GB556369A - Speech transmission system - Google Patents

Abstract

556,369. Composite transmission systems. STANDARD TELEPHONES & CABLES, Ltd. (Western Electric Co., Inc.). May 22, 1942, No. 6997. [Class 40 (iv)] In a phantom-circuit system, low frequency signals are transmitted over the side-circuits with the aid of low-pass filters, shunt elements of which are centre-tapped to provide terminals for the phantom connections. As shown in Fig. 1, low-frequency telegraph signals from instruments associated with the respective wires of the upper side-circuit are applied across condensers 52, 53 and derived from condensers 111, 112 to enable them to be by-passed round a speech repeater. The condensers form low-pass filters with inductances such as 156, 172 and the by-path may include further filter sections, Fig. 2 (not shown). The phantom circuit is connected to common points of the condenser pairs. In another arrangement, Fig. 4, there is only one D.C. channel which uses all four wires in parallel and 20-cycle signalling sources are connected as shown to the side and phantom circuits. In a similar arrangement, Fig. 3 (not shown), the 20-cycle signal current for the phantom is applied to the transmission circuit over the D.C. connection. In other arrangements, Figs. 5 and 6 (not shown) respectively, the arrangements resemble those in Figs. 1 and 4 but make use of combined line and hybrid transformers. Modifications of the arrangement shown in Fig. 6 are illustrated by Figs. 8 and 9 (not shown). To eliminate interference between telegraph channels due to the coupling between windings such as 54, 55 of the line transformer, a counter-coupling may be provided by means of a transformer 241, 242, Fig. 10, having, if desired, a third winding 243 associated with a variable resistance to adjust the countercoupling. In another arrangement with the same object, Fig. 7 (not shown), the line side of the line transformer is shunted by an inductance to make it inefficient at the telegraph frequencies.