GB376245A - Telephone transmission system - Google Patents

Abstract

376,245. Correcting inductive disturbance. STANDARD TELEPHONES & CABLES, Ltd. and LATIMER, K. E., Columbia House, Aldwych, London. Jan 5, 1931, No. 355. [Class 40 (iv).] Far end cross-talk between two cable circuits having substantially equal transmission times is balanced, for both directions of transmission and with either circuit as the disturbing circuit, by inserting condensers, inductances or resistances separated by distances long compared with a wavelength from the unbalances which they correct, e.g. at the ends of the cable or of repeater sections. The invention is useful in two-wav carrier systems with carriers of different frequency for the two directions. There is no near end cross-talk in such systems. If the transmission times are unequal, they are first equalized by cross-connections in a repeater section so that the products of total capacity and inductance are the same for each circuit. The transmission time of a phantom circuit may be made equal to that of the side circuits by adjusting the average inductance of the phantom coils, the phantom to side capacity ratio in loading sections being made constant by T- splicing a small length of spiral-four cable in the section to serve as additional capacities since the phantom to side capacity in this type of cable is high. According to another method, each circuit in a loading section comprises substantially equal portions of multiple twin and spiral-four ; a cable having half its quads of multiple twin and half spiral-four may be used. The phases of cross-talk due to capacity unbalance and mutual inductance differ by a small angle and this difference may be neglected by using either capacity or inductance for balancing, or it may be compensated by using both capacity and inductance for balancing or by using two condensers and a phase-shifting device as shown in Fig. 4. Circuits 1, 2, 3 have terminal equipments 7, 8, 9, and phase-shifting devices 4, 5, 6, so that as between circuits 1 and 2, for example, balancing currents introduced into circuit 2 by condensers 10, 11 differ in phase. Resistance unbalance cross-talk between phantom and side circuits may be corrected by resistance in one wire of the side circuit. The terminal impedance should match the line since reflection in a disturbing circuit gives rise to cross-talk, and this effect will be noticeable if the cross-talk testing set and terminal equipment have different impedances. A slight want of matching may however be used for phase shifting, the devices 4, 5, 6, Fig. 4, being in this case constructed as attenuating pads having the same impedance as the lines. The balancing condensers may be mounted in a rack 3, Fig. 3, each cable pair being connected to a pair of vertical wires 7 or 8 and a pair of horizontal wires 5 or 6. The balancing condenser 4 between any two circuits is conveniently situated with regard to the intersection of the horizontal and vertical wires. In a cable containing two-wire and four-wire circuits, the two-wire circuits may be in the outer layers and the four-wire circuits inside. Usually, the fourwire high-quality circuits, if unscreened, are outside where there is least cross-talk. By placing them inside and balancing them according to the invention, these high-quality circuits are not so subject to external disturbance, suffer less capacity change during cable laying, and the two circuits of the four-wire circuit may be readily screened from each other. Reduction of noise interference may be effected by arranging that the earth circuits have the same transmission velocity as the metallic circuits.