GB651461A - Improvements in or relating to alternating current signalling systems - Google Patents

Abstract

651,461. Voice frequency receivers. STANDARD TELEPHONES & CABLES, Ltd. April 30, 1947, No. 11605. [Class 40 (iv)] In a voice frequency receiver comprising rectifying means to produce from the incoming signal, two D.C. signals of different amplitude having different times of build up and decay and rectifying means to derive a guarding potential, the outputs from the former two are connected in series opposition to give a nett signal of similar duration to the initially received signal, and the guard potential renders an apparent signal ineffective if frequencies other than signal frequencies are present and exceed a predetermined limit. The D.C. signals of different amplitude are obtained by means of rectifier networks VD1, VD2, Fig. 4, which are fed from the secondaries T1a, T1b of the input transformer T1, T1a, having twice as many turns as T1b and being tuned by a condenser C2. The voltage across A-B (curve (iv), Fig. 5), is used to charge a condenser C1 through a resistance R1 which is shunted by a rectifier W1 to provide a quickdischarge circuit and consequently the voltage across E-F (curve (vi)) rises more slowly than that across C-D (curve (v)) but decays more rapidly. In a complete two-frequency receiver, Fig. 14, the pairs of rectifier networks VD1, VD2 and VD3, VD4 are fed respectively by input transformers T1, T2, while a further network VD5 fed by an input transformer T3 for non-signalling frequencies is connected in parallel with VD2, VD4 via rectifiers, so that the guard potential produced is ineffective until it exceeds the potential developed by VD2 or VD4. Rectifiers W5 and W6 feed part of the back bias from VD2 to VD4 and vice versa to oppose potentials due to the filter inefficiency in the non-working circuit when only one circuit is working. Rectifiers W2 serve to hold the points C at earth potential when no signal is being received but are biassed to be ineffective when the potential across Rc which also provides standing bias to V2 and V3 is overcome by the output VD2 or VD4. The potential across resistance RD backbiasses VD2, VD4 to compensate for the difference in contact potential between the rectifiers of the signal and back-bias circuits at low inputs. When the receiver is connected in the receiving channel of a 4-wire line where a common speech and signal amplifier is used, as described in Specification 651,457, the speech output transformer and the transformers T1, T2, T3 are all in series and interference by speech received backwards is prevented by the high impedance of the amplifier valve, Fig. 15 (not shown), or the speech output transformer may be tapped to form a hybrid circuit, Fig. 16 (not shown). The Provisional Specification describes another arrangement in which the rectifier networks are all in series and the primaries of the transformers T1, T2 and T3 are tuned.