Toffolo, Franco;

Noise detector

Noise detector for detecting noise pulses in a non-limited radio broadcast signal modulated on a carrier, comprising an amplitude detection circuit and coupled thereto a first signal comparison circuit having a signal input and a threshold input, a first threshold signal depending on the average input level of the noise detector and a second threshold signal depending on the noise pulse repetition frequency being applied to this threshold input. To limit the repetition frequency of the detection output signals of the noise detector without reducing the input sensitivity, the threshold level of the noise detector is controlled in forward direction in dependence on the field strength and the noise pulse repetition frequency. To that end the amplitude detection circuit is connected through a signal input to an input of the noise detector and through an envelope output to the signal input of the first signal comparison circuit and also to a signal input of a second signal comparison circuit, this envelope output also being coupled to a first low-pass filter for forming the first threshold signal, which first threshold signal is applied to the threshold input of the first signal comparison circuit and a threshold input of the second circuit, this second circuit being coupled through a second low-pass filter for forming the second threshold signal to the threshold input of the first circuit.

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What is claimed is:

1. A noise detector for selecting noise pulses from a non-limited FM radio broadcast signal modulated on a carrier comprising:

a logarithmic level detection circuit having an input for receiving said modulated signal and an output for supplying the logarithm of the envelope of said modulated signal

a first lowpass filter having an input connected to said output of said logarithmic level detection circuit and an ouput for producing a signal representing the average value of said envelope;

a threshold generating circuit connected to said outputs of said logarithmic level detection circuit and said first lowpass filter which generates first and second threshold levels, said first threshold level being generated during a first signal condition when said lowpass filter output signal is less than said logarithmic detection circuit output signal, said second threshold level being generated during a second signal condition when said lowpass filter signal is greater than said logarithmic level detection output signal;

a first signal comparison circuit having a signal input, and a threshold input and an output, said signal input being connected to said output of said logarithmic level detection circuit, said threshold input being connected to said output of said threshold generating circuit for receiving said first or second threshold level signals, said first signal comparison circuit supplying a detection signal when a signal amplitude value at said signal input exceeds the amplitude value of one of said threshold level signals.

2. A noise detector of claim 1 wherein said first threshold level is proportional to the summation of said first lowpass filter output signal and the integral of said lowpass filter output signal.

3. A noise detector of claim 1 wherein said second threshold level is proportional to said lowpass filter output signal.

4. A noise detector of claim 1 wherein said threshold generating circuit comprises:

a second signal comparison circuit having a signal input, and a threshold input and an output, said signal input being connected to said output of said logarithmic level detection circuit, said threshold input being connected to said output of said first lowpass filter, said second signal comparison circuit supplying an output signal when a signal amplitude value at said signal input exceeds the amplitude value of said threshold level input;

a second lowpass filter having an input connected to said output of said second signal comparison circuit and an output supplying an integrated signal; and

an adder circuit having one input connected to said second lowpass filter for receiving said integrated signal and a second input connected to said output of said first lowpass filter said adder circuit adding said output signal of said first lowpass filter to said integrated signal, said adder circuit supplying an output signal representing said first threshold level.

5. A noise detector of claim 4 wherein said threshold generating circuit further comprises another adder circuit having one input connected to said output of said first lowpass filter and a second input for receiving a fixed threshold value, said another adder circuit adding said fixed threshold value to said output signal of said first lowpass filter, said fixed threshold value effecting a D.C. shift of said signal representing said average value at an output of said another adder circuit.

6. A noise detector as claimed in claim 5 wherein said fixed threshold value exceeds the average noise level in said signal applied to said signal input of said second signal comparison circuit.

7. A noise detector as claimed in claim 6, wherein said fixed threshold value is about 1 dB.

8. A noise detector as claimed in claim 7, wherein said second lowpass filter has a 3 dB-cut-off frequency of 0.1 kHz.

9. A noise detector as claimed in claim 4 further comprising a monostable multivibrator connected between said second signal comparison circuit and said second low-pass filter and a threshold circuit connected between said second low-pass filter and the threshold input of said first signal comparison circuit for delaying an increase of said threshold level signal until noise repetition frequency has increased to about 5 kHz.

10. A noise detector as claimed in claim 1 wherein said logarithmic level detection circuit comprises a logarithmic converter circuit having an output signal proportional to the logarithm of its input and an amplitude detection circuit having an input connected to said output of said logarithmic converter.

11. A noise detector for selecting noise pulses from a non-limited FM radio broadcast signal modulated on a carrier comprising:

a logarithmic level detection circuit having an input for receiving said modulated signal and an output for supplying the logarithm of the envelope of said modulated signal

a first lowpass filter having an input connected to said output of said logarithmic level detection circuit and an output for producing a signal representing the average value of said envelope;

a first adder circuit having one input connected to said output of said first low pass filter and a second input for receiving a fixed threshold value, said adder circuit adding said fixed threshold value to said average value signal, said fixed threshold value effecting a D.C. shaft of said signal representing said average value at an output of said adder circuit;

a signal comparison circuit having a signal input, and a first threshold input and an output, said signal input being connected to said output of said logarithmic level detection circuit, said first threshold input being connected to said output of said first adder circuit for receiving a first threshold level signal, said signal comparison circuit supplying an output signal when a signal amplitude value at said signal input exceeds amplitude value of said threshold level signal;

a second lowpass filter having an input connected to said output of said signal comparison circuit and an output supplying an integrated output signal;

a second adder circuit having one input connected to said second lowpass filter for receiving said integrated signal and a second input connected to said output of said first adder circuit for receiving a signal representing said first threshold level signal, said second adder circuit adding said first threshold level signal to said integrated signal, said second adder circuit supplying an output signal representing a second threshold level, said second threshold signal depending on a noise pulse repetition frequency; and

another signal comparison circuit having a signal input, and a second threshold input and an output, said signal input being connected to said output of said logarithmic level detection circuit, said second threshold input being connected to said output of said second adder circuit for receiving said second threshold level signal, said another signal comparison circuit supplying a detection signal when a signal amplitude value at said signal input exceeds the amplitude value of said second threshold level signal.