Patent GB-572002-A (GB)
Title: Improvements in telephone or like signalling systems
IPC: H04Q1/45
Filed: 1944-02-23
Abstract: 572,002. Automatic exchange systems. AUTOMATIC TELEPHONE & ELECTRIC CO., Ltd., GILLINGS, C., and BEALE, C. E. Feb. 23, 1944, No. 3338. [Class 40 (iv)] In a system employing voice-frequency signalling over junctions, each train of direct current impulses is converted at the outgoing end of a junction into a number of VF impulses one more than the number of D.C. impulses, the first and last impulses comprising two frequencies and any intervening impulses a single frequency only. At the incoming end of the junction, the first two VF impulses exercise only a preparatory function, any succeeding impulses are repeated as D.C. impulses and at the end of the train a further D.C. impulse is generated locally to make up the same number as in the original train. Two frequencies X, Y are employed, and for convenience an impulse of frequency X is termed an X impulse and one comprising both frequencies an XY impulse. At the outgoing end, D.C. impulses are received by relay A and a pair of relays EB, EM are so arranged that EB is energized at each break and shortcircuited at each make, whereas EM is energized at each make and short-circuited at each break, and therefore their successive operation by the first impulse sends an XY impulse over the junction, relay SP being connected up by EB. Succeeding impulses are repeated by A as X impulses, and at the end of the train the relapse of EB, EM in turn sends an additional XY impulse. If the digit is 1, only the two XY impulses are sent. At the incoming end, a preliminary seizing signal of frequency X operates relays BZ, NN (not shown), and the former connects up DCC, CC, AA, while the latter prepares the incoming selector. The first XY impulse releases DCC, AA momentarily and brings about the operation of EBB, EMM which are controlled by AA in the same way as EB, EM at the outgoing end are controlled by A. A subsequent X impulse operates MC which connects up DZ, SG, M, PD. Relay DZ locks and holds EBB independently of AA, Relay SG inserts the combined loss pad and termination YA ... YE, while relays M, PD open the talking path and complete the impulse loop, over which all the succeeding impulses are repeated by AA. The final XY impulse releases DCC as well as AA and DZ therefore falls back and restores the original circuit of EBB, so that EBB, EMM relapse in turn on the continued operation of AA and generate an additional impulse in the outgoing loop. Relays M, PD, SG fall back and restore the conditions that existed before the impulse train after a period sufficient to cover a hunting operation. In the case of the digit 1, the second XY impulse operates MC, but the release of DCC prevents the operation of DZ, and only the impulse generated by the relapse of EBB, EMM is sent. It should be noted that if the interval between the first XY impulse and the subsequent impulse is long enough to release EBB, EMM, another XY impulse must be received to re-prepare the circuit, since an X impulse affects AA only when EMM is up. Relay MC is given an operating lag to check the duration of the second incoming impulse, while relay CC which responds to a long supervisory XY signal guards against spurious XY impulses of excessive length.