Why don't voice radio communications use FM?

On Mon, 04 Sep 2006 09:37:53 +0200, Mxsmanic
wrote:

Larry Dighera writes:

You haven't adequately demonstrated the need for a less noisy method
of aviation communications, in my opinion.

I'm not trying to demonstrate need, I'm trying to demonstrate
desirability.

But yours is the
first complaint about white noise present in aviation radio
communications impacting air safety, that I have heard in my 36 years
of being an airman, and I question its validity.

It's not just white noise; it's the poor quality of audio generally.


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!

If you compare "like for like" they are both clear and almost
identical quality under normal signal levels. It's only when they
become weak that AM slowly degrades, soon after that point FM will
just stop working.

Don't compare broadcast quality FM with AM. Broadcast FM uses about
15KHz audio bandwidth and likely to be 50KHz or 75KHz deviation, that
gives a channel width of about 130KHz or 180KHz wide (if I remember
correctly) that's why you get low noise in the system. Compare this to
communication quality FM which is likely to have only about 3 to 5KHz
deviation and you'll see a large difference. The bandwidth used is
then much less, but still wider than AM. As I've previously stated an
AM transmitter with 3KHz audio bandwidth has an RF bandwidth of only
6KHz. An FM transmitter using only 6KHz bandwidth will not work as
well as you seem to imply and it requires a wider channel width unless
you reduce the deviation even more and sacrifice the benefits of FM..

Even commercial broadcast AM only uses a narrow audio bandwidth. I'm
not sure but I believe the audio bandwidth of AM broadcast is about
6KHz. On top of that many use very sophisticated audio compressors
which increase the audio level drastically so everything sounds
louder. It means the transmitters are almost fully modulated most of
the time which gives good signal to noise ratio. That helps when
listening in high noise environments like a car. You loose the dynamic
range and distort the signal but it improves readability. Broadcast FM
also uses compression but not as much so less distortion. Classical
radio stations want to retain the dynamic range so should not use
compression.

- snip-

It's hard to put a price on safety. Some people care a lot about it,
some people care very little about it.


Safety is not all about using radios. I've had a transmitter audio
failure within a military controlled area. It was a non-event. Simply
squawked 7600 listened on the receiver and replied with mike clicks.
Got an IFR clearance (even in VFR) and landed with the green lights.

Like others have said this is going nowhere and there are too many
unsupported facts and misunderstandings. If you're a pilot you'll know
AM works well.

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.

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

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.

I believe FM's principle advantage over AM is superior immunity to certain
classes of noise - audio fidelity per se should be equal. Here's what my
copy of "The ARRL Handbook for Radio Amateurs, 2002" says about FM:

"The primary advantage of FM is its ability to produce a high signal-to-
noise ratio when receiving a signal of only moderare strength. This has
made FM popular for mobile communications services and high-quality
broadcasting. However, because of the wide bandwidth required and the
distortion suffered in skywave propagation, the use of FM has generally
been limited to frequencies higher than 29 MHz."

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.

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
--
The monkey turns the crank and thinks he's making the music.
for Email, make the obvious change in the address.

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 Mon, 04 Sep 2006 17:59:41 -0000, Jim Logajan
wrote:

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.

I believe FM's principle advantage over AM is superior immunity to certain
classes of noise - audio fidelity per se should be equal. Here's what my
copy of "The ARRL Handbook for Radio Amateurs, 2002" says about FM:

"The primary advantage of FM is its ability to produce a high signal-to-
noise ratio when receiving a signal of only moderare strength. This has
made FM popular for mobile communications services and high-quality
broadcasting. However, because of the wide bandwidth required and the
distortion suffered in skywave propagation, the use of FM has generally
been limited to frequencies higher than 29 MHz."

Yes you're correct "audio fidelity per se should be equal" but you
also mention bandwidth, that's critical. There are some advantages
with FM, until the signal gets weak! The other reason for FM is you
can easily modulate the carrier at low level. With AM you need a
higher power modualtor which uses more power.

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 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

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