W7TI wrote:
In the USA, the FCC used to prohibit simultaneous amplitude and
frequency modulation. I did a search of Part 97 rules and I don't see
that exact wording now, but I would still tread lightly in this area.

Sure, I'm not advocating anybody go out and purposely modulate their FM
signals.

Provided all the sidebands are confined to a band no wider than
conventional AM, you probably won't be bothered by Uncle Charlie but
caution is advised.

Indeed.

One interesting commercial system out there is the Motorola C-QUAM
(compatible quadrature amplitude modulation) stereo method, used in the US
for commercial AM stereo broadcastings. Since it has to be backwards
compatible with envelope detector-based AM receivers, the resultant output
is -- I would suggest -- something similar to amplitude modulated phase
modulation! (I think it's actually quite clever...)

---Joel Kolstad