Why don't voice radio communications use FM?

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

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.

I believe you'll find Mr. Weir* will take issue with your statement
above.

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.

Where do you get these unsupported statistics?

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.

Now we're getting somewhere. So that we are all on the same page,
here's a definition of White noise:

http://en.wikipedia.org/wiki/White_noise
[There's an audio sample here also, so you can actually hear it]
White noise is a random signal (or process) with a flat power
spectral density. In other words, the signal's power spectral
density has equal power in any band, at any centre frequency,
having a given bandwidth. White noise is considered analogous to
white light which contains all frequencies.

An infinite-bandwidth white noise signal is purely a theoretical
construct. By having power at all frequencies, the total power of
such a signal is infinite. In practice, a signal can be "white"
with a flat spectrum over a defined frequency band.


http://en.wikipedia.org/wiki/White_noise_machine
A white noise machine is a device that produces a sound that is
random in character, somewhat like air escaping from a balloon.
They generally do not produce actual white noise, which has a
harsh sound, but more often pink noise, whose power rolls off at
higher frequencies, or other colors of noise. They are often used
to protect privacy by masking distant conversations, say in a
psychiatrist's waiting room, and are also sold as sleep aids.

White Noise Machines produces a sound like the gentle whoosh.
Since the sound is absolutely constant but has no structure, the
brain simply tunes it out - just like you have tuned out the noise
from the fans in your computer. You hear the fans, but how often
do you actually notice them?

So, there is a hiss in your receiver when you listen to aviation
communications, and you find it masks intelligibility. Now that you
mention it, I suppose you are correct, because when I turn the Squelch
down, I hear a hiss. The volume of the hiss is much greater when
there is no radio signal present, because the AGC/AVC circuits are
operating at maximum amplification; when a radio signal is present,
that hiss is significantly diminished in volume to the point, that in
36 years of aviation experience, I've never found it an issue.

Perhaps the hiss to which you object is unique to your radio receiving
equipment. How many aviation radios have you had the opportunity to
listen to? Have you found the same objectionable hiss in all aviation
radio receivers you've used?

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.

So you are proposing that the worldwide aviation community re-equip
all their aircraft and facilities with FM, and that all aviation
stakeholders bear the cost of those conversion, so that you won't hear
a hiss?

Do you really believe that what you propose will pay dividends
commensurate with its cost?

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

In the case of transponders, they were not necessary to operate within
the NAS. Anyone who believes that radio communication is not
necessary to operate within the NAS isn't operating in Class B, C, or
D airspace without prior permission, and the flight mission is not
really meaningful in the sense of accomplishing a meaningful result
like transportation.

In the case of VORs replacing Radio Range and NDBs, those are not
_two-way_ communications, so they are in a different class than
aviation radio communications.

Concurrent operation of differing radio based communication systems is
possible, but to concurrently operate two incompatible aviation
communication systems isn't practicable, because it would require
_all_ air and ground systems to be equipped with both AM and FM
equipment simultaneously and instantly. If not, FM transmissions
would not be received by those stations not equipped with FM
receivers, and vice versa.

For situational awareness, it is vital for all participants to know
what the others in the vicinity are doing by hearing their
instructions and intentions over the radio. For example, when I'm VFR
en route, and hear a military transport "cleared for the approach" to
an airport across whose instrument approach path I'm about to
traverse, although the transmission isn't directed to me, it provides
me with safety information that may be vital to my visually acquiring
conflicting air traffic.

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"?

You'll find it difficult to find a pilot who regards today's NAS as
armaturely designed. Are you familiar with TERPS?**



* http://www.rst-engr.com/rst/about_us...%20Engineering


**
http://www.faa.gov/about/office_org/...afs400/afs420/