"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

Jeff Liebermann wrote:
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.

Dang it...my memory is slipping. What did Motorola call their noise
filtering circuit on their old low band Motracs. Extenders[tm]? As I recall
it was a simple noise blanker.

DTC hath wroth:

Jeff Liebermann wrote:
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.

Dang it...my memory is slipping. What did Motorola call their noise
filtering circuit on their old low band Motracs. Extenders[tm]? As I recall
it was a simple noise blanker.

"Extender". It was a 2nd almost identical receiver, tuned to a nearby
empty frequency. If there was any impulse (ignition) noise, both
receivers would detect the pulses. The 2nd (Extender) receiver would
block the IF signal in the main receiver for the duration of the
pulse. This resulted in a "hole" in the receive IF and audio, but it
was far less noticeable than if the pulse were allowed to be heard.
The tricky part of the design was getting the delays nearly identical
in the two receivers. It also made the 80D/140D/Motrac/Motran radios
rather huge and heavy. Extenders were considered a "standard option"
on most Low Band (30-50Mhz) radios as this is where the ignition noise
is the worst.

The more generic term "noise blanker" applies to this scheme, as well
as a mess of others that detect in a single receiver or blank in the
IF or audio. GE decided that "extender" was a good name for their
mobile repeater, and called it a "mobile extender" or more commonly
just "extender".
http://www.mbay.net/~wb6nvh/chpradio.htm
Egads. I'm cleaning house and found a large box of 40 year old
Motrash control heads and cables. Want some junk?

There was also a scheme to eliminate ignition noise that involved
running a wire to the points on the distributor. The assumption was
that there was a substantial delay between when the points opened, and
when the spark jumped in the spark plug. This allowed the receiver to
be blanked before the noise pulse arrived, which really improved the
noise blanker performance. I was working on the design when marketing
decided that it should tilt at other windmills. Only a few prototypes
were built, were never patented or produced, and worked really quite
well. Cheaper too. That was all just fine because cars were begining
to use electronic ignition systems, which didn't have easily
accessible ignition points.


--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558

Jeff Liebermann wrote:
Egads. I'm cleaning house and found a large box of 40 year old
Motrash control heads and cables. Want some junk?

Slacker...I tossed out all my old Moto stuff years ago. Last time I played
with the Motorola line was around the Micors came out. i used a few of them
for tower top UHF repeaters

DTC hath wroth:

Jeff Liebermann wrote:
Egads. I'm cleaning house and found a large box of 40 year old
Motrash control heads and cables. Want some junk?

Slacker...I tossed out all my old Moto stuff years ago. Last time I played
with the Motorola line was around the Micors came out. i used a few of them
for tower top UHF repeaters

The local hams still have ancient junk pretending to be repeaters and
such.
http://www.LearnByDestroying.com/k6bj/
I built most of it out of Micor mobiles and base stations. Since the
Micor stake pin connectors are chronically intermittent, there's a
large rubber hammer in each rack to bang on each radio to reseat the
connectors.

Since no sane person keeps such old parts around, I get to stock old
radios and pieces at my house. When the local comm shop cleaned out
their ancient Motorola parts pile, I ended up with most of their old
parts. I think it's time for a general purge, which means either eBay
or the scrap metal recyclers.

What I find amusing is that many police and fire departments rebuild
antique or vintage police cars and engines. They eventually want a
genuine Motorola twin coffin or 80D radio for the vehicle. I've
supplied about 4 of these radios (working) for various projects. The
first step is to spray the crumbling rubber insulated wiring with
clear acrylic to prevent further deterioration. Getting the radios
working is fairly easy as I have all the old test sets and some docs.
The fun part for me is watching the current crop of comm techs trying
to install the monster case in the vehicle. What do I do with all
this big fat cable is usually the first question.

--
Jeff Liebermann
150 Felker St #D http://www.LearnByDestroying.com
Santa Cruz CA 95060 http://802.11junk.com
Skype: JeffLiebermann AE6KS 831-336-2558