"Brenda Ann" hath wroth:
"Jeff Liebermann" wrote in message
.. .
I'll make it really simple for you. FM is "hi-fi", while AM is noisy
"no-fi". Don't you want to be cool strutting down the street with
your iPhone watching HDTV with 7.1 sound? It wouldn't do to have it
sound like the typical AM broadcast station. For decent quality, you
gotta have FM.
FM is not inherently any more 'hi-fi' than AM.
FM was invented by Edwin Armstrong specifically to eliminate the noise
problems of AM broadcasting. What I think you might be referring to
is the huge ****ing match between Armstrong and John Carson over
whether FM was any better than FM in the 1930's. The consensus is
that very narrow band FM isn't that much better than AM (of equal
occupied bandwidth), but wide band FM (as used in broadcast FM and TV)
is far better than AM for just about everything.
http://fecha.org/armstrong.htm
Fidelity is a product of
bandwidth, not modulation type.
Correct. Actually, it's also a function of modulation linearity
(distortion and intermod) and encoding method (dynamic range), but I
don't wanna slither down that diversion. Pretend I didn't mention it.
AM is not even so susceptible to noise as
the frequency goes up, since the energy of the noise pulses goes down
logarithmically as frequency goes up.
If you're thinking of impulse noise, you're mostly correct. However,
there are plenty of other sources of AM noise available. For example,
the typical VHF aircraft radio requires substantial filtering of the
magneto to avoid hash. Same with any onboard motor. If you've ever
tried to install a TV (VSB is a form of AM) in a vehicle, you'll also
find that ignition and motor noise can be a problem.
Also, your statement isn't quite right. I think what you meant to say
is that as the frequency increases, the energy produced by an impulse
source, in a given bandwidth, goes down. Even that's not accurate as
I have a fluorescent lamp calibrated noise source that's quite noisy
well into the GHz range.
AM is used for aeronautical
communications very successfully for several reasons, one of which is the
LACK of 'capture effect'.
The FAA, FCC, and various manufactories have tried to move aircraft
radios away from AM and towards FM several times in the past 30 year
or so. They failed mostly due to international WRC reluctance to swap
out expensive radios. It took literally forever to get GPS receivers
TSO approved and about 15 years for nav/com radios to go from 50KHz to
25KHz channel spacing, and that was just the FAA. Where else can you
find an industry, where progress is somewhat retarded by a regulatory
agency of the federal government?
I listen to a mix of VHF aircraft AM channels and FM ham and public
safety channels on my scanner almost constantly. It's easy to
recognize the AM stations by their uniformly crappy audio.
Most domestic ground to ground airport traffic is now all FM, as is
military ground to ground and ground to air. The reason is that it's
difficult to find a decent AM aircraft band walkie talkie. So, they
use commercial FM radios. The only AM walkie talkies are used by
experimental aviation and ultralights, some of which do not have much
of an electrical system that can handle the grossly inefficient AM
transmitters.
Also, nobody really cares about the "capture effect" as the tower
usually has multiple receiver sites and can generally deal with
simultaneous transmit collisions. However, they do care about the
heterodynes produced by simultaneous transmissions, which obliterate
both transmissions. With FM, they could use commercial receiver
voting systems and largely eliminate the problem.
There are still some frequencies where AM will be
more susceptible to interference than FM, but FM would still suffer, for
instance the segment between 1330-1400 MHz which is the natural frequency of
Hydrogen (lots of that around).
If my AM or FM receiver is sensitive enough to hear something in the
"water hole", it would be attached to a very big dish antenna.
--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060
http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558