Why don't voice radio communications use FM?

writes:

FM offers better quality that AM when signal is relatively strong (or
signal to noise ratio is high). As the signal strength decreases, there
is a point when the quality is identical in both cases and then the
quality of FM deteriorates _rapidly_, while AM is still usable.
So, it might be a safety issue - using FM would effectively filter out
the weakest stations that could be heard if AM was used.

Under what conditions would you need to hear really weak AM stations?

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Mxsmanic wrote:
writes:

FM offers better quality that AM when signal is relatively strong (or
signal to noise ratio is high). As the signal strength decreases, there
is a point when the quality is identical in both cases and then the
quality of FM deteriorates _rapidly_, while AM is still usable.
So, it might be a safety issue - using FM would effectively filter out
the weakest stations that could be heard if AM was used.

Under what conditions would you need to hear really weak AM stations?

Do you know what an NDB is?

So, it might be a safety issue - using FM would effectively filter out
the weakest stations that could be heard if AM was used.
Under what conditions would you need to hear really weak AM stations?

Let me think. If my radio died and all I had was a handheld I would
really want to be heard... even with this tiny antenna.

Mind you, this is just a theory. Most of my time in the air (and 100%
of my solo time) was in gliders with no radio at all. I just happen to
know a bit about the technical side of radio :-)

Bartek

On 8 Sep 2006 12:58:47 -0700, 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?

FM offers better quality that AM when signal is relatively strong (or
signal to noise ratio is high). As the signal strength decreases, there

For voice communications you want the frequency response narrow.
Probably from a low of 200 or 300 HZ to a high or 2500 or 3,000 Hz.
This is where the majority of the intelligence is located. So,
communications is definitely not hi-fi. Keeping the signal narrow also
improves the signal to noise ratio.

is a point when the quality is identical in both cases and then the
quality of FM deteriorates _rapidly_, while AM is still usable.
So, it might be a safety issue - using FM would effectively filter out
the weakest stations that could be heard if AM was used. Just a theory
;-)

There is also "capture ratio" or the ratio of signal strengths where
both of two stations transmitting can be heard. Not understood, but
heard. With AM this is about 12:1 and with FM about 2:1. (Jim, did I
get those ratios correct?) The means the weaker station is completely
gone in FM where in AM the tower would know the other station was
stepped on. With FM, if the stronger station starts first and talks
as long or slightly longer than the weaker station there will be no
indication the weaker one even transmitted.



Bartek
Roger Halstead (K8RI & ARRL life member)
(N833R, S# CD-2 Worlds oldest Debonair)
www.rogerhalstead.com

"Mxsmanic" wrote in message
...
Which means that a single break in a single wire disables multiple
aircraft systems. Brilliant.

Versus a break in a wire causing a spark that could result in a fire?

I could never find a justification for those home remote control
systems that used house wiring, and they weren't always reliable,
anyway.

Completely different scenario... With the home remote control systems,
you're still using the larger diameter 110VAC wiring... There are two many
other things on the line for it to handle signals very well... I've tried
them before and I often has lights that wanted to turn on even if there was
no command initiated to tell them to go on... A properly designed system is
no more complex than the wiring to the headlights of your car... You are
basically operating the equivalent to a relay, except that instead of a
lower amperage current going to the switch, the "switch" sends a message to
the light in a particular format to tell it to go on...

"Grumman-581" writes:

Versus a break in a wire causing a spark that could result in a fire?

Anything that carries electricity can create sparks, even at low
voltages. And you can use low-voltage for control systems even
without TCP/IP.

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Emily writes:

Do you know what an NDB is?

Yes. How many commercial flights use NDB routing today? And who
suggested using FM for an NDB?

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writes:

Let me think. If my radio died and all I had was a handheld I would
really want to be heard... even with this tiny antenna.

Then use AM on your handheld.

Mind you, this is just a theory. Most of my time in the air (and 100%
of my solo time) was in gliders with no radio at all.

I was wondering about gliders not long ago. I take it they don't have
radios? Wouldn't it be safer to have a radio powered by a battery or
something, just in case? Radios can be made very light so weight
would not be an issue.

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Roger (K8RI) writes:

For voice communications you want the frequency response narrow.
Probably from a low of 200 or 300 HZ to a high or 2500 or 3,000 Hz.
This is where the majority of the intelligence is located.

You need up to 7 KHz or so for sibilants and some other phonological
features which are occasionally phonemic. This high-end response
makes it possible to distinguish between 'f' and 's' in
communications.

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B A R R Y writes:

How about VOR ID's, RCO's, distant TWEBs and FSS stations?

For automated signals (not voice), distance is important. For voice,
I'm not sure that distance matters that much, since if they are far
enough away to be hard to hear, they are probably not an immediate
hazard, and controllers have facilities that provide adequate coverage
for the area that they control.

Do you fly?

As a pilot, only in simulation. As a passenger, only when absolutely
necessary.

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