Re: Analog Cell Phones: Why AM is better than FM

Radium <glucegen1@gmail.com> wrote in news:1180056431.463704.325710
@q19g2000prn.googlegroups.com:

>> > Why do analog cell phones use FM? Why not AM?
>
>> Do you even know if they do?
>
> Yes. I read about it.

Noise. Electrical impulse noise, such as spark plug noise and computer
radiation and leaky insulators from overhead powerlines is amplitude
modulated. Being AM, this noise is detected by the AM detector in your
AM radio. It has nothing to do with frequency or range. Airplanes from
108 to 138 Mhz in the VHF band, is one of the oldest users of VHF. All
airplane radios in this VHF band are AM, not FM. It's just that way
because it, at one time, would have cost aviation too much to convert to
the newfangled FM system Mr Armstrong invented. It's still AM. It's
also line-of-sight, because it's on VHF which isn't reflected by the
ionosphere like frequencies below 30 Mhz are. This phenomenon has
nothing to do with how it's modulated.

Analog cellphones, like the IMTS and MTS "Carphones" before them, use FM
because FM is more immune to AM noise. Certain FM detectors, by their
very nature, cancel out all the AM noise fed to them by the receiver IF
strip. Since WW2, mobile radios always used FM for this reason. Using
it on Carphones was simply good sense. It still is. Your CDMA, TDMA,
PCS or whatever cellphone you carry is STILL an FM radio, at heart, but
this FM radio is now modulated with modem tones, similar to what you hear
if you pick up a landline telephone when a dialup modem is using its
line...except your phone shares the system and channel with many other
users so its transmitter is only on in tiny pulses of data. Data, 1s and
0s, is a DC pulse and cannot be transmitted over the air, directly, so it
is converted by a "modem" to a wide band of tones that modulate an FM
transmitter...for the same reason AMPS used FM....noise immunity on weak
signals.


>
>> > Microwave-frequencies can be done in AM just as well
>> > as FM? Why not use AM?
>
>> Why not use FM?

See above....It's all about noise immunity.

>
> FM is limited to line of sight. AM provides the ability to converse
> over significantly longer distances than FM.

Simply not true. If FM is used at 10 Mhz, it propagates just as far as
AM...but without the noise. But, alas, FM has another
problem...bandwidth.

If you modulate an AM transmitter with a single tone, 3 signals come out
of it. The carrier frequency the transmitter is tuned to is always
transmitted, continuously. The tone mixes, in a non-linear RF stage. RF
mixing always produces two "products"...the sum of the carrier + the
frequency of the modulated tone...and the difference of the carrier - the
modulated tone. If we are transmitting on 1.000 Mhz, with a 1000 Hz
audio tone modulating the transmitter, you get a Lower Sideband of
..999Mhz, 1.000Mhz and 1.001 Mhz. The bandwidth occupied is only 2 Khz of
the RF spectrum. The bandwidth of an AM transmitter is twice the
frequency of the highest modulating tone. AM radio broadcasting LIMITS
the audio frequencies fed to the transmitters to 5 Khz. AM broadcast's
bandwidth (and channel spacing in the USA) is 10 Khz... 800 Khz, 810
Khz, 820 Khz etc. Europe and Asia use 9 Khz channel spacing to get more
channels. The audio bandwidth is limited to around 4.5 Khz, lower
fidelity, to accomplish this without undue interference. AM transmitters
used only for voice transmissions usually have an audio bandwidth from
300 to 3000 Hz, making their bandwidth only 6 Khz. CB radio is a good
example. Due to how cheap CB is manufactured, their transmitter's
carrier frequency isn't very accurate. When detected, this results in a
beat note you can hear, that howling when hundreds of skip CB station are
all received at once, rendering it pretty useless. At night, when the
atmosphere reflects the AM broadcast band, AM stations also have a beat
note you can "hear". They're accurate to +/- 20 Hz, but are much more
accurate than that to reduce channel interference. You hear a very low
warbling like a Leslie speaker on a Hammond Organ makes as the signals
aid then cancel each other due to this difference note.

The sunspot cycle is at a very low point, right now, so Ham Radio in the
HF band is pretty poor. But, when the 10 meter band is open (28-29.7
Mhz) you can hear very long range FM stations near the upper end of the
band. Hams have "repeater" stations on just a few "channels", by
gentlemen's agreement, between 29.5 and 29.7 Mhz output (They listen 100
Khz below their output where we talk to them.) I've use 10 Meter FM
repeaters in Europe, Africa, Japan and Australia on the other side of the
planet from South Carolina, my home, since around 1970. Great fun HF FM.

How far a signal can be heard is very dependent on the frequency of the
signal and time-of-day because of the layers of supercharged ions in the
ionosphere over your heads, right now. That's what makes the signals
reflect off these very high layers at the lower end of the RF
spectrum....as the Earth turns under these variable layers that depend
solely on the ions streaming off the sun for their existence. Many more
layers trail the Earth in the shadow the Earth creates to the solar wind,
than on the sunny side. These layers do not rotate with the planet. We
turn under them. Generally speaking, in the day, frequencies that
reflect off the sun-side layers are from about 7 Mhz to about 25 to 40
Mhz, depending on how thick the layers are and the solar activity, which
varies nearly like a sine wave in 11 year sunspot cycles. At night,
different layers AT DIFFERENT ALTITUDES create different reflectors, many
of them, that reflect different frequency bands. Your AM broadcast radio
has no reflectors in the daytime when you only hear local stations. At
night, special stations on "clear channels", reserved for them alone to
provide long range AM radio to the countryside, pumping 50,000 watts into
massive antenna arrays, some with 3 to 16 towers, can be heard a thousand
miles away. Good examples are WSM, 650 Khz, in Nashville, WLW, 700 Khz
in Ohio, WWL, 870 in New Orleans. If they were on FM, you'd hear them
just fine, but we'd only have a few stations...why?

FM's spectrum is much more complex and WIDER than AM's. Notice on your
FM radio the stations are a whopping 200 Khz apart! There are two
reasons for this, one economic and one fidelity. FM broadcasting has an
audio bandwidth of 50 to 15000 Hz. It is transmitted in a very wideband
way with the carrier swinging very far from its resting frequency so you
can go buy a really cheap FM radio, with really cheap electronics in it,
and listen to the constant blather of commercials that broadcasting in
America has become. Two things effect the spectrum bandwidth of
FM....The highest audio frequency, 15Khz, and how hard you drive the
transmitter away from its carrier frequency (or change its phase, which
looks just like FM, too.) 15 Khz is the audio freq allowed on FM
broadcasting. 75 Khz is its "deviation". This produces a huge load of
detectable sidebands by even the cheapest detectors for high fidelity
sound, as wide as human ears can hear actually, no matter what the stereo
industry advertises...(c; Can you hear your picture tube analog TV
screaming? It's screaming all the time its running at 15,575 Hz and you
don't even hear it. So, why buy a stereo that can reproduce 25,000 Hz?
It's crazy!...(c; 15 Khz audio at 75 Khz deviation needs around 200 Khz
of bandwidth, a crazy amount. If we used that on the old AM band, we'd
get 1650-550 khz = 1100 Khz for the whole band...divided by 200 = only 5
channels! As much as Clear Channel Communications would love to own all
5, FCC has other ideas...(c; That's why FM isn't on the lower
frequencies...bandwidth.

>
>> > At the receiving end, the carrier signal should be
>> > amplified prior to demodulation.
>
> The purpose of this is to "DX". This allows communications over even
> longer distances than without the DX.
>
> For best results, longwave frequencies [around 150 KHz] should be used
> along with DX.
>
>

The signal received by any "radio" receiver is miniscule and too small to
power germanium diode or tube diode "detectors". That's the only reason
radios have amplifiers, to make the lowest signals big enough to drive
detector diodes. It's that simple. Hazeltine Research Corp, NYC, solved
another problem related to how wide a bandwidth a cheap receiver listened
to, to stop the radios from listening to 5 stations at once, after quite
a few stations got on the air in the 1920's/30's. It was called the
"superheterodyne" to impress the stupid public fascinated with Flash
Gordon....on the radio, of course! If we "convert" the high frequency
signal from the station to a lower frequency signal the industry, at
first, decided would be 260 Khz, but was later changed to today's 455 Khz
"IF Frequency", you could narrow the bandwidth of the signal to just ONE
station at a time, fed to the detector diodes. Because FM is a wider
bandwidth service, 10.7 Mhz is the IF frequency of your FM radio, netting
us an easy-to-achieve 200 Khz receiver bandwidth. (FM mobile radios,
including cellphones use two conversions....10.7 then 455 Khz to get
narrow band FM channels...one at a time. These fixed frequency
amplifiers can be made VERY high gain because you don't have to tune them
to any other frequency...so these "Superheterodyne" receivers are very
sensitive...way down into the natural noise level. That hasn't changed
since the 1930's when they were produced. Your cellphone's IF amp has a
special type of ceramic filters which is very cheap to produce in the
Chinese slave factories.

FM is all about the NOISE.....or rather, the lack of it.

Larry
--
Grade School Physics Factoid:
A building cannot freefall into its own footprint without
skilled demolition.

Nice writeup, but I do have to take exception to one thing:

AM broadcast audio bandwidth is *not* limited to 5 KHz. It is limited to 10
KHz by the NRSC AM Bandwidth and pre-emphasis standard. Prior to NRSC, it
used to be unlimited, with a *minimum* audio bandwidth of 5 KHz. Some
stations are voluntarily reducing their bandwidth to 6 - 8 KHz in an attempt
to reduce interference in congested markets.

AM channels are separated by 10 KHz, but the FCC tries to allocate AM
broadcast stations locally on alternate or second alternate channels to
avoid interference. The interference is more noticeable at night, when local
signals skip long distances. Then, you hear the 10 KHz whistle due to
carriers 10 KHz up or down the dial, as well as "monkey chatter" due to the
inverted sidebands of adjacent stations.

"Larry" wrote ...
[long and excelent treatise on FM vs. AM and other topics]

Larry, you've been trolled by "Radium".
Many of us have dramatically increased the SNR of these
newsgroups by simply plonking him/her/it.

Larry,

I agree with your general point that FM doesn't propagate any
differently than AM and that the key difference is noise immunity.

However, I think:

> Your CDMA, TDMA, PCS or whatever cellphone you carry is STILL an FM
> radio, at heart,

is inaccurate.

I wouldn't consider the underlying modulation type of CDMA FM.
CDMA2000 uses QPSK as its modulation. That's not FM.

However, I would agree that the old US TDMA standard (IS-54/IS-136),
which uses pi/4 DQPSK is a type of FM.

Standard GSM uses GMSK (Gaussian Minimum Shift Keying), which is also
a type of "digital" FM, but the new high-speed GSM channels, the
so-called EDGE signals, are 3pi/8-shifted 8PSK and thus again are not
really FM.

The exceptions I've noted are phase-modulated signals, and since phase
and frequency modulation share the constant modulus property, they
both have noise immunity against impulsive, AM noise. So the idea that
they all resist noise is correct, but calling them FM is not accurate,
in my opinion.

I'm coming from [rappaport] and [proakiscomm].

--Randy

@book{rappaport,
title = "Wireless Communications: Principles and Practice",
author = "Theodore S. Rappaport",
publisher = "Prentice Hall",
edition = "second",
year = "2002"}

@BOOK{proakiscomm,
title = "{Digital Communications}",
author = "John~G.~Proakis",
publisher = "McGraw-Hill",
edition = "fourth",
year = "2001"}

--
% Randy Yates % "Bird, on the wing,
%% Fuquay-Varina, NC % goes floating by
%%% 919-577-9882 % but there's a teardrop in his eye..."
%%%% <yates@ieee.org> % 'One Summer Dream', *Face The Music*, ELO
http://home.earthlink.net/~yatescr

"Richard Crowley" <rcrowley@xp7rt.net> wrote in news:135dsro7vv2bvb5
@corp.supernews.com:

> Larry, you've been trolled by "Radium".
> Many of us have dramatically increased the SNR of these
> newsgroups by simply plonking him/her/it.
>
>

Done...(c;

I've been attacked by better...

Larry
--
Grade School Physics Factoid:
A building cannot freefall into its own footprint without
skilled demolition.

> Just out of personal preference, I'd much rather have more
> interference than less bandwidth.

Similarly, out of personal preference, I'd much rather have more cats than
less toothpaste.

"Radium" <glucegen1@gmail.com> wrote in message
news:1180133162.767788.59280@x35g2000prf.googlegroups.com...
> Just out of personal preference, I'd much rather have more
> interference than less bandwidth.

Bandwidth being more important for mobile phone conversations than lower
noise/interference in your opinion?
:-)

MrT.

On May 27, 10:03 pm, "Mr.T" <MrT@home> wrote:

> Bandwidth being more important for mobile phone conversations than lower
> noise/interference in your opinion?

Bandwidth is more important for any/everything than lower noise/
interference in my opinion.

I like listening to long distance magnetic disruptions originating
from the objects in outer space significantly further than our solar
system.

Which brings up another question.

Let's say I am in a space station which has a supercooled 150 KHz DX
analog receiver that receives the magnetic fields [while ignoring the
electric fields] of extremely weak 150 KHz AM analog carrier signals.
In addition, this receiver is so sensitive and powerful that it can
clearly pick up AM carrier waves as weak as 10^-10,000 watt [i.e. 10-
to-the-power-NEGATIVE-10,000 watt]. Also, this receiver has an
astronomically-powerful amplifier which amplifies the extremely-soft
carrier waves until the resulting modulation signals will be just loud
enough for the human ear to detect. Following this amplification, the
carrier waves are demodulation to modulation waves - the stuff we
"hear" - and then sent to loudspeaker so those onboard can hear those
sounds. In addition, the audio devices filter out modulation signals
that are below 20 Hz or above 20 KHz, as the human ear only responds
to 20-20,000 Hz.

If I am on this spaceship, what will I hear on the radio? My guess is
that I would hear long-distance magnetic disruptions.

On May 27, 10:03 pm, "Mr.T" <MrT@home> wrote:

> Bandwidth being more important for mobile phone conversations than lower
> noise/interference in your opinion?

Bandwidth is more important for any/everything than lower noise/
interference in my opinion.

I like listening to long distance magnetic disruptions originating
from the objects in outer space significantly further than our solar
system.

Which brings up another question.

Let's say I am in a space station which has a supercooled 150 KHz DX
analog receiver that receives the magnetic fields [while ignoring the
electric fields] of extremely weak 150 KHz AM analog carrier signals.
In addition, this receiver is so sensitive and powerful that it can
clearly pick up AM carrier waves as weak as 10^-10,000 watt [i.e. 10-
to-the-power-NEGATIVE-10,000 watt]. Also, this receiver has an
astronomically-powerful amplifier which amplifies the extremely-soft
carrier waves until the resulting modulation signals will be just loud
enough for the human ear to detect. Following this amplification, the
carrier waves are demodulation to modulation waves - the stuff we
"hear" - and then sent to loudspeaker so those onboard can hear those
sounds. In addition, the audio devices filter out modulation signals
that are below 20 Hz or above 20 KHz, as the human ear only responds
to 20-20,000 Hz.

If I am on this spaceship, what will I hear on the radio? My guess is
that I would hear long-distance magnetic disruptions.

On Mon, 28 May 2007 09:21:37 -0700, Radium wrote:

> On May 27, 10:03 pm, "Mr.T" <MrT@home> wrote:
>
>> Bandwidth being more important for mobile phone conversations than
>> lower noise/interference in your opinion?
>
> Bandwidth is more important for any/everything than lower noise/
> interference in my opinion.
>
> I like listening to long distance magnetic disruptions originating from
> the objects in outer space significantly further than our solar system.
>
> Which brings up another question.
>
> Let's say I am in a space station which has a supercooled 150 KHz DX
> analog receiver that receives the magnetic fields [while ignoring the
> electric fields] of extremely weak 150 KHz AM analog carrier signals. In
> addition, this receiver is so sensitive and powerful that it can clearly
> pick up AM carrier waves as weak as 10^-10,000 watt [i.e. 10-
> to-the-power-NEGATIVE-10,000 watt]. Also, this receiver has an
> astronomically-powerful amplifier which amplifies the extremely-soft
> carrier waves until the resulting modulation signals will be just loud
> enough for the human ear to detect. Following this amplification, the
> carrier waves are demodulation to modulation waves - the stuff we "hear"
> - and then sent to loudspeaker so those onboard can hear those sounds.
> In addition, the audio devices filter out modulation signals that are
> below 20 Hz or above 20 KHz, as the human ear only responds to 20-20,000
> Hz.
>
> If I am on this spaceship, what will I hear on the radio? My guess is
> that I would hear long-distance magnetic disruptions.

Most likely you would hear interference from nearby (in cosmic terms)
powerline systems. This noise will be attenuated by the
ionosphere but some will get through, and there's plenty of it
where it came from.

As to tropospheric bending ("skip"), this is a function of the weather
and frequency but not modulation. I received FM and TV stations
800+ miles away one summer morning in the 60s, and this is nowhere
near a record.

Not counting exotic modes, the most efficient voice modulation
is suppressed carrier single sideband, a form of AM.

--
/u/caf/signature.txt