> It's not possible to encode a higher-frequency signal over a lower frequency carrier..
Actually it is possible. AM modulation is just a matter of multiplying two frequencies together. It's just that the lower sidebands wrap around DC. It's a bit harder to demodulate, but not impossible.
>.. AM broadcasts are in the medium-wave (MW) band...And I'd suspect that fidelity above 10kHz audio is not especially high even at those transmission frequencies.
In the transmitter the fidelity is limited by the bandwidth of the modulator, and in the Receiver by the bandwidth of the IF stage. You can actually get FM like audio from a BC AM radio, but not with cheap and nasty receivers designed for the American market. In Europe many HiFI AM receivers have an optional "wide" IF filter, and the transmitters can transmit 15KHz audio.
"In the transmitter the fidelity is limited by the bandwidth of the modulator, and in the Receiver by the bandwidth of the IF stage."
This applies to heterodyne systems with the bandwidth limitation coming from deliberate filtering.
My understanding is the channel width, set out in the broadcast standard and license, is the bottleneck and not so much the limitations of the electronics. FM must be under 150khz (5 times the maximum source bandwidth of 15khz, then doubled for upper and lower sidebands) and MW-AM must be under 9/10khz (limiting the source to around 5 kHz). So without the transmitter broadcasting outside its allocation, I don't understand how changing the receiver could improve the audio quality.
I could be wrong. Do you have a source on 15khz audio bandwidth over MW-AM?
There is no direct connection between the channel spacing, and the allowed TX bandwidth.
If the bandwidth is wider than the channel spacing (for AM BC), all that happens is that adjacent stations hear a little "monkey chatter". And that is minimised because most stations are arranged on a geographic grid which prevents a local station having another local on an adjacent channel.
In Australia the local Government ABC stations most definitely do transmit 15KHz wide audio.
A quick search turned up this article from September 1980 ETI magazine which discusses stations having 30KHz total bandwidth. https://imgur.com/a/gVpSwoo
Whatever, just tuning around with a high-end SDR shows many stations at 20KHz bandwidth.
The Commercial stations have a problem in that they use aggressive audio processing in order to maximise their coverage, given their licensed power output. So as soon as the commercials decided to concentrate on talk shows, they wound down their bandwidth.
AM modulation is just a matter of multiplying two frequencies together.
Pardon my limited knowledge here, but isn't amplitude modulation actually the practice of varying the amplitude of the carrier frequency alone?
FM, frequency modulation, by contrast does vary the frequency of the carrier itself, which can be modeled mathematically as a function of two (or more) frequencies being combined as with Fourier analysis.
I think there is some confusion here mixing up the time and frequency domains. The poster above is correct that AM is achieved by multiplying the intelligence with a carrier. This changes a carrier's amplitude which must (as indicated as a Fourier series) generate sidebands, which occupy frequencies around the carrier frequency. FM is produced differently but also generates (wider) sidebands.
Actually it is possible. AM modulation is just a matter of multiplying two frequencies together. It's just that the lower sidebands wrap around DC. It's a bit harder to demodulate, but not impossible.
>.. AM broadcasts are in the medium-wave (MW) band...And I'd suspect that fidelity above 10kHz audio is not especially high even at those transmission frequencies.
In the transmitter the fidelity is limited by the bandwidth of the modulator, and in the Receiver by the bandwidth of the IF stage. You can actually get FM like audio from a BC AM radio, but not with cheap and nasty receivers designed for the American market. In Europe many HiFI AM receivers have an optional "wide" IF filter, and the transmitters can transmit 15KHz audio.