681,002. Radio signalling; frequency modulation. CURRY, P. May 16, 1949, No. 13020/49. Class 40 (v). Two carrier waves frequency modulated with symmetrical saw-tooth waveforms and having different upper and/or lower frequency limits and unequal frequency excursions are coincidentally transmitted by radio or wire to a receiver, with one at least amplitude or frequency modulated additionally with the signal to be communicated and the other used at the receiver as a control wave for deriving the signal. The double modulation ensures secrecy. A symmetrical saw-tooth generator 20ab of frequency about 10 kc/s. cathode modulates through an amplifier and cathode followers 34a, 34b a pair of blocking oscillators 12a, 12b, to produce modulation of the quench intervals, i.e. frequency modulation of the quench frequency, between 50-35 kc/s., harmonics of which are passed through a high-pass filter to amplifiers and detectors with resonant networks 38a, 39a, and 38b, 39b of narrow bandwidth, centred about 7650 kc/s. and 8500 kc/s., respectively. Each channel (A and B) network is thus intermittently energized by successive harmonics, 153rd to 217th and 170th to 240th, respectively, of the varying blocking oscillator frequencies. This energization after detection by diodes 41a, 41b, and filtering, produces a sinusoidal wave frequency modulated between 900-1800 kc/s. and 1000-2000 kc/s. in channels A and B, respectively, with a symmetrical saw-tooth modulation waveform, approximately linear. Each wave is then passed through a band-pass filter to output valves 48a, 48b, one or both of which are suppressor grid amplitude modulated, e.g. in phase opposition, by the modulation signal to be transmitted, fed to a transformer 19ab, each output valve feeding a common aerial or line. The saw-tooth generator 20ab may be a gas-filled triode, the amplitude of which and therefore frequency limits of the waves A and B are determined by a double diode 31ab producing voltages of opposite sign alternately across its load circuit applied to the triode control grid, the voltages being derived from tuned circuits 27ab, 28ab resonant to about 900 and 1800 kc/s. connected to the output of channel A. Receiver. At the receiver a further wave C, frequency-modulated between 1500-2500 kc/s., is produced with regular saw-tooth modulation waveform by means similar to that producing waves A and B at the transmitter. The frequency sweep of wave C is synchronized with that of waves A and B by beating wave C with frequencies 1800 kc/s. and 900 kc/s. derived alternately from control wave A to produce two difference frequencies 700 kc/s. and 600 kc/s. which alternately, through a double diode rectifier, initiate the charge and discharge cycles of the gas-filled triode by acting on its control grid. Wave B (1000-2000 kc/s.) is then beat with the cophasally produced wave C (1500- 2500 kc/s.) to produce a wave of frequency 500 kc/s. free of frequency modulation having the amplitude modulation of wave B, from which is then demodulated the transmitted intelligence. Alternatively the wave C can be synchronized at the receiver by an operator to maintain a constant frequency difference from wave A (then B being the control wave), provided A also is modulated with intelligence. By detuning the saw-tooth generator slightly by a push switch initial synchronism of wave C is effected. In a modification, Fig. 4 (not shown), a switch permits the signal modulation to be applied in push-pull as additional frequency modulation to the grid circuits of blocking oscillators 12a, 12b, in lieu of amplitude modulation. At the receiver the beat frequency of 500 kc/s. is then frequency-modulated with the signal, and the latter is derived by having the succeeding R.F. circuits off-tuned to one side of the resonance curve.