Why don't voice radio communications use FM?

Mxsmanic wrote:
Jim Logajan writes:

details snipped

Maybe someday the FAA and/or ICAO will consider replacing analog radios
with a more capable digital system....

All very interesting, but one of the criteria that any new system
would have to satisfy is that it would have to work in parallel with
the existing system. Adding features to the new system that are not
available in the old system would create dangerous differences between
the two. Seeing fancy displays in the ATC or tower for the lucky
digital users won't help deal with traffic from old AM users, and it
might even confuse things enough to cause problems.

A highly advanced solution would require replacing everything at once,
which isn't going to happen. A simpler solution that just provides
better quality audio could coexist with older systems without a
problem.

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

Analog cell phones are being replaced with digital cell phones, so I fully
expect the same co-existence can be done with a changeover from analog
aviation radio to digital radio. 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.

On Sat, 02 Sep 2006 21:06:47 -0000, Jim Logajan
wrote in :

Analog cell phones are being replaced with digital cell phones, so I fully
expect the same co-existence can be done with a changeover from analog
aviation radio to digital radio.

They operate on different frequency bands, so that is not a good
analogy unless you can get the FCC to commit to allocating frequency
spectrum for aviation use.

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.

Because it is unlikely the FCC will agree to allocate additional
frequency spectrum for the proposed new communications system.

Larry Dighera wrote:
Because it is unlikely the FCC will agree to allocate additional
frequency spectrum for the proposed new communications system.

The frequency allocation would need to be changed or added to on an
international scope, so I believe the operative organizations would be the
ITU and the ICAO or IATA - the FCC would simply enforce the change within
the U.S. Like you, I would have thought new allocations or changed
allocations would be hard, but changes are made every four years and in the
GHz range they seem to been more readily done; e.g.:

http://www.boeing.com/connexion/news...r_030707j.html

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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Mxsmanic wrote:
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.

Regarding your argument in the paragraph above and the one below...

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.

....honestly don't make any sense to me. In the first paragraph you see no
problem with two transmitters being used to transmit the same thing using
different frequencies and different modulation techniques, and in the
second paragraph you do. I think you could turn the first paragraph into
the second or vice-versa with appropriate special pleading - which is why
I'm confused about why you find a switch from AM to FM a better transition
than any other transition. I guess I just don't see what you see.

Jim Logajan writes:

...honestly don't make any sense to me. In the first paragraph you see no
problem with two transmitters being used to transmit the same thing using
different frequencies and different modulation techniques, and in the
second paragraph you do.

The second instance involves additional or different information being
transmitted over one channel, but not the other. The first instance
involves only a reduction of noise; the information content is the
same in both channels.

I think you could turn the first paragraph into
the second or vice-versa with appropriate special pleading - which is why
I'm confused about why you find a switch from AM to FM a better transition
than any other transition. I guess I just don't see what you see.

I don't know if it's better than any other transition; I just think
that something should be done to improve the archaic system that
exists now.

--
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