GB513533A - Improvements in or relating to electric signalling systems - Google Patents

Abstract

513,533. Multiplex systems. STANDARD TELEPHONES & CABLES, Ltd., and TERRY, V. J. April 8, 1938, No. 10857. [Class 40 (iv)] [Also in Group XL] In a carrier system in which speech is transmitted on a single side-band and the carrier is keyed at a low frequency for the transmission of dialling or supervisory signals or for telegraphy, une speech is demodulated by the addition of a locally-produced carrier of the same frequency as the original carrier, whilst for the other signals a locally-produced carrier of a different frequency is used, the difference frequency being then,rectified and used to control a relay. In the multi-channel system shown, Fig. 1, speech and signals modulate the carrier O and the filter SF1 transmits one speech side-band and both signal side-bands. At the receiving end, these are applied through a bridge BN (to prevent interference between the two carrier sources) to demodulators DMf1, DMs1. Speech is demodulated by the addition of carrier O1 of the same frequency as O and passed through an amplifier A with a filter Fx to remove any low frequencies corresponding to the signals. In the demodulator DMs1 a source On of a different frequency is added and this may be the carrier for one of the other channels or for the return path of the same channel. The difference frequency with signal side-bands is selected by a narrow band-pass filter RFs1, rectified at D and applied as grid bias to the amplifier A to control a relay R in the anode circuit. Fig. 2 shows a suitable circuit for filter RFs1, detector D, and amplifier A. The demodulated speech is applied over transformer T1 to the grid of valve V1 and the rectified signals are applied over resistances D2, D1 as grid bias to control relay R. Transformer T1 is inefficient for low frequencies and resistances D2, D1 render the signal path inefficient for high frequencies. The amplifier may comprise a pair of valves connected in push-pull for the speech and in parallel for the signals or vice versa. The Provisional Specification refers to arrangements for ordinary telephone repeater systems in which the signals are transmitted on a carrier just above the speech range, the same valves being used for the speech currents and the rectified signals. In two-wire circuits, different carrier frequencies are used for the two directions and singing in intermediate repeaters is prevented by a band-stop filter in one path and a low-pass filter in the other.