. Subscriber carrier terminal, SCT, Fig. 3.- Frequency F2 modulated with ringing current from the exchange is filtered, demodulated and amplified to operate and release relay 38 at 20 cycles per sec. Relay 38 in combination with battery 46 and condensers 48 generates ringing current in the subscriber's line. When the subscriber replies, relay 35 operates in the loop and starts both oscillations 26, 27 generating frequencies F1, F2. Relay 35 also connects the subscriber's line to the hybrid coil H2 so that, with relay 34 normally unoperated, a two-way connection is completed between the subscriber's line and the power line L. The frequency F1 from oscillator 26 is modulated at 23 and fed through a filter and a coupler to the power line. The oscillator 27 is not used at this stage. When a subscriber makes a call, relay 35 operates providing that relay 37 is not operated. Oscillators 26, 27 are started and frequency 19 is transmitted to the exchange as described for an incoming call. Relays 36, 37 operate respectively whenever frequencies F2, F1 are on the power line so long as relay 35 is not operated first. If both frequencies F1, F2 are on the line when a subscriber attempts to make a call, relay 35 cannot operate, the hybrid coil is not connected and the subscriber cannot listen in. In a revertive call, when ringing ceases and the calling subscriber lifts his receiver, the frequency F2 has been cut off in the exchange terminal and F1 is transmitted by the called subscriber terminal. If the called and calling subscriber are on the same terminal F1 continues to be transmitted to mark the line busy at the exchange. If they are on different terminals, relay 37 is operated in the calling terminal, relay 34 operates in series with one winding of 35 when the line is looped, and the transmitting and receiving frequencies are interchanged to permit speech between the two party-line subscribers. Relay 35 then follows and completes the speech connection through the hybrid coil.