GB415717A - Improvements in echo suppressors or other similar blocking arrangements for telephone and like transmission systems - Google Patents

Abstract

415,717. Wired wireless systems. TELEFONAKTIEBOLAGET L. M. ERICSSON, 33, Kungsgatan, and LAURENT, T., 8, Karlsviksgaten, both in Stockholm. Feb. 28, 1933, No. 6143. [Class 40 (v).] In a system employing the same channels for telephony and superaudible signalling such as carrier telegraphy, a channel is adapted to be blocked as regards telephonic currents only by a short:circuit across the output end of a filter bridged across the channel and passing the speech band. Fig. 1 shows the simplest form of the invention. A preferred arrangement shown in Fig. 3 employs a differential transformer T with two bridged filters F, F<1> which have the same impedance as regards telephonic currents, but as regards signal currents, F is of high impedance while F<1> is of low impedance. The filter F is adapted to be short-circuited at its output end by a contact S, while the filter F<1> is permanently short-circuited at its output end. The impedance characteristics of the two filters of the forms shown are illustrated in Figs. 4, 5, from which it is seen that w1 is the most suitable frequency for the signalling currents. If desired, the connections of the line l1 and the filter F and of the line l2 and the filter F<1> may be interchanged. The short-circuit across the filter F may be applied through a transformer N, Fig. 7, by means of two neon tubes g as in Specification 308,816, [Class 40 (iv)]. Since this short-circuit is not complete however, a small balancing resistance R<1> is connected across the output end of the filter F<1>. In order that reflection shall not occur at the input of the differential transformer T, the bridge connection is dimensioned so that its input impedance corresponds to the line impedance. Fig. 8 shows an arrangement for obtaining this result for both positions of the contact U, the resistances R, R<1> which terminate the filters when U is operated being equal to the characteristic impedances of the filters over the speech range. Greater attenuation for speech currents may be introduced by connecting at the output ends of the filters resistances which may be included in the end resistances.