Why don't voice radio communications use FM?

On Mon, 04 Sep 2006 19:47:13 +0200, Mxsmanic
wrote:

writes:

You don't seem to understand there is NO quality difference in audio
quality between FM & AM, unless you're equipment is faulty and
introducing distortion. I've used AM & FM with amateur radio and been
a professional Broadcast Engineer for 30 years so believe me you are
wrong!

Odd that FM seems to sweep AM out of so many markets, then. All the
FM transmissions I've heard were superior to AM.

Because it's simple to modulate at low level and generally uses a
wider bandwidth. It is only superior in signal to noise as long as the
receiver detector is limiting but once the signal drops it's useless.
I don't know if you would get much multipath distortion from the
ground to an aircraft but if you do you would get noticable
distortion.

I cannot remember the exact figures but I seem to remember
communication quality AM is about 8db better than the equivalent FM.
SSB is about 13db better than FM. Remember you need also to specify
power.

On Mon, 04 Sep 2006 18:15:29 GMT, Jose
wrote:

Mxsmanic wrote:
Odd that FM seems to sweep AM out of so many markets, then. All the
FM transmissions I've heard were superior to AM.

That's the reason for your error. You are relying on your experience
with broadcast radio, without compensating for other differences.

Yes, thank you for understanding that!

Broadcast FM has huge bandwidth. (look it up - don't just take it from me)

Broadcast AM has tiny bandwidth. (look that up too)

It is the bandwidth difference that makes broadcast FM look better than
broadcast AM, not the encoding difference.

Think of it this way - they make concrete highways and dirt roads.
Station wagons are allowed only on dirt roads, sedans are allowed only
on concrete highways. I can get from NY to CA much faster in a sedan.

Are sedans inherenly faster?

Jose

At this point I'll give up with the troll but I hope those of you who
are pilots heave learned something useful.

On Tue, 05 Sep 2006 00:49:23 +0100, wrote:

I cannot remember the exact figures but I seem to remember
communication quality AM is about 8db better than the equivalent FM.
SSB is about 13db better than FM. Remember you need also to specify
power.

I think you have identified precisely the reason why aircraft use AM,
or in some cases SSB, instead of FM. We're going for distance, not
quality. Voice's narrow range of frequencies doesn't require broadband
to useful information. Under many circumstances, punching through with
a noisy, barely readable message is preferable to no message at all.
Flying an ordinary instrument approach might exceed the limits of
narrowband FM, especially if the aircraft happens to be encased in
ice. Over mountains, it's easy to exceed the limits of VHF
communications with ATC, getting a weak, "radar service terminated..."
until you get closer to the airport or to a RCO. I don't think we're
willing to go to the extreme of SSB, so AM offers a reasonable
compromise for most operations.

On the issue of intelligibility, I've always found that a more
important factor is equipment, not modulation. Old, decrepit
radios in need of maintenance send and receive poorly. I've flown
airplanes where one radio was loud and clear and the other was
unreadable. Bad microphones may not give the transmitter much to work
with either. There are still airplanes flying with cheap (relatively)
carbon hand mikes. I was flying with such a mike, from a major name in
aircraft communications, when ATC refused to handle my flight because
of the poor communications. Some headset mikes can be really bad too.
Some aren't even noise canceling, and sometimes even noise canceling
isn't enough, as evidenced by some transmissions I've heard from
helicopters. It should also be noted that communication mikes have a
different frequency response from the broadcast mikes they use at the
FM broadcast station. The frequency response helps to emphasize those
frequencies that will punch the signal out over a distance.

A good AM radio with a good mike ought to give quite satisfactory
results. Unfortunately, sometimes FAA's radios aren't that good
either. I once reported poor transmissions to a controller. He thanked
me, made a switch, and I was then able to report 5x5 to him. Unicom
operators' radios are sometimes virtually useless.

RK Henry

Mxsmanic,

But knowing Airbus, and its tendency to
install bleeding-edge gadgets on its aircraft in a desperate attempt
to have _something_ that Boeing does not,

Examples?

There's enough to worry about in commercial aviation as it is.

Examples?

Oh, I forgot, you don't back up your "statements" with fact.

--
Thomas Borchert (EDDH)

Mxsmanic,

This is no reflection on Windows; it's just that the
operating system is designed for general information processing use in
homes, offices, and schools ... not for process control, real-time
systems, or mission-critical applications.


Says who? You?

--
Thomas Borchert (EDDH)

On Tue, 05 Sep 2006 06:34:33 GMT, RK Henry
wrote:

On Tue, 05 Sep 2006 00:49:23 +0100, wrote:

I cannot remember the exact figures but I seem to remember
communication quality AM is about 8db better than the equivalent FM.
SSB is about 13db better than FM. Remember you need also to specify
power.

I think you have identified precisely the reason why aircraft use AM,
or in some cases SSB, instead of FM. We're going for distance, not
quality. Voice's narrow range of frequencies doesn't require broadband
to useful information. Under many circumstances, punching through with
a noisy, barely readable message is preferable to no message at all.
Flying an ordinary instrument approach might exceed the limits of
narrowband FM, especially if the aircraft happens to be encased in
ice. Over mountains, it's easy to exceed the limits of VHF
communications with ATC, getting a weak, "radar service terminated..."
until you get closer to the airport or to a RCO. I don't think we're
willing to go to the extreme of SSB, so AM offers a reasonable
compromise for most operations.

On the issue of intelligibility, I've always found that a more
important factor is equipment, not modulation. Old, decrepit
radios in need of maintenance send and receive poorly. I've flown
airplanes where one radio was loud and clear and the other was
unreadable. Bad microphones may not give the transmitter much to work
with either. There are still airplanes flying with cheap (relatively)
carbon hand mikes. I was flying with such a mike, from a major name in
aircraft communications, when ATC refused to handle my flight because
of the poor communications. Some headset mikes can be really bad too.
Some aren't even noise canceling, and sometimes even noise canceling
isn't enough, as evidenced by some transmissions I've heard from
helicopters. It should also be noted that communication mikes have a
different frequency response from the broadcast mikes they use at the
FM broadcast station. The frequency response helps to emphasize those
frequencies that will punch the signal out over a distance.

A good AM radio with a good mike ought to give quite satisfactory
results. Unfortunately, sometimes FAA's radios aren't that good
either. I once reported poor transmissions to a controller. He thanked
me, made a switch, and I was then able to report 5x5 to him. Unicom
operators' radios are sometimes virtually useless.

RK Henry

Thank you RKH, I wasn't going return to this thread but you have
detailed exactly what I was trying to put across.

SSB is by far the best for long range communication but requires very
accurate tuning, unless you leave a small amount of carrier and allow
the receiver to do it for you. At least with AM the carrier is
transmitted along with the signal so tuning is relatively unimportant.
You only have to listen to two stations transmitting at the same time
and you can hear the inaccuracy in the form of the heterodyne. If they
were both on frequency you would not hear the hetrodine.

David

FM receivers typically use a PLL (phase lock loop) circuit for
demodulation. In the presence of multiple received signals the largest
amplitude one will be locked to and thus demodulated. In AM receivers
all signals present at the detector above a certain noise threshhold
are detected. This feature, while often noiser, allows your friendly
controller to hear two guys calling simultaneously. For voice
transmissions, narrow band FM can be used and equivalent bandwidth
results.

Sorry if this is addressed somewhere in this thread already. I didn't
wade through all of it.

Thomas Borchert writes:

Says who? You?

I and Microsoft, as well as anyone else familiar with Windows
internals.

--
Transpose mxsmanic and gmail to reach me by e-mail.

Mxsmanic,

I and Microsoft, as well as anyone else familiar with Windows
internals.


Sources? (I'll be really interesetd in one for the "anyone else" part)

Windows isn't Windows, as you try to make it in your generalization -
which in itslef implies a certain cluelessness with the topic.
Furthermore, these units are certified to certain reliability levels,
which are well defined. Your sweepingly broad statements just aren't
true.

--
Thomas Borchert (EDDH)

In article ,
Thomas Borchert wrote:

Sources? (I'll be really interesetd in one for the "anyone else" part)

Windows isn't Windows, as you try to make it in your generalization -
which in itslef implies a certain cluelessness with the topic.
Furthermore, these units are certified to certain reliability levels,
which are well defined. Your sweepingly broad statements just aren't
true.

What reliability levels are available for which windows products that would
be appropriate for a critical application? I assume you aren't talking about
Windows XP versions intended for desktop/laptop computers.

I wasn't aware that microsoft had any OS that would "certifiable" to DO-178B
level C or above (for those that are familar with 178B, please forgive the
shorthand wrt certification and levels), never mind any of the European
safety standards.

--
Bob Noel
Looking for a sig the
lawyers will hate