GB474021A - Improvements in or relating to multi-channel signal transmission systems - Google Patents

Abstract

474,021. Multiplex systems. SIEMENS & HALSKE AKT.-GES. June 16, 1937, No. 16756. Convention date, June 18, 1936. [Class 40 (iv)] In a multiplex system employing two modulation stages, the first carrier frequency being the same for all the channels, and one side band being filtered out after each modulation, the carrier frequencies are so chosen in relation to each other and to the width of the speech bands that the upper and lower side bands resulting from the second modulation form two substantially continuous bands with a gap between them, so that the final filtering operation can be performed by a single filter for all the channels. In Fig. 1, the speech signals, which are limited to the range 0-3 kc. by filters AF1-3, modulate a carrier S, for example 6 kc. in modulators M11-13 and the lower side bands are eliminated by filters BF1-3. These bands within the range 6-9 kc. entermodulators M21-23 which are supplied with different carriers T1-3, for example 27, 30, 33 kc. The combined outputs ‹ comprise lower side bands 18-21, 21-24, 24-27 7 kc. and upper side bands 33-36, 36-39, 39-42 kc., and the lower side bands are eliminated bv a high-pass or band-pass filter GF (see Figs. 2, 3). The general conditions for this result are that S must be an integral multiple of the original band width a and that T1, 2, 3 ... must be consecutive integral multiples of a. If there are a considerable number of channels, they may be divided into several groups each of which share a common filter GF. Moreover two neighbouring groups may be arranged to share a single band-pass filter GF, which selects the upper side bands of the lower frequency group and the lower side bands of the upper frequency group (see Fig. 5).