Telstra''s Experimental Broadband Network

An FM demodulation system having means for noise reduction. The system has a signal path which includes a limiter, an FM demodulation circuit and a gate circuit, and a control circuit which includes a differentiator, two mono-stable-multivibrators, an adder and a gate signal generator. The input of this control circuit is coupled to the output of the limiter and the output of the control circuit is coupled to the control terminal of the gate circuit so as to apply a gate signal to the gate circuit. The gate circuit in the signal path operates in such a way that a demodulated signal is passed to the output terminal of the system when the gate signal is not present and the demodulated signal is prevented from passing to the output terminal when the gate signal is present. The control circuit generates the gate signal when a dropout in the FM signal occurs in the following manner. The signal from the limiter is differentiated by the differentiator and is formed into a narrow pulse train having positive and negative polarities. The positive spikes and the negative spikes of this pulse train trigger the respective ones of two mono-stable-multivibrators, and the outputs from these multivibrators are added by the adder. The output pulse-width of these multivibrator is controlled so that a marker signal appears at the output of the adder when carrier dropout in the FM signal occurs. The gate signal generator generates the gate signal from the marker signal and the resulting gate signal is fed to the control terminal of the gate circuit. a result, a noise component does not appear at the output terminal.