Patent GB-484016-A (GB)
Abstract: 484,016. Exchange systems. STANDARD TELEPHONES & CABLES, Ltd. Dec. 15, 1936, Nos. 34440 and 34441. Convention dates, Dec. 18, 1935 and Dec. 20, 1935. [Class 40 (iv)] In a carrier system using a number of channels, certain channels are used only for calling. The invention is described with reference to a telephone exchange system in which carrier channels play the part of inter-exchange junctions. Two systems are described. System using separate frequencies for sending and receiving. As shown in Fig. 1, the insertion of a plug PL in the outgoing jack of an idle both way trunk circuit, operates relays RL1, RL2, thus causing the carrier frequency employed by this circuit for sending in outgoing calls to be generated and sent out over the filter BF2. At all other similar offices connected to the common medium this frequency passes the filter TC of an identical circuit and operates RL4 to light a busy lamp BL. The calling operator then selects an idle order channel by operating its key KY. Relays RLO at all the other stations using the order channel light the busy lamps BL2 and the calling operator then dials information as to the wanted number and the chosen communication channel to the call indicators CI. The appropriate operator then plugs into the incoming jack IJ of the selected communication channel, operating RL5 and RL2 therein. The incoming carrier frequency then passes to the demodulator DM1, operating RL3 to light the supervisory lamp SL1, and the oscillator OS sends out the sending frequency used for incoming calls. At the originating station, this frequency passes to the demodulator DM1 to operate RL3 and light the supervisory lamp SL. At the end of the call the removal of each of the plugs extinguishes the supervisory lamp at the other end, and the removal of the plug at the originating end extinguishes the busy lamps BL at all the other stations. System using the same frequency for both directions, Fig. 2. In this system, each bothway trunk circuit instead of having an oscillator which can produce only two definite frequencies for use respectively in incoming and outgoing calls has a tunable oscillator. When the operator plugs in, a circuit is made for moving on the assigning switch SM3 (which is mechanically connected to the switch SM2) if the chosen circuit happens to be that assigned for incoming calls. The operator then operates the key KY1 to complete an order channel individual to the wanted office. KY1 also operates RL3 which energizes the stepping magnet SM1 of the tuning switch. At. the incoming end, the characteristic carrier current energizes RL7, which energizes the magnet SM1 of the trunk circuit assigned for incoming calls by the switches SM2, SM3. RL3 at the outgoing end and RL7 at the incoming end and consequently the two stepping magnets SM1 are operated together. When the circuits are tuned to a busy frequency the relays RL4, RL5 are operated and the switches continue to step by self-interruption. But when they are tuned to an idle frequency, the relapse of RL5 at each station removes a shunt from RL6 which connects up the oscillator OS1. At the calling end, at which RL1 and RL2 have been operated, RL6 also lights SL1. At the incoming end it lights CL1 which is replaced by SL1 when the operator plugs in to answer the call. The removal of each plug extinguishes the supervisory lamp at the other office.