GB448084A - Improvements in carrier wave signalling systems - Google Patents

Abstract

448,084. Two-way transmission. STANDARD TELEPHONES & CABLES, Ltd., 63, Aldwych, London.-(Assignees of Caruthers, R. S.) Nov. 1, 1935, No. 30224. Convention date, Dec. 14, 1934. Addition to 427,605. [Class 40 (iv)] [See also Group XL] In the two-way carrier transmission system of the parent Specification in which combined modulator-demodulators are provided at two interconnected stations whereas a carrier source is provided at one station only, a twoway amplifier is also used at the station having the carrier source, this amplifier serving as a high frequency amplifier in one direction and a speech frequency amplifier in the other direction. In a modification, the valve acting as the amplifier serves also as an oscillator. In the system shown in Fig. 1, a separate carrier source 40 and amplifier 22 are provided at the left hand station. Speech from jack 31 passes over filter LPF to a bridge 25, is modulated at 33 and after amplification at 22 passes over HPF to the line. The carrier is also amplified at 22 and fed to the line so that the modulated carrier on arrival at the other station is demodulated at 331. Speech in the other direction modulates the carrier applied over the line to 331 and on arrival at the left hand station is demodulated at 33, and amplified at 22. The impedance across the line at point 15 between the filters LPF, HPF approximates to a resistance and is balanced by resistance 21 which may have capacity in series or parallel or be replaced by a network. If filters LPF, HPF do not have the same cut-off frequency, an unbalance may exist over the range between the cut-off frequencies in which case a rejector filter 23 is provided. Where a hybrid coil, Fig. 1A, is used instead of the bridge 25, a choke 26 is used to provide a path for direct current due to the modulator-demodulator. The combined oscillator and two-way speech and carrier frequency amplifier is shown in Fig. 3 which also comprises the bridge 25 and modulatordemodulator 33 of Fig. 1. The pentode 46 oscillates at the desired frequency by virtue of a feed-back path 52 containing series resistance and a tuned shunt circuit 53 to exclude speech and side bands. In order to avoid overload as a combined oscillator and amplifier, the amplitude of oscillation is limited by a further shunt 47 of non-linear characteristic such as a material formed from silicon carbide, or the arrangement of metal rectifiers 63, 64 and adjustable resistances 66 shown in Fig. 3A. The modulator-demodulator 33 may be designed to provide the limiting effect. A voice frequency channel may be operated over the line in known manner.