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.