GB729115A - Improvements in or relating to telephone subscribers' instruments - Google Patents

Abstract

729,115. Anti-side-tone circuits. STANDARD TELEPHONES & CABLES, Ltd. Dec. 6, 1951 [Dec. 6, 1950], No. 29901/50. Class 40(4). In order to improve side-tone balance on lines of differing impedances, a substation circuit of the anti-side-tone type, comprising a transmitter 5, a receiver 6, and normal side-tone balancing means X, is provided with a correcting network Y containing a non-linear element responsive to the alternating current output of the transmitter 5 but independent of the applied unidirectional current, the correcting network Y being so arranged that when the transmitter output is low the impedance of the non-linear element is sufficiently high to constitute effectively an open circuit, but when the transmitter output exceeds a predetermined level the impedance of the non-linear element is relatively low and the correcting network Y when subjected to an applied voltage of any given frequency produces an output current in which the component of the given frequency is substantially the same as would be produced by a reactance and resistance in series. The invention may be applied to a substation of either the common battery type, Fig. 1, or the local battery type, Fig. 2 (not shown). The correcting network Y may comprise a condenser in series with a non-linear element which may consist of two oppositely-poled rectifiers in parallel, a silicon carbide element, a thermistor, or a saturable choke (Figs. 3-6 respectively, not shown). Alternatively the correcting network Y may comprise a transformer 19, Fig. 7, of which the secondary circuit contains a non- linear element 16 in series with a condenser 13 and a resistor 21 in parallel.