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Assume we have a simple AM signal, which is a mix of the carrier wave and upper/lower bands.

If I were to plot its time domain graph, I would get a graph with Y being its amplitude and X as time, similar to an oscilloscope graph. Let's call this graph A.

I can also plot a graph for its generated audio, with some software like "Audacity". This is graph B.

Is graph B the same as graph A? Are they identical? if not then how do they differ? What is their relation?


Why am I asking this question?

Question is related to software defined radio and demodulating RF signals. We can either sample the signal directly using IQ provided by the hardware, OR we could use its generated audio with software like multimon-ng. I'm trying to understand what's the difference between these two methods, is a signals audio the same as the signal itself?

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    \$\begingroup\$ ask yourself this question ... is the modulated carrier within audio frequency range? \$\endgroup\$
    – jsotola
    Apr 25, 2022 at 2:35
  • \$\begingroup\$ If you want to know the difference between modulated and unmodulated signal, Wikipedia has beginner level pictures of AM signals. Whether to demodulate it in hardware or software depends on if you want to use CPU horsepower to demodulate and unless you have specs of the demodulators you can't know which one does a better quality demodulation and even the worse one can be better than what you need. \$\endgroup\$
    – Justme
    Apr 25, 2022 at 7:17

2 Answers 2

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No, the signals are different: the AM signal (AM=amplitude modulation) has a fixed frequency, and its amplitude is modulated by the sound signal, which has a much lower frequency.

If you connect every high peak of the AM signal to the next high peak, you get the envelope; this envelope, translated and centered around 0 volts, is the demodulated signal which can be fed to a speaker (probably after amplification).

This modulation is necessary to transmit an audio signal through a medium unable to make low frequencies travel.

For this kind of modulation, a simple diode and a low-pass filter could be enough to extract the signal from its carrier.

Another possible modulation is FM (frequency modulation) where, again, the carrier has a much higher frequency than the signal it carries but, instead of varying its amplitude, it varies its frequency. Say, for example, 100 MHz -> 0 volts of the signal; 100.1 Mhz -> 3 volts of the output signal; 99.9 Mhz -> -3 volts of the output signal.

In this FM case, the carrier has fixed amplitude (reception problems apart), but even if the signal is not very stable, the output signal is not concerned. This is why FM radios sound better than AM radios.

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  • \$\begingroup\$ A low-pass filter will not work. You need a detector (nonlinear element), followed by a low-pass filter \$\endgroup\$
    – user253751
    Apr 25, 2022 at 11:48
  • \$\begingroup\$ @user253751 your are true, forgotten to write that. I edited the answer. \$\endgroup\$ Apr 26, 2022 at 6:27
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As a visual correlation of your two graphs A and B, the demodulated audio is akin to the envelope of the AM signal.

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