662,930. Electric selective signalling. WESTERN ELECTRIC CO., Inc. Dec. 30, 1949. [Dec. 30, 1948] No. 33366/49. Class 40 (i). [Also in Group XL (c)] A central station calls any one of a number of sub-stations by transmitting a distinctive combination of frequencies, the called station sending back a distinctive signal which establishes communication if it corresponds to the calling signal. General description. Calling sub-station from central station. A four digit number is dialled at 1, Fig. 1, in the normal way, the pulses being applied over a control unit 2 to a dial pulse sender 3 where they are translated into a combination of four frequencies selected from thirty audiofrequencies ; these selected frequencies are combined at 4 and transmitted as modulations on a carrier. At the called station, the demodulated frequencies are applied simultaneously to a control unit 7 which includes four appropriately tuned reeds which operates to complete a circuit to cause amplifier 8 to oscillate to produce four frequencies corresponding to the reed frequencies and therefore to the received frequencies ; these frequencies are fed to 12 for transmission as an answer-back signal; the four reeds also prepare a circuit for the actuation of bell 10 and lamp 168 by a later signal. The answer back signal from 12 is received at the central station at 13 and the four demodulated frequencies are applied to 14 for translation into a four-digital number which is passed through amplifier 15 to the dial pulse sender 3 for comparison with the number dialled at 1. If the two numbers are identical, a check signal over lead 118 is passed to the wave unit 2 to remove the calling signal of the four frequencies and substitute a ringing signal which is received at 6 and operates the bell 10 and lamp 168. When the subscriber at the sub-station lifts his handset, the amplifier 8 is connected between the receiver 6 and the telephone and the transmitter is connected to the microphone. Calling central station from sub-station. The subscriber lifts his handset 11 to complete a circuit for amplifier 8 so that it oscillates to produce four frequencies corresponding to the four reeds at the calling sub-station ; these frequencies are received at 13 and applied to 14 for translation into the corresponding four - digital number which is displayed at 15 or recorded ; the translating unit 14 also produces a signal indicating that it has received exactly four frequencies, and this signal causes the control unit 2 to transmit a check signal which at the substation operates the control unit 7 to put the sub-station in the talking condition. Detailed description. Calling sub-station from central station. The operator at the central station closes key 16, Fig. 7, which connects impulsing dial 18 to relay 163, Fig. 2 ; relay 163 repeats the impulses from dial 18 and passes them over slow release relay 19 for counting by relays in Fig. 2 which operate relays in Fig. 3 to set the relays in Fig. 4 in accordance with the four - digit number that is dialled. As a consequence, the leads 401 ... 404, Fig. 4, are each connected to a different audio-frequency, the four frequencies corresponding to the number dialled, and the four frequencies are amplified at 20, 21, 22, 23, Fig. 5, mixed at 24, 25, 26, modulated onto a carrier at 27 and transmitted. At the called sub-station, Fig. 10, the modulated signal is applied over the closed contacts of released relay 31 to amplifier 32 feeding tuned reeds 35, 36, 37, 38 ; these reeds are tuned to the four frequencies corresponding to the sub-station, and when all are actuated, a positive potential from the junction of resistances 43, 44 is applied to charge condenser 49 and render valve 50 conducting, so that relay 54 is actuated. Relay 54 actuates relay 31 over the contacts of released relays 55, 34, and relay 31 locks and actuates the control circuit 57 to start the transmitter 58. Relay 31 also disconnects the amplifier 32 from the receiver 30, and converts amplifier 32 into an oscillator having four feed-back paths, each comprising one of the tuned reeds 35 ... 38, and a 180 degree phase shifter 60 ... 63, so that a signal comprising the four audiofrequencies is applied to 59 for modulating the carrier and transmission back as an answerback signal. Relay 31 disconnects the feed from valve 50 to render it non-conducting to release relay 54 and also applies the output of receiver 30 to the screen grid of valve 50 over filter 73 and rectifier 74. The release of relay 54 actuates relay 55 which holds over relay 31 and prepares the circuit of relay 56 for actuation by relay 54. At the central station, the answer-back signal is received and demodulated at 75, Fig. 7, and applied to tuned relays 81 ... 95 and 76, Figs. 8, 9 ; two of the relays 81 ... 95 respond to the two frequencies representing the first two digits, and two of a similar set of relays in 76 respond to the two frequencies representing the second two digits. For example, relays 82, 87 respond to render valves 96, 98 conducting to actuate relays 97, 99 which lock over relay 29 of Fig. 7. The lower contacts,of relays in Figs. 8, 9 are so connected that when four of them are actuated relay 142 Fig. 7, is operated to light lamp 163 and indicate that exactly four frequencies have been received in the answer-back signal. Relay 142, over the contacts of relay 17 (actuated by key 16) connects the upper contacts of the relays of Figs. 8 and 9 with the contacts of relays set in Fig. 4 by the calling signal so that if the answer-back signal is identical with the calling signal, a circuit is completed for relay 28 which lights lamp 166 to indicate that the called substation is responding, and substitutes for the calling signal a check frequency from 158. At the sub-station, Fig. 10, the demodulated check frequency passes over filter 73 and rectifier 74 to render valve 50 conducting and actuate relay 54. Relay 54 actuates relay 56 over the contacts of operated relay 55, and relay 56 locks, provides an additional lock for relay 31, and rings the bell 1043 and lights the lamp 1044. The lamp is extinguished by button 1046 or by removal of handset. On lifting handset, switch 167 operates relay 34 which locks and connects the amplifier 32 to the receiver of the handset, and the microphone to the modulator 59. Relay 34 also releases relay 31 and thus removes the tuned reeds from the oscillating circuit. Relay 31 also releases relay 55 and disconnects the amplifier 32 from the rectifier 74 thus rendering valve 50 non-conducting and releasing relay 54. At the central station, the operator opens key 16, Fig. 7, and closes key 159 to operate relay 29 and connect the transmitter 27 and receiver 75 to the line 160. Relay 29 also releases all actuated relays in Figs. 8, 9 while the release of slow-release relay 19, Fig. 2 (when key 16 opens) releases all actuated relays in Figs. 2, 3 and 4. At the end of the call, the operator opens 159, Fig. 7, which restores the idle condition ; the subscriber at the sub-station restores the idle condition by hanging up his handset. Calling central station from substation. When the handset at a sub-station is lifted, relay 31, Fig. 10, is actuated and makes the oscillatory circuit for generating the appropriate four audio frequencies as described above. At the central station, the four audio-frequencies actuate the appropriate relays in Figs. 8, 9 to actuate relay 142, Fig. 7, and light lamp 163 ; relay 142 also operates relay 28 to transmit the check frequency. Since relay 17 is not actuated, the relays in Figs. 8, 9 light appropriate lamps in the assembly 165, 164, Fig. 5, to indicate the number of the calling sub-station. The reception of the check frequency at the sub-station operates as before to institute the talking condition. The operator at the central station, after noting the number displayed in Fig. 5, closes switch 59, Fig. 7, which actuates relay 29 to release the relays in Figs. 8 and 9 and extinguish the lamps in Fig. 5. Specification 649,470 is referred to.