Jim Logajan writes:

Analog AM and FM are fundamentally incompatible with each other.

Not if they are on different frequencies.

Analog AM and digital encoding over spread-spectrum are fundamentally
incompatible with each other.

Analog AM is used for digital spread-spectrum encoding. AM is the
modulation. Digital is the encoding. Spread-spectrum is just a
frequency and bandwidth assignment.

You asked why AM is being used and not FM and all I'm
pointing out is that if you are willing to consider any new system that is
incompatible with an older system (like FM replacing AM), you may as well
do it with something more advanced and capable, like digital packets over
spread spectrum (which could be considered a relative to FM).

Switching from AM to FM doesn't involve incompatibilities. You can
run both in parallel indefinitely, providing identical services (just
as some commercial radio stations have broadcasts on both AM and FM
simultaneously). Introducing FM doesn't obsolete any of the AM
equipment.

Adding all sorts of digital gadgets is quite a different matter. Now
you are adding functionality that will be available only to the
FM/digital community. This introduces potential safety and usability
issues. Stacking transmissions digitally isn't going to work when the
same transmissions must be mirrored on analog AM--and they have to be
if you want to maintain safety and keep controller workload
reasonable.

One does _not_ need to implement any of the fancier capabilities that
I mentioned. I stated them only as what could be easily done once the
capability is in place.

A change from AM analog to anything else would be glacially slow, and
small steps are safest. I see a direct safety benefit in having the
clarity of FM transmission. I don't see a direct safety benefit in
having other unnecessary features, and I do see potential risks.

Analog cell phones are being replaced with digital cell phones ...

Analog cell phones were replaced with digital well over a decade ago
throughout the world, except for a couple of countries.

There would be no need to replace
everything at once and I'm not sure why you think that would need to be the
case.

The need arises as soon as you add new functionality.

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