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How does multiplexing differ from FM signals to AM signals. If both FM and AM shift the message signal in frequency, then is there a difference between multiplexing for FM or AM signal?

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  • $\begingroup$ You've got some misunderstanding regarding what QAM is. It's not a multiplexing method, but a complex constellation class. And I don't think you understand what orthogonality in the context of signals is. I don't think we can fix all the issues with your question at once, I'm afraid. I don't even see what you might be confusing this with. So, I'm going to vote to close this question as "unclear"; please feel encouraged to improve your question, by editing it and clarifying things, but again, I'm afraid you have a basic misunderstanding in all what multiplexing,QAM and what orthogonality is $\endgroup$ – Marcus Müller Apr 17 '17 at 18:49
  • $\begingroup$ Much better! Still, you're confusing things: AM and FM are analog modulations (hence the name, AM – Amplitude Modulation, and FM – frequency modulation). They modulate a carrier. This concept is separate to the concept of multiplexing, which is the procedure of sharing one medium for multiple signals. In the wild, you'll practically only find the FDMA (frequency division multiple access) used for analog signals, but there's no theoretical reason for that – it's just that it's really complicated to build TDMA, CDMA systems in analog domain. $\endgroup$ – Marcus Müller Apr 17 '17 at 19:34
  • $\begingroup$ @MarcusMüller But modulation (AM or FM) helps with multiplexing signals, generally, because we can use different carrier frequencies for different message signals, correct? $\endgroup$ – user281270 Apr 17 '17 at 19:40
  • $\begingroup$ yes, and no. Basically, we have this thing called equivalent base band, which basically says that you can equivalently represent any limited bandwidth around any center frequency in complex baseband, and vice versa. Which means that modulation has nothing to do with the fact that you can have your signal anywhere in the spectrum $\endgroup$ – Marcus Müller Apr 17 '17 at 19:43
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In U.S. broadcast FM, the various subcarriers (pilot tone, stereo L-R, RDS, etc.) are modulated into the upper sidebands (above the 15 kHz L+R/mono audio, but below 100 kHz, for instance 38 kHz DSB for the stereo subcarrier) of the baseband signal before that entire multiplex is used to wideband frequency modulate a single, much higher frequency, FM carrier.

The FM carrier is modulated, but not shifted in center frequency.

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