DTC wrote:> John E. Hadstate wrote: >> >> "DTC" <me@nothingtoseehere.zzx> wrote in message >> news:DWvRj.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. �����������������������������������������������������������������������
Disadvantages of using AM for DSSS/FHSS?
Started by ●April 28, 2008
Reply by ●April 29, 20082008-04-29
Reply by ●April 29, 20082008-04-29
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
Reply by ●April 29, 20082008-04-29
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
Reply by ●April 30, 20082008-04-30
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, > JohnOnly 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.
Reply by ●April 30, 20082008-04-30
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.
Reply by ●May 1, 20082008-05-01
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
Reply by ●May 1, 20082008-05-01
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_RadioYep. 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
Reply by ●May 1, 20082008-05-01
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
Reply by ●May 1, 20082008-05-01
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
Reply by ●May 1, 20082008-05-01
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
