Why don't voice radio communications use FM?

"Larry Dighera" wrote in message
.. .

AM frequencies are currently 25 kHz wide. FM would require more
bandwidth. Regardless, where would you place these newly allocated
frequencies?


On Sat, 2 Sep 2006 09:10:19 -0700, "RST Engineering"
wrote in
:

That's just not true. For a given voice signal, I can squeeze the same
amount of fidelity into an FM channel that I can into an AM channel.

That's the first time I've heard that.

The current actual transmitted bandwidth of a VHF AM signal is about 4 kHz..

Does that mean the highest audio frequency transmitted it 2kHz?

Standard deviation on a VHF FM signal is 3.5 kHz.. Bessel and Armstrong to
the rescue once more {;-)

BTW, the current European channel spacing is 8.3 kHz.. Now THAT's going to
be a challenge for us AMers to meet.


And, I suspect, it would be completely impossible for FM to fit within
8.3 kHz channel spacing with the same fidelity?

That's just not true. For a given voice signal, I can squeeze the same
amount of fidelity into an FM channel that I can into an AM channel.

That's the first time I've heard that.

The first time I heard it was when VHF FM at 2 meters became popular in the
early 1960s. The first time I had it explained using Bessel functions was
as a first year graduate student in the late 1960s. The first time I had a
chance to design with it was my first FCC type acceptance gauntlet in the
mid 1970s.

Take a look at a ham 2 meter rig sometime. Channels are 5 kHz. wide.



The current actual transmitted bandwidth of a VHF AM signal is about 4
kHz..

Does that mean the highest audio frequency transmitted it 2kHz?

No, sorry, I should have been absolutely technically precise. The current
actual transmitted bandwidth of a VHF AM signal is plus/minus 4 kHz.. In
practice, with symmetric modulation ("good" AM or FM) you generally give the
bandwidth as the distance from carrier to one sideband and not sideband to
sideband.

The highest audio frequency that we try to achieve is about 3 to 3.5 kHz,
with rapid rolloff above 2.5 kHz. -- generally 12 to 18 dB/octave cornered
on 2.5 kHz.. Yes, there will be some higher order stuff leaking through;
the idea is to contain as much of it as you can in the filter before it hits
the modulator.


Standard deviation on a VHF FM signal is 3.5 kHz.. Bessel and Armstrong
to
the rescue once more {;-)

BTW, the current European channel spacing is 8.3 kHz.. Now THAT's going
to
be a challenge for us AMers to meet.


And, I suspect, it would be completely impossible for FM to fit within
8.3 kHz channel spacing with the same fidelity?

Easier for FM than AM, but it is a moot point. FM will PROBABLY never
happen on the VHF COM band.

Jim

"Larry Dighera" wrote in message
...

"Larry Dighera" wrote in message
.. .

AM frequencies are currently 25 kHz wide. FM would require more
bandwidth. Regardless, where would you place these newly allocated
frequencies?


On Sat, 2 Sep 2006 09:10:19 -0700, "RST Engineering"
wrote in
:

That's just not true. For a given voice signal, I can squeeze the same
amount of fidelity into an FM channel that I can into an AM channel.

That's the first time I've heard that.

The current actual transmitted bandwidth of a VHF AM signal is about 4
kHz..

Does that mean the highest audio frequency transmitted it 2kHz?

Standard deviation on a VHF FM signal is 3.5 kHz.. Bessel and Armstrong
to
the rescue once more {;-)

BTW, the current European channel spacing is 8.3 kHz.. Now THAT's going
to
be a challenge for us AMers to meet.

I believe they were just implementing that when I left avionics work 20
years ago. The main reason for the relatively wide spacing was poor
frequency stability. The real problems with any changeover would/will be
the large amount of existing infrastructure in place and the need for
radically "better" adjacent channel rejection. And you don't dare to
"improve" the adjacent frequency rejection of the receivers that much untill
you are really sure that the transmitters in service can meet the new
standard ... and so forth ...

And, I suspect, it would be completely impossible for FM to fit within
8.3 kHz channel spacing with the same fidelity?

Wouldn't be much of a problem, IIRC the hams have been doing it forever. I
just don't know of and good reason to choose one modulation scheme over the
other, and certainly not to change from one to the other!

In article ,
"RST Engineering" wrote:

BTW, the current European channel spacing is 8.3 kHz.. Now THAT's going to
be a challenge for us AMers to meet.

huh? Putting FM into 8.33 kHz spacing? or did you mean something else?

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

Larry Dighera writes:

Hence the popularity of Active Noise Reduction headsets.

Then why not apply the same logic to the radio channel itself, and
reduce its noise as well.

Do you use an ANR headset?

No. The source of the noise is not anything around me, it's coming
from the channel itself.

Request 'say again' if in doubt.

Most people guess without realizing it, so they cannot do that.

What would you estimate the cost of re-equipping all aircraft with
such a system might be?

They don't all have to be reequipped at once, any more than everyone
must have a glass cockpit.

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

Mxsmanic,

Do you use an ANR headset?

No. The source of the noise is not anything around me, it's coming
from the channel itself.


ANR headsets enhance speech as well as reducing noise.

--
Thomas Borchert (EDDH)

On Sat, 02 Sep 2006 19:51:30 +0200, Mxsmanic
wrote in :

Larry Dighera writes:

Hence the popularity of Active Noise Reduction headsets.

Then why not apply the same logic to the radio channel itself, and
reduce its noise as well.

I would guess that noise-blanker and noise-limiting circuits are
incorporated in the current radio designs.

Do you use an ANR headset?

No. The source of the noise is not anything around me, it's coming
from the channel itself.

Other than the occasional heterodyne squeal that occurs in the
receiver when two transmitters are transmitting on the same frequency
simultaneously, there shouldn't be any other noise. Ignition noise
should be suppressed by Faraday shielding, and generator/alternator
noise should be bypassed to ground.

What is the nature of the noise you are hearing? Can you describe it?
Is it a hum, pulses, growling, squealing, what?

What would you estimate the cost of re-equipping all aircraft with
such a system might be?

They don't all have to be reequipped at once, any more than everyone
must have a glass cockpit.

Regardless of when it occurs, there will ultimately be an additional
cost.

And to expect the old (current) communications system to remain
operational while the new system you are proposing is operating
concurrently won't be feasible if they use the same frequencies. If
an new alternate frequency band is used for the new communications
system you are proposing, it could work. But getting the FCC to
allocate additional frequency spectrum will probably be opposed,
because the frequency spectrum is a finite resource, and there are
many more services desiring to use it than there is bandwidth
available.

You really should read the information at some of the links I provided
to get an idea of what has been tried, and what is on the FAA's
horizon regarding aviation communications. This topic has been very
thoroughly researched by government personnel and it's unlikely that
you will hit upon a superior system to what the professionals have
examined.

Larry Dighera writes:

I would guess that noise-blanker and noise-limiting circuits are
incorporated in the current radio designs.

You can't actively remove noise over a radio channel because you have
no unique identifier of noise vs. information. Noise-reduction
headsets work because they know what is noise (outside sound) and what
isn't (audio being played through the headset).

Other than the occasional heterodyne squeal that occurs in the
receiver when two transmitters are transmitting on the same frequency
simultaneously, there shouldn't be any other noise. Ignition noise
should be suppressed by Faraday shielding, and generator/alternator
noise should be bypassed to ground.

Anything that isn't signal is noise. AM transmissions are fuzzy and
hard to hear. In fact, aviation AM radio is probably the noisiest
type of radio voice communication still in use. Most other types of
radio communication today are FM.

What is the nature of the noise you are hearing? Can you describe it?
Is it a hum, pulses, growling, squealing, what?

White noise. It doesn't come from anything within the aircraft or
station.

Regardless of when it occurs, there will ultimately be an additional
cost.

Sure, but one that companies and individuals can assume on a phased
basis at their convenience.

The fact that transponders and VORs exist today (when they did not in
the early days of aviation) proves that this works.

And to expect the old (current) communications system to remain
operational while the new system you are proposing is operating
concurrently won't be feasible if they use the same frequencies.

Presumably they would use different frequencies.

If an new alternate frequency band is used for the new communications
system you are proposing, it could work. But getting the FCC to
allocate additional frequency spectrum will probably be opposed,
because the frequency spectrum is a finite resource, and there are
many more services desiring to use it than there is bandwidth
available.

Aviation is a pretty critical use of bandwidth.

You really should read the information at some of the links I provided
to get an idea of what has been tried, and what is on the FAA's
horizon regarding aviation communications. This topic has been very
thoroughly researched by government personnel and it's unlikely that
you will hit upon a superior system to what the professionals have
examined.

How much of aviation was designed by "professionals"?

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

"Mxsmanic" wrote:

I would guess that noise-blanker and noise-limiting circuits are
incorporated in the current radio designs.

You can't actively remove noise over a radio channel because you have
no unique identifier of noise vs. information.

******** again. I have a radio that does actively remove noise--it has a
button to turn the feature on and off, and it works quite well.

I'll say one thing for you, you are fearless in your ignorance.

--
Dan
C172RG at BFM

On Sun, 03 Sep 2006 13:39:09 +0200, Mxsmanic
wrote:

Larry Dighera writes:

--- cut-----

Anything that isn't signal is noise. AM transmissions are fuzzy and
hard to hear. In fact, aviation AM radio is probably the noisiest
type of radio voice communication still in use. Most other types of
radio communication today are FM.


This thread is becoming a lot of guessing and not based on facts!
I doubt anybody has evaluated AM compared to FM in an aircraft so
won't know what the effects are when tried side by side.

There is absolutely no difference in qualilty between AM & FM if they
are designed to the same specification. If you modulate an AM or FM
transmitter with up to 3KHx of audio they will sound identical. What
you put into the transmitter comes out of the receiver assuming there
isn't a fault.

FM maintains a low background noise longer than AM and the only
difference is at low signal levels when FM very quickly becomes
totally unreadable. AM can still be heard and understood, depending on
the ability of the person listening. With all the noise in an aircraft
a little bit of low level background noise is not significant.

Digital has some merit but again when the signal reaches a threshold
it stops completely.

The whole thread is futile as the centre of the 'aviation universe'
may well be the US but you aren't going to get the rest of the world
to change. Even having regultions which are supposed to be accepted
worldwide doesn't work. Most counties have exceptions.

About the only thing which is standard is the use of the English
language. Even then the locals will use their own language! Have you
ever worked controllers with Spanish English, Finnish English,
Canadian English, New Zealand English, Cyprus English, Bahamian,
Caribbean or even Amereican English. That's where the differences can
be heard. AM radio is adequate for the job and if you don't think so
then get your installation checked out by a qualified engineer, you
may be suprised.

As for increasing the number of frequencies Europe has introduced
8.3KHz spacing. Fortunately at the lower GA flight levels it's not
required but the higher commercials now require new radios.

Most radios are dual NAV/COM so not only would you need a new COM but
a new NAV too... It isn't going to happen...!