AM Radio

Start with a band-limited signal.

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a = BandpassFilter[ AudioNormalize@ AudioResample[Import["ExampleData/rule30.wav"], 96000], {Quantity[ 10, "Hertz"], Quantity[5000, "Hertz"]}, 201]

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Periodogram[a, 2000, ImageSize -> Small, PlotRange -> {{0, 10000}, All}]

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Modulate the amplitude with a 22,050 Hz sinusoid to shift the content at the high end of the spectrum. The result should be already inaudible for most people.

In[3]:=

amsignal = ((.5 + a) AudioGenerator[{"Sin", Quantity[22050, "Hertz"]}, Duration@a])/2

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Periodogram[amsignal, 2200, ImageSize -> Small]

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Demodulate the AM signal by multiplying the result by another sinusoid at 22,050 Hz with the same phase.

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result = BandpassFilter[ amsignal*AudioGenerator[{"Sin", Quantity[22050, "Hertz"]}, Duration@amsignal], {Quantity[100, "Hertz"], Quantity[5000, "Hertz"]}, 201]

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Use a nonlinear filter to demodulate an AM signal.

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BandpassFilter[# - Mean@# &@ MaxFilter[amsignal, 4], {Quantity[100, "Hertz"], Quantity[5000, "Hertz"]}, 201]

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