Abstract: A radio broadcasting system is provided for transmitting and receiving through free space a composite signal consisting of a frequency modulated (fm) analog signal and a multicarrier modulated digital signal which is especially adapted to be resistive to multipath degradation. The fm signal and the digital multicarrier modulated signal are fully coherent. Further according to the invention, the digital signal comprises a plurality of carriers having a maximum amplitude at least 20 dB below the unmodulated fm signal and preferably, 30 dB below the analog signal. The multicarrier modulated signal is phase locked according to the invention to the recovered analog fm pilot tone at 19 kHz in the composite baseband spectrum of the fm signal which is at least 20 dB above the multicarrier modulated signal, which enables rapid and reliable acquisition of signal for coherent detection.

Abstract: A streaking or framing image tube includes a photocathode (4), three grid electrodes (1, 2 and 3), a focus electrode (5) and an anode (6) disposed successively along the image tube. The tube is gated by applying a positive voltage pulse first to the second grid electrode (2) and then to the photocathode (4), the delay time between the application of the pulse to the second grid and the photocathode being the gated "on" time of the image tube, while during the application of the pulse to the second grid electrode (2) and the photocathode (4), the potentials of the first and third grid electrodes (1, 3) are held constant at a constant value, and accordingly ultra high speed gating (e.g. allowing the passage of electrons for less than 10.sup.-9 seconds) can be achieved without effect on the magnification or resolution performance of the image tube.