GB582044A - Improvements in multiplex radio systems - Google Patents

Abstract

582,044. Multiplex wireless signalling. PRESS WIRELESS, Inc. Nov. 15, 1943, No. 18998. Convention date, March 12. [Class 40 (v)] In a multiplex system employing a single carrier, one channel for code or facsimile telegraphy is provided by frequency modulation and another channel for voice signals is provided by amplitude modulation, the telegraph signals being rounded to prevent abrupt changes of frequency. In Fig. 1, the carrier generator 2 is frequency-modulated by a modulator 3 controlled by telegraph or like signals from the source 4. The F.M. wave is amplified at 5 and is amplitude-modulated fairly deeply by a modulator 7 controlled by an audio source 6. At the receiver the amplitude modulation is detected by a flat-response receiver 9 preferably wlthout heterodyning and passes to an audio signal reproducer 11. The received wave is also fed to a receiver 10 where it is heterodyned by a source 12 and the output passed through a band-pass filter 13 to a limiter 14, frequency discriminator 15, which may be of the kind described in Specification 580,968, detector 16, and facsimile or telegraph reproducer 17. Before transmission the telegraph signals are rounded to avoid too abrupt changes in frequency. The frequency range after heterodyning by 12 is less than an octave, and the pass band of the filter 13 is correspondingly limited. The receiving arrangement may comprise a pair of diversity-receiving aerials connected to pairs of AM and FM receivers with interconnecting devices for selecting the stronger of the two signals as described for example in Specification 577,247, (Figs. 2 and 3, not shown). In a modified arrangement, the voice signal is frequency-modulated on a subcarrier, and the resultant is amplitude-modulated on the main carrier (Fig. 3, not shown). The limiter 14 of Fig. 1 may be replaced by a radio-frequency limiting stage interposed before the heterodyne stage 12 (Fig. 4, not shown).