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Amplitude modulation uses the modulation of the amplitude of a carrier wave to transmit data while leaving the carrier frequency constant, and in the case of FM it's just the other way round.

enter image description here

Why is a modulation of the frequency in an AM signal not used to transmit additional information, e.g. two audio channels or RDS information?

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QAM modulates both the phase (= frequency) and amplitude of the signal.

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    $\begingroup$ So why is is not widely used instead of AM or FM? $\endgroup$ – Baptiste Candellier Oct 26 '18 at 14:48
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    $\begingroup$ @BaptisteCandellier Oh, but it is... it is for example the modulation used in DVB-T. $\endgroup$ – Radovan Garabík Oct 26 '18 at 14:54
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    $\begingroup$ @RadovanGarabík Modulation of DVB-T within OFDM, which is a whole nother dimension of complexity, but yeah. Mike: Just because your radio doesn't have a knob for something doesn't mean it's not widespread! For example, aside from broadcast radio stations, airplanes and emergency communications, AM as voice communication medium has practically all but died out – only hams use it, and mainly for "legacy" reasons. Generally, yes both AM and FM are terribly inefficient at using the spectrum, which is why modern communication is digital and not analog. Digital achieving higher spectral … $\endgroup$ – Marcus Müller Oct 26 '18 at 15:04
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    $\begingroup$ @BaptisteCandellier, QAM decoding is a fair bit harder than AM or FM decoding. I'm pretty sure it would have been impossible in the 1920s when AM broadcast networks started showing up, or in the 1930s with FM broadcasting. This gave AM and FM about a 30-year head start over QAM modulation. $\endgroup$ – Mark Oct 26 '18 at 19:56
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    $\begingroup$ Analog QAM was used for the failed effort for stereo broadcast AM radio (C-QUAM), using phase to encode the stereo signal in a backwards-compatible manner. It's obsolete now that digital methods (also using QAM) are available, namely HD Radio. $\endgroup$ – user71659 Oct 26 '18 at 21:10
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It turns out that we DO simultaneously use frequency and amplitude modulation: our everyday term for this is "single sideband." As described on Wikipedia, single-sideband is a special case of quadrature modulation in which the quadrature component (Q) is derived directly from the in-phase component (I) of the modulating signal.

This effect becomes evident upon examination of the waveform of an SSB signal produced by modulating tones of 700Hz and 1900Hz: enter image description here

Note that the period of the envelope is 833$\mu$s, a frequency of 1200Hz, which is the difference between the frequencies of the two modulating tones. At the "cusp" of the waveform, it experiences a 180-degree phase reversal, whose accurate reproduction is essential to a clean output. The mathematical combination of these amplitude and phase modulation components produces the two tones that are offset from the frequency of the carrier wave.

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The two different techniques are not just different ways of modulating the signal. As you can see from the frequency units in your examples, FM carrier frequency is about two orders of magnitude higher than AM.

This means that FM can modulate a much higher signal frequency than AM, and this leads to greater fidelity (i.e. sound quality).

The FM carrier frequency is also a lot less susceptible to interference effects (particularly atmospheric). This is because it is fairly easy for random attenuation, reflection and refraction to change the amplitude of a signal (thus interfering with AM) but more difficult for it to change the frequency.

When FM became popular in the 1970s, it was regarded as a major technology leap - similar to DAB today. So much so, that Steely Dan wrote a song about it.

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    $\begingroup$ FM is by no means an invention of the 1970s. It's been patented in the US in 1933, it's been in wide use in Europe starting the years after WW2 (i.e. by the the 1960s, it was the standard in broadcasting), BBC started broadcasting its main programming via FM in 1955. $\endgroup$ – Marcus Müller Oct 26 '18 at 15:07
  • $\begingroup$ and DAB isn't a technical leap today. In fact, DAB, a 1990's technology (read: 25+ years old) has been superseeded by DAB+ quite a while ago, there's little DAB adoption at all, and adoption of DAB+ is pretty slow, which has led to the situation that technologically, you'd solve a lot of things more modern by now, while most Germans still don't have a DAB+ receiver (most of them are in cars, by the way), and even fewer actually use that, because the internet has made nice audio quality ubiquitous, and DAB+ isn't that cool technologically. $\endgroup$ – Marcus Müller Oct 26 '18 at 15:10
  • $\begingroup$ The frequencies in the example are just the bands that are reserved for public broadcast - that has nothing to do with the quality of FM over AM per se. The sound quality depends on the sampling rate (44kHz), which is much smaller than the carrier frequency of AM or FM (hundreds of kHz to hundreds of MHz). Also, it doesn't answer the question. I know that FM is more suitable for suppressing noise, but the question was if it is possible to transmit two separate audio signals (for example) with a single carrier wave that is modulated both in FM and AM, regardless of which works better. $\endgroup$ – ahemmetter Oct 26 '18 at 18:22
  • $\begingroup$ @ahemmetter I'd venture that the OP is talking about public broadcast, not the fundamentals of the technology per se. Not sure why you mention sampling rate... that term relates to digitisation; AM and FM (for public broadcast) are both analogue technologies. I wouldn't say FM suppresses noise, it's more that it is less susceptible to noise than AM. Not being funny, but were you actually around in the 70s? Did you listen to AM and FM radio? $\endgroup$ – Oscar Bravo Oct 29 '18 at 14:41
  • $\begingroup$ @MarcusMüller Thanks for informing me about the early start of FM. I've edited my answer to point out it only became widespread in the 70s. For example, BBC Radio 1 was still using AM until the 1980s. Not so sure DAB is quite the flop you make it out to be; all new cars have DAB(+) as standard and all radio-alarms in the shops are DAB, at least around where I live. There's also pressure from governments to shut down FM so they can sell off the spectrum to mobile telephony et al. $\endgroup$ – Oscar Bravo Oct 29 '18 at 14:50

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