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Why don't voice radio communications use FM?



 
 
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  #11  
Old September 2nd 06, 05:10 PM posted to rec.aviation.piloting
RST Engineering
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Posts: 1,147
Default Why don't voice radio communications use FM?

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.

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

Jim




"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?



  #12  
Old September 2nd 06, 05:20 PM posted to rec.aviation.piloting
Larry Dighera
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Posts: 3,953
Default 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?

  #13  
Old September 2nd 06, 05:38 PM posted to rec.aviation.piloting
Bob Noel
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Posts: 1,374
Default Why don't voice radio communications use FM?

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

  #14  
Old September 2nd 06, 05:55 PM posted to rec.aviation.piloting
Ron Natalie
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Posts: 1,175
Default Why don't voice radio communications use FM?

James Robinson wrote:
Mxsmanic wrote:

Perhaps this is a naive question, but: Why don't voice radio
communications for aviation use FM radio instead of AM radio?


I understand it is because of a characteristic of FM called "capture
effect" that blanks out weaker transmissions when two radios transmit at
the same time. The listener would have no idea that a second, weaker
transmission was being made.

No actually, it's just historical. Early av radio used AM, and for that
reason we still do.
  #15  
Old September 2nd 06, 05:56 PM posted to rec.aviation.piloting
RST Engineering
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Posts: 1,147
Default Why don't voice radio communications use FM?



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


  #16  
Old September 2nd 06, 05:58 PM posted to rec.aviation.piloting
Steven P. McNicoll[_1_]
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Posts: 660
Default Why don't voice radio communications use FM?


"Ron Natalie" wrote in message
m...

No actually, it's just historical. Early av radio used AM, and for that
reason we still do.


Didn't all early radio use AM?


  #17  
Old September 2nd 06, 06:03 PM posted to rec.aviation.piloting
[email protected]
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Posts: 91
Default Why don't voice radio communications use FM?

On Sat, 2 Sep 2006 12:02:26 -0400, "John Gaquin"
wrote:


"Mxsmanic" wrote in message

....that improper and misunderstood radio
communication is a leading cause of accidents,


Cite, please.

... I can barely understand what I hear on the radio.


I suspect the reasons for this relate more to the environmental effects and
quality of the speakers, etc., than to the nature of AM transmissions.


The previous comment re capture effect of FM is valid. i.e. the
strongest signal wins. This is desireable for broadcast radio but not
aviation.

With FM the signal remains much clearer until the point where it
suddenly becomes unreadable when itl becomes weak.

With AM is that readability gradually reduces as the signal gets
weaker. If you open the squelch you can often still read AM when FM
would be unreadable.

The audio bandwidth for acceptable communication is 3KHz. When
modulating an AM transmitter you have two sidebands. One up to -3KHz
the other up to +3KHz so transmitted bandwidth is 6KHz.

With an FM transmitter the bandwidth will still be 6KHz plus the
deviation of the system. In addition the sidebands theoretically
extend to infinity but they become rapidly weaker.

To get the best signal to noise ratio with FM you need higher
deviation. If you try increase the number of FM frequencies you need
to reduce the deviation. That in turn would reduce its effectivness.

As for the original comments I would suggest there's something wrong
if AM is not clear.

Could be poor hearing, inadequate headset, turning up the volume
causing overload of either headset or receiver audio. Ignition or
alternator interference distorting the received signal, poor
transmitter, poor microphone, poor microphone technique.
Sorry but the problem is NOT AM!
  #18  
Old September 2nd 06, 06:18 PM posted to rec.aviation.piloting
Peter Dohm
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Posts: 1,754
Default Why don't voice radio communications use FM?

"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!



  #19  
Old September 2nd 06, 06:42 PM posted to rec.aviation.piloting
Mxsmanic
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Posts: 9,169
Default Why don't voice radio communications use FM?

Steven P. McNicoll writes:

Wouldn't that reduce the available frequencies?


For a given audio bandwidth, FM tends to require somewhat more radio
bandwidth, as I recall, but the audio bandwidth of aviation radio is
already so limited that I don't think this would be an issue. The
gain in clarity would outweigh any loss of audio fidelity, assuming
that the same channel widths were used.

If frequencies were reallocated (instead of allocating new ones), that
would be different. That would also obsolete older equipment much
more quickly, which might not be acceptable. But there must be some
space somewhere that could be added to the frequencies, or perhaps
some band so rarely used that it could be reassigned.

--
Transpose mxsmanic and gmail to reach me by e-mail.
  #20  
Old September 2nd 06, 06:44 PM posted to rec.aviation.piloting
Jim Logajan
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Posts: 1,958
Default Why don't voice radio communications use FM?

Mxsmanic wrote:
Perhaps this is a naive question, but: Why don't voice radio
communications for aviation use FM radio instead of AM radio? I
realize there's substantial inertia in the installed base of AM
equipment, but surely one could allocate some new frequencies to FM
and use them in parallel for some years to ease the transition.


If one were to mandate a replacement technology, it would be far far more
effective to use the packet-based mechanisms that digital cellular phone
technology and 802.11 wireless Ethernet (aka WiFi) rely on. Both these
technologies turn over the job of transmission collision resolution to
chip logic and take humans out of the loop. And it is possible to put
audio over WiFi using Voice over IP (VoIP) technology.

Such a system would be incredibly flexible. If one had, say, ten planes
in the air and they all started to talk to ATC at once, a packet-based
system would make it possible to do any of the following:

1) Clearly deliver only one of the voice signals to the controller and
provide a visual display that indicated 9 other planes had attempted to
speak also. It could even provide audio or visual feedback to the other 9
pilots that their transmissions were not delivered - or it could
automatically sequence the delivery of the transmissions to the
controller if the transmissions were not too lengthy.

2) If multiple controllers were available, the audio from several of the
planes could be routed to multiple controllers with no impact on audio
fidelity as far as the controllers or pilots are concerned.

3) Once you go packetized audio, you can put all sorts of useful stuff in
the packets for presentation to the other end - such as aircraft number,
the location and velocity vector from the aircraft's GPS or
altimeter/DG/airspeed indicator, and so on. A pilot could key the mike
and make a request without needing to ID themselves or their position -
that information would be extracted from the audio packet's header and
automatically presented on either a simple display to the controller or
mapped to a fancy map display.

The technical issues have been pretty much solved and commoditized in
both the WiFi VoIP and digital cellular realms. It is my humble opinion
that the radio technology currently being used for aviation
communications is now less reliable and useful than even that used in
home WiFi networks.

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




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