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Suppose one radio station emits an amplitude modulated (AM) signal at 100 MHz, and a nearby station emits a frequency modulated (FM) signal at 100 MHz.

Is it possible to separate these two signals, and listen to only one at a time, or is there irreversible interference between them?

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    \$\begingroup\$ Fun fact: an AM receiver of suitable bandwidth can typically receive FM intelligibly if tuned such that the modulation moves up and down the filter skirt. Such a detector doesn't display the capture effect (or noise suppression) of a more sophisticated FM receiver, so can potentially permit hearing both signals at once; but distinguishing them is harder. \$\endgroup\$ – Chris Stratton Oct 9 '17 at 22:51
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    \$\begingroup\$ I think that is called slope detection. \$\endgroup\$ – st2000 Oct 9 '17 at 22:56
  • \$\begingroup\$ You can also transmit two entirely distinct signals, for example one on an upper sideband and another on a lower, or use various sub-carriers, for example TV sound was traditionally FM modulated at a subcarrier offset from the main amplitude modulated video signal. \$\endgroup\$ – Chris Stratton Oct 9 '17 at 22:57
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    \$\begingroup\$ This is just an interesting thought exercise right? Otherwise it is pointless. That said, I think a key constraining point is missing from the question from oversight or just not-understanding. That is to stipulate if the two carriers (given no modulation) are in synchronization and in phase with one another. If not, then, that fact in it's self makes the question too open for even a pure thought experiment. \$\endgroup\$ – st2000 Oct 9 '17 at 23:01
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    \$\begingroup\$ @st2000 I'm not sure I agree that synchronization matters much, at least not as long as each transmitter's center frequency is relatively stable with at most a slow thermal drift and one is willing to use moderately sophisticated detectors, and not just a diode rectifier. \$\endgroup\$ – Chris Stratton Oct 9 '17 at 23:20
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there will be interferance the FM and AM station will beat together. if the two sources be coerced to share a single transmitter that modulates both amplitude and frequency the signals could be later extracted with little cross-talk.

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Sure this is possible. The analog TV signals had AM video, PM chroma at 3.58MHz above the video carrier, and FM audio at 4.5MHz above the video carrier.

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    \$\begingroup\$ Yes, but those aren't actually "at the same frequency" as in the premise of the question. Rather, they are offset to make it easier. \$\endgroup\$ – Chris Stratton Oct 10 '17 at 18:02

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