Patent GB-731912-A (GB)
Title: Improvements in or relating to electronic switching systems
Inventor: SCOWEN FRANK
IPC: H04Q11/02
Filed: 1949-03-29
Abstract: 731,912. Automatic exchange systems. POSTMASTER GENERAL. March 20, 1950 [March 29, 1949], No. 6906/50. Divided out of 8518/49. Class 40 (4). [Also in Group XL (c)] The Provisional Specification describes an automatic exchange in which signals from subscriber's lines are caused to modulate associated carrier frequencies and applied to a common path feeding a number of link circuits one of which selects signals from the desired line by applying an appropriate frequency to demodulate the incoming signals. As shown, when a calling subscriber closes his loop the D.C. potential developed across relay A, Fig. 2, unbalances the modulator 5 which thereupon transmits a carrier frequency fo (common to all lines) via the filter 10 to the modulator 13 which is fed with frequency f6 characteristic of the subscriber's line. The output is fed to the common filter 14 which passes the lower side-bands only to all the links, Figs. 3, 4. Relay A in operating applies battery to the marking lead M6 and in the link pre-selected by the switch W the corresponding gas-tube V7 is fired. The variable attenuator U8, U9, R22 is thereby unblocked to pass frequency f6 via the amplifier 23 to the demodulator 32 and the modulator 33. The former therefore passes frequency fo to the detectors U4, U5. The former provides A.V.C. bias for the demodulator 32 and the latter causes V2 to conduct. Relay C operates, followed by D which steps the link preselecting switch W to enable the next free link. Relay D also unbalances the modulator 41 which applies frequency fo to the modulator 33 whence the signal is conveyed to the demodulator 16 in the subscriber's line circuit. After demodulation the frequeney fo is rectified and causes V1 to conduct, bringing up B which disconnects the marking wire M6 to prevent the calling line seizing another link. Dial tone, normally connected to leads 42, is fed over the modulators 41, 33, the demodulator 16 and rectifier U3, valve V1 and the hybrid 3 to the calling subscriber. The dial breaks are repeated as interruptions of carrier frequency current fo to rectifier U5 and thence via V2 to the relay C which steps the switch Y. During dialling, slow-release relays E, G and J operate in succession, E bringing up H, which removes the dial tone. If the wanted line is busy its lead in the bank of Y2 is earthed (over a3 or b3) and F comes up to connect busy tone to leads 42 and to prevent operation of the connector, Fig. 4. If the wanted subscriber is idle however, when G and J release the valve V8 corresponding to the wanted subscriber is triggered over wiper Y3 and thereby unblock an attenuator to allow the carrier frequency of the wanted subscriber to pass to the modulator 58 and demodulator 57. Current of frequency fo is thereby allowed to pass to rectifier U3, thereby enabling valve V1 in the called subscriber's line circuit to bring up B which connects up ringing current. When the called subscriber replies this is removed by the operation of relay A. Speech takes place in the one direction over modulators 5, 13 in the calling subscriber's line circuit, demodulator 32 and rectifier U5, valve V2, leads 39, 40 and modulator 59, 58 in the link circuit; and demodulator 16, rectifier U3 and valve V1 in the called subscriber's line circuit. The speech in the reverse direction is similarly treated. When the calling subscriber hangs up modulator 5 ceases to transmit frequency fo which causes V2 to cut off, the resulting rise in anode potential operating through a time delay circuit to fire V3 (previously extinguished by the firing of V7). Gas valve V7 is thereby extinguished. The rise in anode potential of V2 is also passed over the phantom of wires 39, 40 to fire V4 which extinguishes V8 and the link circuit is thus returned to normal. The Complete Specification relates only to the gate circuit comprising valve V7 and the attenuator network U8, U9, R22 (see Group XL (c)).