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  #1 (permalink)  
Old 04-28-2008, 09:26 PM
Green Xenon [Radium]
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Default Disadvantages of using AM for DSSS/FHSS?

Hi:

Most cell phones and wireless internet routers, modems, and access
points that use spread spectrum usually broadcast and receive their data
on FM-radio waves. Just out of curiosity, I ask, why not use AM?

Let's say a DSSS/FHSS type of spread-spectrum is transmitted and
received using the AM radio waves in the UHF spectrum [i.e. spread info
for transmission throughout the UHF band and receive AM radio waves
throughout all UHF frequencies]. What would be the disadvantages of this?

Normally DSSS and FHSS are transmitted/received on FM radio waves. So I
ask what would be the disadvantages of using AM instead of FM for this?

AM radio tends to be more vulnerable to unwanted magnetic disruptions
than FM, however this only affects analog reception. Digital reception
on AM should be unaffected even by the strongest-interfering analog RF
magnetic signal. Right?


Thanks,

Radium

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  #2 (permalink)  
Old 04-28-2008, 11:12 PM
clay@claysturner.com
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Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

On Apr 28, 5:26*pm, "Green Xenon [Radium]" <gluceg...@excite.com>
wrote:
> Hi:
>
> Most cell phones and wireless internet routers, modems, and access
> points that use spread spectrum usually broadcast and receive their data
> on FM-radio waves. Just out of curiosity, I ask, why not use AM?
>
> Let's say a DSSS/FHSS type of spread-spectrum is transmitted and
> received using the AM radio waves in the UHF spectrum [i.e. spread info
> for transmission throughout the UHF band and receive AM radio waves
> throughout all UHF frequencies]. What would be the disadvantages of this?
>
> Normally DSSS and FHSS are transmitted/received on FM radio waves. So I
> ask what would be the disadvantages of using AM instead of FM for this?
>
> AM radio tends to be more vulnerable to unwanted magnetic disruptions
> than FM, however this only affects analog reception. Digital reception
> on AM should be unaffected even by the strongest-interfering analog RF
> magnetic signal. Right?
>
> Thanks,
>
> Radium


Cellphones moved away from FM when they dropped analog service. IDEN
phones use a 4 subcarrier 16-QAM, but they are channelized and not
spread spectrum.

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  #3 (permalink)  
Old 04-28-2008, 11:55 PM
John Monro
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Default Re: Disadvantages of using AM for DSSS/FHSS?

Green Xenon [Radium] wrote:
> Hi:
>
> Most cell phones and wireless internet routers, modems, and access
> points that use spread spectrum usually broadcast and receive their data
> on FM-radio waves. Just out of curiosity, I ask, why not use AM?
>
> Let's say a DSSS/FHSS type of spread-spectrum is transmitted and
> received using the AM radio waves in the UHF spectrum [i.e. spread info
> for transmission throughout the UHF band and receive AM radio waves
> throughout all UHF frequencies]. What would be the disadvantages of this?
>
> Normally DSSS and FHSS are transmitted/received on FM radio waves. So I
> ask what would be the disadvantages of using AM instead of FM for this?
>
> AM radio tends to be more vulnerable to unwanted magnetic disruptions
> than FM, however this only affects analog reception. Digital reception
> on AM should be unaffected even by the strongest-interfering analog RF
> magnetic signal. Right?
>
>
> Thanks,
>
> Radium


Hi,

FM demodulation produces a handy 'capture' effect. If a weak and a
strong signal are present together, then the recovered baseband signal
from the weaker FM signal is greatly reduced. This is useful in
rejecting interference from adjacent stations on the same frequency.

With AM, the demodulated baseband signals are present in amplitudes that
are proportional to their RF amplitudes. In addition, the AM carriers
will 'beat' together to produce an additional and unwanted
tone-modulation of the received RF signal.

Regards,
John

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  #4 (permalink)  
Old 04-29-2008, 12:52 AM
Green Xenon [Radium]
Guest
 
Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

John Monro wrote:
> Green Xenon [Radium] wrote:
>> Hi:
>>
>> Most cell phones and wireless internet routers, modems, and access
>> points that use spread spectrum usually broadcast and receive their
>> data on FM-radio waves. Just out of curiosity, I ask, why not use AM?
>>
>> Let's say a DSSS/FHSS type of spread-spectrum is transmitted and
>> received using the AM radio waves in the UHF spectrum [i.e. spread
>> info for transmission throughout the UHF band and receive AM radio
>> waves throughout all UHF frequencies]. What would be the disadvantages
>> of this?
>>
>> Normally DSSS and FHSS are transmitted/received on FM radio waves. So
>> I ask what would be the disadvantages of using AM instead of FM for this?
>>
>> AM radio tends to be more vulnerable to unwanted magnetic disruptions
>> than FM, however this only affects analog reception. Digital reception
>> on AM should be unaffected even by the strongest-interfering analog RF
>> magnetic signal. Right?
>>
>>
>> Thanks,
>>
>> Radium

>
> Hi,
>
> FM demodulation produces a handy 'capture' effect. If a weak and a
> strong signal are present together, then the recovered baseband signal
> from the weaker FM signal is greatly reduced. This is useful in
> rejecting interference from adjacent stations on the same frequency.
>
> With AM, the demodulated baseband signals are present in amplitudes that
> are proportional to their RF amplitudes. In addition, the AM carriers
> will 'beat' together to produce an additional and unwanted
> tone-modulation of the received RF signal.
>
> Regards,
> John



But if the signal is digital, won't it remain immune to EMI/RFI [analog
disruption] even if received on AM? DSSS and FHSS and digital. So I
would think that the analog magnetic interferences wouldn't affect it.

Also, doesn't FM have the disadvantages in that it hogs more bandwidth
than AM?

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  #5 (permalink)  
Old 04-29-2008, 01:01 AM
Green Xenon [Radium]
Guest
 
Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

clay@claysturner.com wrote:
> On Apr 28, 5:26 pm, "Green Xenon [Radium]" <gluceg...@excite.com>
> wrote:
>> Hi:
>>
>> Most cell phones and wireless internet routers, modems, and access
>> points that use spread spectrum usually broadcast and receive their data
>> on FM-radio waves. Just out of curiosity, I ask, why not use AM?
>>
>> Let's say a DSSS/FHSS type of spread-spectrum is transmitted and
>> received using the AM radio waves in the UHF spectrum [i.e. spread info
>> for transmission throughout the UHF band and receive AM radio waves
>> throughout all UHF frequencies]. What would be the disadvantages of this?
>>
>> Normally DSSS and FHSS are transmitted/received on FM radio waves. So I
>> ask what would be the disadvantages of using AM instead of FM for this?
>>
>> AM radio tends to be more vulnerable to unwanted magnetic disruptions
>> than FM, however this only affects analog reception. Digital reception
>> on AM should be unaffected even by the strongest-interfering analog RF
>> magnetic signal. Right?
>>
>> Thanks,
>>
>> Radium

>
> Cellphones moved away from FM when they dropped analog service. IDEN
> phones use a 4 subcarrier 16-QAM, but they are channelized and not
> spread spectrum.



Yeah but even if the signal being transmitted/received is digital, it's
carrier wave is still analog. Right?

AFAIK, there is no such thing as a digital carrier wave. The carrier
wave is always analog just like a cable link is also always analog. The
signal transmitted through the analog medium maybe digital, though.

If a PCM signal [digital] is transmitted on an AM carrier wave [analog],
the AM wave's peak-to-peak amplitude will vary according to the PCM
signal in the following manner:

1. A positive amperage of the PCM signal will cause the AM carrier
wave's peak-to-peak amplitude to increase while a negative amperage
[i.e. going below the x-axis when graphed] will cause a decrease the AM
carrier's peak-to-peak amplitude.

2. A increase in frequency of the PCM signal will cause the AM carrier's
peak-to-peak amplitude to vary more rapidly while a decrease in the PCM
signal's frequency will cause the AM wave's peak-to-peak amplitude to
vary more slowly.

That's how I understand it. The PCM signal is digital but causes a
measurable affect on the analog AM carrier wave. Upon reception, an
AM-demodulator can retrieve this PCM signal and play it back. I could be
wrong though.

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  #6 (permalink)  
Old 04-29-2008, 01:03 AM
Green Xenon [Radium]
Guest
 
Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

Green Xenon [Radium] wrote:

> DSSS and FHSS and digital.


Sorry, that should read "DSSS and FHSS *are* digital."

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  #7 (permalink)  
Old 04-29-2008, 02:41 AM
DTC
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Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

Don't feed the troll

Bob Myers wrote:
Radium has never actually had an original
question. All of his questions are either rehashing very
well-understood situations, or are utterly nonsensical and
therefore not answerable AS legitimate questions.

RHF wrote:
Since Radium is an ignorant idiot, he has no clue what group would
be appropriate for his moronic questions.

Porgy Tirebiter wrote:
You are a uneducated troll....
You blabber on and on about nothing. No one is interested in your
opinions. Discover women or masturbation, stop playing on Usenet,
we are not amused.

Jim wrote:
Belief doesn't make reality.
A hundred years or so of experments say that's utter, babbling,
nonsense.
I'd suggest fluphenazine and haloperidol.
Babbling, delusional nonsense.
Seek help.
There are drugs that may help your condition if you are treated
early enough.
Radium is a silly-***, ignorant child.

DTC (that's me) wrote
This thread reminds me of situations where someone acquire a little
knowledge of something and extrapolates it into areas of which they have
a limited skill set for understanding them.


Another example of Radium's mastery of trolldum is:

Radium wrote:
Can the Spin Exchange Relaxation Free Magnetometer
be used to receiver distant AM radio signals in which the
carrier frequency is 150 KHz?

What if the carrier frequency is 44.1 KHz [for the same
reason CDs use a 44.1 KHz sample rate]? 40 KHz?

Thanks a bunch,

Radium

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  #8 (permalink)  
Old 04-29-2008, 09:34 AM
John E. Hadstate
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Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?


"Green Xenon [Radium]" <glucegen1@excite.com> wrote in message
news:48164106$0$5109$4c368faf@roadrunner.com...

> AM radio tends to be more vulnerable to unwanted magnetic
> disruptions than FM, however this only affects analog
> reception. Digital reception on AM should be unaffected even
> by the strongest-interfering analog RF magnetic signal.
> Right?


I'm not sure how. All transmission is analog until the
received signal is digitized. FM (and it's step-brother, FSK)
offer better immunity to impulse noise than AM at the expense
of spreading the transmitted power over the redundancy that
exists in their spectra.



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  #9 (permalink)  
Old 04-29-2008, 09:36 AM
John E. Hadstate
Guest
 
Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?


"DTC" <me@nothingtoseehere.zzx> wrote in message
newsWvRj.11932$V14.1307@nlpi070.nbdc.sbc.com...
> Don't feed the troll


Some of you "Don't feed the troll" freaks are more
objectionable than the OP. At least he asked a sensible
question (whether or not he understands the answer). What,
exactly, did you contribute?



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  #10 (permalink)  
Old 04-29-2008, 01:39 PM
DTC
Guest
 
Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

John E. Hadstate wrote:
>
> "DTC" <me@nothingtoseehere.zzx> wrote in message
> newsWvRj.11932$V14.1307@nlpi070.nbdc.sbc.com...
>> Don't feed the troll

>
> Some of you "Don't feed the troll" freaks are more objectionable than
> the OP. At least he asked a sensible question (whether or not he
> understands the answer). What, exactly, did you contribute?


Obviously you haven't kept up with his dribble over the past year.

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  #11 (permalink)  
Old 04-29-2008, 03:22 PM
Jerry Avins
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Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

DTC wrote:
> John E. Hadstate wrote:
>>
>> "DTC" <me@nothingtoseehere.zzx> wrote in message
>> newsWvRj.11932$V14.1307@nlpi070.nbdc.sbc.com...
>>> Don't feed the troll

>>
>> Some of you "Don't feed the troll" freaks are more objectionable than
>> the OP. At least he asked a sensible question (whether or not he
>> understands the answer). What, exactly, did you contribute?

>
> Obviously you haven't kept up with his dribble over the past year.


Past *year*? Where were you before that?

Jerry
--
Engineering is the art of making what you want from things you can get.
ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ ŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻŻ

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  #12 (permalink)  
Old 04-29-2008, 04:09 PM
Jeff Liebermann
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Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

On Mon, 28 Apr 2008 14:26:35 -0700, "Green Xenon [Radium]"
<glucegen1@excite.com> wrote:

>Most cell phones and wireless internet routers, modems, and access
>points that use spread spectrum usually broadcast and receive their data
>on FM-radio waves. Just out of curiosity, I ask, why not use AM?


I'll ignore cellular modulation methods because you already beat that
to death in sci.electronics.design. I'll assume that you've had a
brain transplant and are now capable of understanding various answers
to your marginal questions.

802.11 wireless modulation is pure FM for 1 and 2Mbits/sec data rates.
5.5 and 11 add AM to the FM modulation, thus yielding higher bits/Hz
modulation density, which is a measure of spectral efficiency.
Therefore, 802.11 wireless data does use some AM.

6Mbits/sec thru 54Mbits/sec use OFDM, which are 48 orthogonal
(non-overlapping) QAM (quadrature amplitude modulation) carriers. Each
carrier is modulated by both FM and AM. Each carries a part of the
data stream which when reassemble, yields even higher bits/Hz
modulation density and efficiency. Quadrature implies phase
modulation (or FM if you prefer). The AM in QAM implies that it's
also amplitude modulated.

>Let's say a DSSS/FHSS type of spread-spectrum is transmitted and
>received using the AM radio waves in the UHF spectrum [i.e. spread info
>for transmission throughout the UHF band and receive AM radio waves
>throughout all UHF frequencies]. What would be the disadvantages of this?


The FCC would have you arrested and returned to wherever you escaped
from. Modulation methods and technology have evolved over the years
to satisfy many conflicting requirements. Pure AM is one of the
oldest, and has the advantage of simplicity. Everything else about it
is inferior to other modulation methods. FAA/FCC regulations
protecting obsolete technology has insured that it's still in use in
the aviation and broadcast business.

Pure AM has a number of disadvantages, depending on what you're trying
to accomplish. The big one is that it's grossly inefficient in both
the utilization of power and spectral efficiency. The carrier hogs at
least half the available power. AM has its place, but there are
better ways.

I'll skip all the intermediate modulation schemes and jump directly to
the present. Microprocessors have become so cheap, that it's now
economical to do DSP (digital signal processing) in every radio.
Digital has the huge advantage of offering error correction, noise
immunity, bandwidth compression, and high spectral efficiency. Schemes
have been devised that will extract useful audio or data from signals
that are well below the thermal noise floor. You can't do that with
AM (or FM). If you attempted to do Wi-Fi using pure AM technology,
the data thruput would be horrible and/or the error rate would be
hideous.

>Normally DSSS and FHSS are transmitted/received on FM radio waves.


DSSS are pure FM at 1 and 2Mbits/sec data rates. 5.5 and 11Mbits/sec
are a mixture of AM and FM. OFDM is neither pure AM or FM. It's QAM
modulation, a form of PSK (phase shift keying), with multiple
sub-carriers to reduce the effects of frequency selective fading.

>So I
>ask what would be the disadvantages of using AM instead of FM for this?


Lousy spectral efficiency, difficult linearity requirements, slow AGC,
excessive bandwidth, high error rate for data, and it's not
fashionable. I suppose there's some benifits to resurrecting stone
age technology, so that we don't forget our past, but I wouldn't want
to live like a cave man. That's essentially what you're suggesting by
reverting to pure AM technology.

>AM radio tends to be more vulnerable to unwanted magnetic disruptions
>than FM, however this only affects analog reception.


Magnetic? Wave a magnet around your cherished AM BCB (broadcast band)
radio. Hear anything different? How are magnets causing disruptions?

>Digital reception
>on AM should be unaffected even by the strongest-interfering analog RF
>magnetic signal. Right?


No. Magnets don't affect radio waves much. There is Faraday
rotation, which causes EM (electro-magic) waves to change polarization
in the presence of a magnetic field. That has some effect on
microwave signals and is used to good advantage in RF circulators and
isolators. However, for RF below about 1GHz, it's a non-issue.

So what are you proposing this time? Cellular service at HF
frequencies? Ultra high index of modulation AM schemes? Long range
cellular where you would have only a few simultenous users on a given
continent? Laws to limit acronyms to two letters?

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

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  #13 (permalink)  
Old 04-29-2008, 04:56 PM
glen herrmannsfeldt
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Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

Green Xenon [Radium] wrote:
(snip)

> Yeah but even if the signal being transmitted/received
> is digital, it's carrier wave is still analog. Right?


> AFAIK, there is no such thing as a digital carrier wave. The carrier
> wave is always analog just like a cable link is also always analog. The
> signal transmitted through the analog medium maybe digital, though.


> If a PCM signal [digital] is transmitted on an AM carrier wave [analog],
> the AM wave's peak-to-peak amplitude will vary according to the PCM
> signal in the following manner:


> 1. A positive amperage of the PCM signal will cause the AM carrier
> wave's peak-to-peak amplitude to increase while a negative amperage
> [i.e. going below the x-axis when graphed] will cause a decrease the AM
> carrier's peak-to-peak amplitude.


That is one way. Usually you will also need some synchronizing
information to know which bits are which. Sometimes it is done
as a subcarrier, modulating a carrier synchronous to the bit rate,
and then using that to amplitude modulate the real carrier.

> 2. A increase in frequency of the PCM signal will cause the AM carrier's
> peak-to-peak amplitude to vary more rapidly while a decrease in the PCM
> signal's frequency will cause the AM wave's peak-to-peak amplitude to
> vary more slowly.


> That's how I understand it. The PCM signal is digital but causes a
> measurable affect on the analog AM carrier wave. Upon reception, an
> AM-demodulator can retrieve this PCM signal and play it back. I could be
> wrong though.


You could also generate a pulse for 1 and no pulse for 0, though
there are synchronizing problems with that. Next easiest is
a pulse for 1, no pulse for zero, and a clock pulse half way
in between. (The coding used on single density floppy disks.)

You need some kind of modulation for an AC coupled signal path.

-- glen


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  #14 (permalink)  
Old 04-30-2008, 09:31 AM
kronecker@yahoo.co.uk
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Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

On Apr 29, 11:55 am, John Monro <johnmo...@optusnet.removethis.com.au>
wrote:
> Green Xenon [Radium] wrote:
> > Hi:

>
> > Most cell phones and wireless internet routers, modems, and access
> > points that use spread spectrum usually broadcast and receive their data
> > on FM-radio waves. Just out of curiosity, I ask, why not use AM?

>
> > Let's say a DSSS/FHSS type of spread-spectrum is transmitted and
> > received using the AM radio waves in the UHF spectrum [i.e. spread info
> > for transmission throughout the UHF band and receive AM radio waves
> > throughout all UHF frequencies]. What would be the disadvantages of this?

>
> > Normally DSSS and FHSS are transmitted/received on FM radio waves. So I
> > ask what would be the disadvantages of using AM instead of FM for this?

>
> > AM radio tends to be more vulnerable to unwanted magnetic disruptions
> > than FM, however this only affects analog reception. Digital reception
> > on AM should be unaffected even by the strongest-interfering analog RF
> > magnetic signal. Right?

>
> > Thanks,

>
> > Radium

>
> Hi,
>
> FM demodulation produces a handy 'capture' effect. If a weak and a
> strong signal are present together, then the recovered baseband signal
> from the weaker FM signal is greatly reduced. This is useful in
> rejecting interference from adjacent stations on the same frequency.
>
> With AM, the demodulated baseband signals are present in amplitudes that
> are proportional to their RF amplitudes. In addition, the AM carriers
> will 'beat' together to produce an additional and unwanted
> tone-modulation of the received RF signal.
>
> Regards,
> John


Only if the power of the co-channel is very much less otherwise you
get complete destruction of both to a point when they flip over and
you hear the other channel. Power saving is the big thing with FM.

K.

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  #15 (permalink)  
Old 04-30-2008, 09:34 AM
kronecker@yahoo.co.uk
Guest
 
Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

On Apr 29, 12:52 pm, "Green Xenon [Radium]" <gluceg...@excite.com>
wrote:
> John Monro wrote:
> > Green Xenon [Radium] wrote:
> >> Hi:

>
> >> Most cell phones and wireless internet routers, modems, and access
> >> points that use spread spectrum usually broadcast and receive their
> >> data on FM-radio waves. Just out of curiosity, I ask, why not use AM?

>
> >> Let's say a DSSS/FHSS type of spread-spectrum is transmitted and
> >> received using the AM radio waves in the UHF spectrum [i.e. spread
> >> info for transmission throughout the UHF band and receive AM radio
> >> waves throughout all UHF frequencies]. What would be the disadvantages
> >> of this?

>
> >> Normally DSSS and FHSS are transmitted/received on FM radio waves. So
> >> I ask what would be the disadvantages of using AM instead of FM for this?

>
> >> AM radio tends to be more vulnerable to unwanted magnetic disruptions
> >> than FM, however this only affects analog reception. Digital reception
> >> on AM should be unaffected even by the strongest-interfering analog RF
> >> magnetic signal. Right?

>
> >> Thanks,

>
> >> Radium

>
> > Hi,

>
> > FM demodulation produces a handy 'capture' effect. If a weak and a
> > strong signal are present together, then the recovered baseband signal
> > from the weaker FM signal is greatly reduced. This is useful in
> > rejecting interference from adjacent stations on the same frequency.

>
> > With AM, the demodulated baseband signals are present in amplitudes that
> > are proportional to their RF amplitudes. In addition, the AM carriers
> > will 'beat' together to produce an additional and unwanted
> > tone-modulation of the received RF signal.

>
> > Regards,
> > John

>
> But if the signal is digital, won't it remain immune to EMI/RFI [analog
> disruption] even if received on AM? DSSS and FHSS and digital. So I
> would think that the analog magnetic interferences wouldn't affect it.
>
> Also, doesn't FM have the disadvantages in that it hogs more bandwidth
> than AM?


Ye cannae re-write the laws of physics captain - digital or analogue
FM is the same. You do the Maths and see.

Digital comms still breaks down except it has error-correcting codes
built in - so you don't see it! Ocassionally you may well find that
your cell-phone losses connection too. It's all or nothing with
digital.

K.

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  #16 (permalink)  
Old 05-01-2008, 05:16 AM
glen herrmannsfeldt
Guest
 
Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

Jeff Liebermann wrote:
(snip)

> I'll skip all the intermediate modulation schemes and jump directly to
> the present. Microprocessors have become so cheap, that it's now
> economical to do DSP (digital signal processing) in every radio.
> Digital has the huge advantage of offering error correction, noise
> immunity, bandwidth compression, and high spectral efficiency. Schemes
> have been devised that will extract useful audio or data from signals
> that are well below the thermal noise floor. You can't do that with
> AM (or FM). If you attempted to do Wi-Fi using pure AM technology,
> the data thruput would be horrible and/or the error rate would be
> hideous.


Somehow this reminds me that there have been discussions on
using digital techniques to demodulate standard AM and FM radio
signals. Now that we have digital radio, what seems to be
called HD radio (radio stations seem to advertise it pretty often).

http://en.wikipedia.org/wiki/HD_Radio

I don't remember much discussion here about it.

-- glen


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  #17 (permalink)  
Old 05-01-2008, 05:37 AM
Jeff Liebermann
Guest
 
Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

On Wed, 30 Apr 2008 21:16:04 -0800, glen herrmannsfeldt
<gah@ugcs.caltech.edu> wrote:

>Jeff Liebermann wrote:
>(snip)
>
>> I'll skip all the intermediate modulation schemes and jump directly to
>> the present. Microprocessors have become so cheap, that it's now
>> economical to do DSP (digital signal processing) in every radio.
>> Digital has the huge advantage of offering error correction, noise
>> immunity, bandwidth compression, and high spectral efficiency. Schemes
>> have been devised that will extract useful audio or data from signals
>> that are well below the thermal noise floor. You can't do that with
>> AM (or FM). If you attempted to do Wi-Fi using pure AM technology,
>> the data thruput would be horrible and/or the error rate would be
>> hideous.


>Somehow this reminds me that there have been discussions on
>using digital techniques to demodulate standard AM and FM radio
>signals.


Ah, topic drift...

SDR (software defined radio) uses digital techniques to demodulate
just about anything. Even CW (Morse code) can be demodulated and
decoded. For example:
<http://www.rfspace.com/SDR-IQ.html>
<http://www.amqrp.org/kits/softrock40/>
You can even do it with a PC sound card using Linrad, SD-Radio, and
other software.

>Now that we have digital radio, what seems to be
>called HD radio (radio stations seem to advertise it pretty often).
>http://en.wikipedia.org/wiki/HD_Radio


Yep. It's the new name for Ibiquity (IBOC) radio:
<http://www.ibiquity.com>
I've played with commercial products. It's nifty. Good quality
stereo crammed into a 9KHz AM broadcast channel.

In Canada and Europe, they have a different system called DRM (digital
radio mondiale).
<http://www.drm.org>
I have a downconverter (simple mixer from 455KHz to about 12Khz) hung
on my HF ham rig and use a sound card to do the demodulation. The
catch is that I had to pay $50 for a codec license. Grrr....
<http://www.drmrx.org>
<http://www.drmrx.org/receiver_mods.html>
I couldn't get the open source DRM software to work for me:
<http://drm.sourceforge.net>

Anyway, the point is that it's easy and common enough to demodulate AM
with digital techniques.

>I don't remember much discussion here about it.


Where is here? I'm in the Santa Cruz, CA area, and we discuss just
about anything.

--
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# 831-336-2558 jeffl@comix.santa-cruz.ca.us
# http://802.11junk.com jeffl@cruzio.com
# http://www.LearnByDestroying.com AE6KS

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  #18 (permalink)  
Old 05-01-2008, 09:43 AM
John Monro
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Default Re: Disadvantages of using AM for DSSS/FHSS?

kronecker@yahoo.co.uk wrote:
> On Apr 29, 11:55 am, John Monro <johnmo...@optusnet.removethis.com.au>
> wrote:
>> Green Xenon [Radium] wrote:
>>> Hi:
>>> Most cell phones and wireless internet routers, modems, and access
>>> points that use spread spectrum usually broadcast and receive their data
>>> on FM-radio waves. Just out of curiosity, I ask, why not use AM?
>>> Let's say a DSSS/FHSS type of spread-spectrum is transmitted and
>>> received using the AM radio waves in the UHF spectrum [i.e. spread info
>>> for transmission throughout the UHF band and receive AM radio waves
>>> throughout all UHF frequencies]. What would be the disadvantages of this?
>>> Normally DSSS and FHSS are transmitted/received on FM radio waves. So I
>>> ask what would be the disadvantages of using AM instead of FM for this?
>>> AM radio tends to be more vulnerable to unwanted magnetic disruptions
>>> than FM, however this only affects analog reception. Digital reception
>>> on AM should be unaffected even by the strongest-interfering analog RF
>>> magnetic signal. Right?
>>> Thanks,
>>> Radium

>> Hi,
>>
>> FM demodulation produces a handy 'capture' effect. If a weak and a
>> strong signal are present together, then the recovered baseband signal
>> from the weaker FM signal is greatly reduced. This is useful in
>> rejecting interference from adjacent stations on the same frequency.
>>
>> With AM, the demodulated baseband signals are present in amplitudes that
>> are proportional to their RF amplitudes. In addition, the AM carriers
>> will 'beat' together to produce an additional and unwanted
>> tone-modulation of the received RF signal.
>>
>> Regards,
>> John

>
> Only if the power of the co-channel is very muchless otherwise you
> get complete destruction of both to a point when they flip over and
> you hear the other channel.


Neither AM nor FM can cope with co-channel interference when the powers
are roughly equal.

Power saving is the big thing with FM.
> K.


It all depends on what baseband Signal-to-Noise ratio is acceptable.
Very high S/N applications (e.g. Hi-Fi music) require an AM RF-level
that is high relative to the FM RF-level. For low S/N applications
(e.g. voice communication) the positions are reversed.

Regards,
John



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  #19 (permalink)  
Old 05-01-2008, 01:24 PM
Randy Yates
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Default Re: Disadvantages of using AM for DSSS/FHSS?

Jeff Liebermann <jeffl@cruzio.com> writes:
> [...]
> Where is here? I'm in the Santa Cruz, CA area, and we discuss just
> about anything.


What about what you smoke? ...



--A Former Bay Area Resident

--
% Randy Yates % "She has an IQ of 1001, she has a jumpsuit
%% Fuquay-Varina, NC % on, and she's also a telephone."
%%% 919-577-9882 %
%%%% <yates@ieee.org> % 'Yours Truly, 2095', *Time*, ELO
http://www.digitalsignallabs.com

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  #20 (permalink)  
Old 05-01-2008, 02:05 PM
msg
Guest
 
Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

Jeff Liebermann wrote:

<snip>

>>Now that we have digital radio, what seems to be
>>called HD radio (radio stations seem to advertise it pretty often).
>>http://en.wikipedia.org/wiki/HD_Radio

>
>
> Yep. It's the new name for Ibiquity (IBOC) radio:
> <http://www.ibiquity.com>
> I've played with commercial products. It's nifty. Good quality
> stereo crammed into a 9KHz AM broadcast channel.


<snip>

Think twice about promoting IBOC; anyone who values the AM BCB spectrum
and the ability to hear low power, adjacent channel and DX AM radio
should loudly object to this fiasco.

Michael

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  #21 (permalink)  
Old 05-01-2008, 05:05 PM
Jeff Liebermann
Guest
 
Posts: n/a
Default Re: Disadvantages of using AM for DSSS/FHSS?

On Thu, 01 May 2008 09:05:08 -0500, msg <msg@_cybertheque.org_> wrote:

>Jeff Liebermann wrote:
>
><snip>
>
>>>Now that we have digital radio, what seems to be
>>>called HD radio (radio stations seem to advertise it pretty often).
>>>http://en.wikipedia.org/wiki/HD_Radio

>>
>>
>> Yep. It's the new name for Ibiquity (IBOC) radio:
>> <http://www.ibiquity.com>
>> I've played with commercial products. It's nifty. Good quality
>> stereo crammed into a 9KHz AM broadcast channel.

>
><snip>


>Think twice about promoting IBOC; anyone who values the AM BCB spectrum
>and the ability to hear low power, adjacent channel and DX AM radio
>should loudly object to this fiasco.
>
>Michael


Ah yes. After technology comes the politics. I'll resist jumping in
with both feet.

Incidentaly, the HD in HD Radio is "hybrid digital" not "high
definition" (or "heavy duty). Marketing at work.
<http://en.wikipedia.org/wiki/Hd_radio>

It's an old axiom, that you don't get something new, without losing
some of the old. In this case, you get hi-fi, low noise, better
frequency response, 2 or more audio channels, data, realtime traffic
updates, additional services, and possibly profitability for AM
stations. What you lose is some legacy AM compatibility and increased
adjacent channel garbage along with the usual added complexity. In my
never humble opinion, it's a reasonable trade. While I got my start
in radio listening to DX stations on my parents Grundig radio, I would
personally be willing to lose some of that, in favor of technical
progress and a radical improvement in performance and features.

Note that in hybrid mode, as commonly used by US AM BCB stations, the
digital portion of the xmit power is only about 1% of the total
xmitted station power, where most of the power is also in the adjacent
channels. There isn't going to be much 2nd adjacent channel (also
known as alternate channel) splatter from that low power level. It's
kinda academic because few AM receivers have the IF selectivity
necessary to reject the adjacent channels anyway.

However, the adjacent 9KHz channels will certainly be polluted. From
a recent FCC report:
<http://www.fcc.gov/mb/audio/digital/>
<http://www.fcc.gov/fcc-bin/audio/FCC-07-33A1.pdf>
According to McLarnon, the hybrid IBOC AM system creates two
new "stations" in the first adjacent channels, each with a total
power of -16 dBc. He states that for a 50 kW station, each
would therefore be 1250 watts and current allocation rules provide
protection of +6 dB D/U for first adjacent channels. According
to McLarnon, if a station currently at +6 dBD/U adds IBOC, it
creates a new source of cochannel interference to first adjacent
channels at +22dB D/U. He believes that this is significant since
it is 4 dB more interference power than is permitted by the
Commission’s allocation rules for co-channel stations. McLarnon
further states that the majority of existing allocations were
created when first adjacent protection was only 0 dB D/U, and
this figure still applies to the Canada-US bilateral agreement
on AM broadcasting.

I do have some not very nice things to say about the FCC endorsing a
proprietary technology, from a single vendor (formerly Lucent), and
other administrative oddities. Also, the FCC ignored European Eureka
147 technology:
<http://www.worlddab.org>
and went it's own non-compatible way.

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

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