In frequency-modulated signals, particularly in the frequency-analog measuring technique, only the zero crossings of the signals or the intervals between the zero crossings are evaluated. In order to prevent or at least to reduce interferences which occur meanwhile, the receiver is blocked for a given period after each zero crossing. In order to generate a blocking signal having an optimum duration, the values of the duration of the preceding signal zero crossing intervals are stored individually and the minimum duration to be expected of the next signal zero crossing interval is calculated therefrom. In the same manner the maximum duration to be expected of the next signal zero crossing interval can be determined, at the end of which maximum duration a substitution signal can be applied to the receiver when no zero crossing of the input signal has meanwhile been detected. As a result, the adverse effects of for instance short transmission interruptions can be eliminated.
Disclosed is a muting circuit, in which the generation of click noise is prevented in that the muting operation is caused to occur at an instant when the instantaneous level (i.e., waveform level) of the input audio signal becomes zero for the first time since the appearance of a muting signal.
A digital frequency demodulator includes a first counter and a second counter measuring the instantaneous and mean values of the period of a frequency modulated signal. A comparator compares the values measured and creates a binary signal of the same frequency as the modulation signal. The measurement of the instantaneous value of the frequency modulated signal is effected by counting the number of reference clock pulses during a period of the modulated signal, and the measurement of the mean value is effected by counting the number of pulses of a clock signal of frequency N times lower than the reference clock during N periods of the modulated signal.
