GB603586A - Multiplex radio signalling system - Google Patents

Abstract

603,586. Multiplex radio signalling. FINCH, W. H. July 2; 1945, No. 16753. Convention date, Dec. 30, 1944. [Class 40 (v)] A signalling system comprises means for generating a frequency modulated signal having a predetermined frequency swing, and means for generating picture modulated signals operating within the guard bands provided between the frequency - modulated signnal and the next frequency-modulated signal channel. The channel allotted to an F.M. transmission, Fig. 1, has a centre carrier, the swing of the F.M, wave about the carrier being represented as a zig-zag line. A guard band is provided on either side of the band occupied by the F.M. signals, and two facsimile channels are provided at the extreme edges of the allotted channel. In a modification, the frequency swing of F.M. wave is reduced, enabling guard bands to be provided between the facsimile channels and the edges of the allotted channel. The centre carrier frequency is provided by master oscillator 10, Fig. 4, and fed through amplifier 11 to a modulator 12, which is supplied with audio signals, and via amplifier 14 to the aerial system as a frequency modulated transmission. Part of the output of master oscillator 1.0 is combined with the output of side-channel oscillator 16 in a mixer 15 to give the carrier frequencies for the facsimile channels. The lower side channel carrier frequency is separated in filter 17, and fed through phase-shifter 19 to modulator 20, which is supplied with facsimile signals from scanner-amplifier 23, which is similar to that described in U.S.A. Specification 2,047,863. The ouput of modulator 20 is fed to mixer 21 and via amplifier 27 to the antenna system to give a facsimile channel using single side-band transmission, the lower side-band only being transmitted. Similarly, the upper side channel carrier is separated in filter 15, and fed through phase-shifter 24, modulator 25, mixer 26 and amplifier 27 to the antenna system to give a single side-band facsimile transmission, using the upper side band only. In a modification, double side-band facsimile signals are sent in the lower side-channel, while the upper side-channel is used to send synchronising signals for the facsimile transmission The master oscillator and side channel oscillator may be crystal controlled, the spacing of the side channels depending on the frequency of the latter. The facsimile transmission 'may use amplitude or frequency modulation. In another embodiment, the carrier frequency is displaced from the centre of the allotted channel, and one facsimile channel is provided, spaced from the edge of the allotted channel and from the F.M. transmission by suitable guard bands.