GB472816A - Improvements in or relating to carrier signalling systems - Google Patents

Abstract

472,816. Multiplex transmission. GENERAL ELECTRIC CO., Ltd., WESTELL, E. P. L., and LEY, D. A. Feb. 28, 1936, No. 6025. [Class 40 (iv)] [Also in Group XL] In a multiplex two-wire or four-wire carrier system, each channel comprises a two-stage modulator M1 .. M12 in which the speech first modulates a frequency which is the same for all channels and is comparatively low but is at least twice the highest speech frequency, one of the side-bands then modulating a carrier characteristic of the channel and one of the resulting side-bands being selected for transmission and amplified at 1A .. 12A. The odd and even channels are associated with the multiplex line through separate impedances 25, 26 as described in Specification 405,026, [Group XL], to limit inter-channel interference. The carriers at one end are derived from a common oscillator O and harmonic producer H which comprises a frequency doubler followed by a square-wave generator of the kind described in Specification 428,708, [Group XL], and those at the other end are derived from currents transmitted with the signals and acting also to control the gain of the amplifiers on the route, e.g. as described in Specification 463,409. The stand-by and main oscillators may be at opposite ends. Demodulators D1 .. D12 are constructed and arranged similarly to the modulators. To save apparatus, pairs of second modulators may use the same carrier, upper and lower side-bands respectively being selected for transmission. Two-stage modulators and demodulators. In one arrangement, Fig. 2 (not shown), both stages comprise two-rectifier arrangements, but as shown in Fig. 3A the first modulator comprises a four-rectifier arrangement which is very much less favourable to third order modulation products than the tworectifier arrangement and so makes it possible to use a simpler filter at 12. The demodulators Fig. 3B are identical except for the situation of the potentiometers for balancing out residual carrier current. By suitably mismatching the impedance of the low-pass input filter 9 to the impedance of the modulator 11 and the band-pass filter 12, and by mismatching the input impedance of filter 12 to the modulator 11 and the filter 9 and by suitable design of the filter 12 the circuit can be made to be free from or to compensate for distortion. Attenuation pads may be associated with the first modulators to limit the degree of mismatching and with the second modulators to compensate for distortion.