Why don't voice radio communications use FM?

On Sat, 02 Sep 2006 21:08:49 GMT, B A R R Y
wrote:

On Sat, 02 Sep 2006 18:03:31 +0100, wrote:


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.

That's the way I always understood it. AM transmissions can be pulled
out of background noise. FM is more difficult, as it cuts out before
it's unreadable.

I would imagine digital would be the worst. Digital is either
decodable and there or nothing and silence.

This is all based on my experiences with aircraft AM radios, FM
business radios, broadcast and satellite TV, and radio, so it might
be all wrong. G

You're spot on with your comments. My experience is amateur radio with
AM/FM & SSB plus business radios, broadcast radio & TV FM/AM from
longwave to SHF.

Given the choice SSB gives best low signal readability but not very
suitable for normal aviation. The problem with comparisons is a 10watt
AM transmitter puts out 2x sidebands of 2.5watts (max). All of the
intelligence is available in one 2.5watt sideband, the rest is to make
the signal easier to decode. The equivalent 10watt FM transmittter
uses the full 10 watts but that's getting too technical:-)

At the end of the day if AM equipment is working properly there's not
a problem and there's no reason to change every transmitter in the
world. Probably the main problem is aircraft noise and poor hearing
along with microphone technique and peoples accents! I've flown mamy
aircraft in a number of countries and can't say there's a problem with
AM. More likely to have a problem with a controller rattling out an
instruction too fast. I doubt I've had more than a handful of
transmissions, in 15 years, I'd give worse than readability 4. Almost
always 5.

Normal communication quality is up to 4KHz audio response. As you get
older you loose the high frequencies anyway so forget hi-fi! My
hearing is only good to around 6KHz but when I was younger was around
16KHz. You only require 3KHz audio bandwidth and if pushed for maximum
readability and least bandwidth 2KHz but it sounds very harsh though
very readable.
The worst transmissions in the UK are the military who sometimes sound
like they're using throat mikes. Myself and another aircraft were
working one military controller who was almost impossible to
understand. I could just make him out but the commercial aircraft gave
up. I'd say readability 2.

David