Patent GB-326570-A (GB)
Title: Improvements in or relating to telephone systems
IPC: H04Q1/453
Filed: 1928-12-20
Abstract: 326,570. Standard Telephones & Cables, Ltd., and Skillman, T. S. Dec. 20, 1928. Exchange systems. - Calling &c. signals and switch-controlling impulses are transmitted over long-distance junctions by alternating-current pulses consisting of combinations of different voice-frequencies, means being provided to prevent faulty operation if the same frequencies occur during speech. Signalling arrangements, Fig. 1. The apparatus shown is arranged at each end of a longdistance junction. 20-cycle ringing current applied from the left causes the operation of relays 5, 3, 4, whereby the frequency-combination W, X, Y is sent out over the junction. At the incoming end the response of relays WV, XV, YV short-circuits all the energized windings of relav 2 so that the latter releases and causes relay 1 to apply 20-cycle current to the local circuit. A further relay 2V tuned to a fourth voicefrequency controls a fourth winding of relay 2 to prevent faulty operation of relay 1, it being assumed that any speech-wave which simultaneously operates all three relays WV, XV, YV will also operate ZV. Relays 1, 4 connect suitable terminating resistances across the junction during signalling periods. Signalling and impulsing arrangements, Figs. 2 and 3. On plugging into jack J, relays 20, 10, 12, 14 operate and during the interval before the relapse of relay 13, relay 17 transmits a calling pulse of frequencies W, X, Y over the junction. Relays WVI, XVI, YVI respond, causing the release of relay 22 which connects earth over its back contact to operate relay 34 and impulse-repeating relay 31 which connect 25 to the a-wire. Dial impulses from the cordcircuit are repeated by relay 10 to relays 15, 16 which together cause a pulse of frequency-combination W, X, Z to be transmitted over the junction for each impulse. Relays WVI, XVI, ZVI respond to each impulse causing the deenergization of relay 31 which repeats the impulses over the a-wire to local selectors. When the called subscriber replies relay 26 operates and relays 28, 32 cause a pulse of frequencies W, Y, Z to be sent back. The response of relays WVQ, YVO, ZVO releases relay 6 which by operating relay 8 gives answering supervision in the cord. When the called subscriber hangs up, the release of relays 26, 28 causes relay 33 to transmit a pulse of frequencies W, X, Z, which releases relays 7, 8. When the jack J is unplugged, the release of relays 20, 12 sends a pulse W, Y, Z, which effects the release of relays 21, 34, 35. False impulsing is prevented by relay 35 which is operated during conversation by relay 26 and is also operable, prior to the reply of the called subscriber, by relay YVI which may be expected to operate in the event of the simultaneous accidental operation of all the impulsing relays WVI, XVI. ZVI. The sluggishness of relays 6, 7 prevents faulty operation of the signalling apparatus shown in Fig. 2. Relays 11 and 27 operate whenever any of the voice-frequency relays in Figs. 2 and 3 respectively pull up and thus prevent any alteration in the state of relays 12 or 28 respectively during receipt of signals. This prevents one of the clearing signals being lost if release of the connection should occur simultaneously at both ends of the junction. Signals may be transmitted by one voicefrequency only, the remaining frequencies being used for guard purposes.