Patent GB-699901-A (GB)
Title: Improvements in or relating to telecommunication exchange systems
Inventor: FLOWERS THOMAS HAROLD
IPC: H04Q1/446
Filed: 1948-12-31
Abstract: 699,901. Automatic exchange systems. POSTMASTER GENERAL. March 7, 1950 Dec. 31, 1948], No. 33618/48. Class 40 (iv). In a signalling system in which D.C. impulses are transmitted as corresponding V.F. impulses, the calling termination, Fig. 1, includes apparatus which delays the transmission of each V.F. impulse so that isolation of the D.C. and V.F. circuits in response to the first D.C. impulse in a train is completed and the consequent transient disturbance in the V.F. circuit dies away before V.F. signal transmission commences, while the receiving termination, Fig. 3, includes apparatus which tests the duration of the first V.F. impulse in a train and only repeats this and the subsequent impulses as D.C. impulses if the test is successful. The calling termination also includes a fault indicating circuit which is completed on seizure and is only disabled by a V.F. signal from the receiving termination. The V.F. signals are transmitted over a 4-wire circuit L1-L4 which is coupled to 2-wire D.C. signalling circuits by hybrid coils T1, T11, voicefrequency receivers VFR and blocking amplifiers V being provided in the receive paths. Calling termination. On seizure of the 2-wire circuit over selectors (not shown), relay A pulls up, followed by PA and B. Relay PA connects a V.F. source f1 to the outgoing circuit and a forward calling signal is transmitted which is terminated by B. A gating circuit consisting of rectifiers QA-QD and resistors R1, R2 connected between transformers T3 and T4 has a reversible bias potential applied by contact a1 over a time-delay network N and functions as a V.F. impulsing relay, the circuit becoming conductive to voice frequencies whenever relay A relapses. Incoming D.C. impulses are repeated by A while relays B and C function as release and dialling relays, relay C transferring the outgoing circuit to transformer T3. Since relay C does not pull up immediately in response to the first D.C. impulse a transient condition is established in the outgoing circuit. The delay network N retards the establishment of a conductive condition between transformers T3 and T4 so that interference at the receiving termination due to simultaneous reception of the transient and the first V.F. pulse is prevented. Further transients are not transmitted since relay C remains operated throughout the impulsing period. At the termination of the call relay A falls back and a long V.F. forward clearing signal is transmitted by the delayed relapse of relays B and C. Receiving termination. The V.F. impulses are repeated by a contact x11 in the voice frequency receiver VFR. Relay BS operates only when both windings are energized and relays BR, BS operate and lock at the beginning and end, respectively of a long impulse constituting the forward calling signal. BR pulls up a high-speed relay HS which establishes a D.C. calling condition. A polarized timing relay PB is normally back with both windings energized but operates when the lower winding is de-energized and the current in the upper winding reversed. Since BS is initially unoperated, the forward signal does not pull up PB. When a V.F. impulse train is received, PB operates to repeat the first impulse over relay HS only if the pulse lasts long enough for current reversal to be effected in the upper winding through a delay network N11 before x11 relapses. Slow-release relay CR is pulled up by PB and remains operated for the duration of each impulse train. The second and subsequent impulses in each train are repeated in the D.C. circuit by PB and HS after delay by network N11 without discrimination, since the lower winding of timing relay PB is disabled by CR. A gating circuit comprising resistors R11, R21 and rectifiers QA1-QD1 blocks the direct transmission path over transformer T31 to the D.C. circuit, while x11 is operated during receipt of V.F. impulses. When a prolonged impulse such as a forward clearing signal is received, relays BR and BS fall back and CT operates from wire P so that a backward signal from a voice frequency source f<SP>2</SP> is transmitted to the calling termination. According to the Specification, the capacity and inductance delay network shown may be replaced by a resistor-capacitor charging circuit and the time delay may be augmented by biasing the second winding of PB. The Provisional Specification includes the feature whereby the resistor and rectifier network is maintained non-conducting throughout an impulse train by a rectifier-controlled condenser charging circuit connected across the bias leads. Fault indicating. At the calling termination the momentary operation of PA pulls up relay T which locks to contact x in the voice frequency receiver. Lamp LA is therefore lit until T is disabled by a backward V.F. signal.