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#1
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Why don't voice radio communications use FM?
Larry Dighera wrote:
Because it is unlikely the FCC will agree to allocate additional frequency spectrum for the proposed new communications system. The frequency allocation would need to be changed or added to on an international scope, so I believe the operative organizations would be the ITU and the ICAO or IATA - the FCC would simply enforce the change within the U.S. Like you, I would have thought new allocations or changed allocations would be hard, but changes are made every four years and in the GHz range they seem to been more readily done; e.g.: http://www.boeing.com/connexion/news...r_030707j.html |
#2
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Why don't voice radio communications use FM?
Jim Logajan writes:
Analog AM and FM are fundamentally incompatible with each other. Not if they are on different frequencies. Analog AM and digital encoding over spread-spectrum are fundamentally incompatible with each other. Analog AM is used for digital spread-spectrum encoding. AM is the modulation. Digital is the encoding. Spread-spectrum is just a frequency and bandwidth assignment. You asked why AM is being used and not FM and all I'm pointing out is that if you are willing to consider any new system that is incompatible with an older system (like FM replacing AM), you may as well do it with something more advanced and capable, like digital packets over spread spectrum (which could be considered a relative to FM). Switching from AM to FM doesn't involve incompatibilities. You can run both in parallel indefinitely, providing identical services (just as some commercial radio stations have broadcasts on both AM and FM simultaneously). Introducing FM doesn't obsolete any of the AM equipment. Adding all sorts of digital gadgets is quite a different matter. Now you are adding functionality that will be available only to the FM/digital community. This introduces potential safety and usability issues. Stacking transmissions digitally isn't going to work when the same transmissions must be mirrored on analog AM--and they have to be if you want to maintain safety and keep controller workload reasonable. One does _not_ need to implement any of the fancier capabilities that I mentioned. I stated them only as what could be easily done once the capability is in place. A change from AM analog to anything else would be glacially slow, and small steps are safest. I see a direct safety benefit in having the clarity of FM transmission. I don't see a direct safety benefit in having other unnecessary features, and I do see potential risks. Analog cell phones are being replaced with digital cell phones ... Analog cell phones were replaced with digital well over a decade ago throughout the world, except for a couple of countries. There would be no need to replace everything at once and I'm not sure why you think that would need to be the case. The need arises as soon as you add new functionality. -- Transpose mxsmanic and gmail to reach me by e-mail. |
#3
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Why don't voice radio communications use FM?
Mxsmanic wrote:
Switching from AM to FM doesn't involve incompatibilities. You can run both in parallel indefinitely, providing identical services (just as some commercial radio stations have broadcasts on both AM and FM simultaneously). Introducing FM doesn't obsolete any of the AM equipment. Regarding your argument in the paragraph above and the one below... Adding all sorts of digital gadgets is quite a different matter. Now you are adding functionality that will be available only to the FM/digital community. This introduces potential safety and usability issues. Stacking transmissions digitally isn't going to work when the same transmissions must be mirrored on analog AM--and they have to be if you want to maintain safety and keep controller workload reasonable. ....honestly don't make any sense to me. In the first paragraph you see no problem with two transmitters being used to transmit the same thing using different frequencies and different modulation techniques, and in the second paragraph you do. I think you could turn the first paragraph into the second or vice-versa with appropriate special pleading - which is why I'm confused about why you find a switch from AM to FM a better transition than any other transition. I guess I just don't see what you see. |
#4
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Why don't voice radio communications use FM?
On Sat, 02 Sep 2006 20:19:15 +0200, Mxsmanic wrote:
Jim Logajan writes: details snipped Maybe someday the FAA and/or ICAO will consider replacing analog radios with a more capable digital system.... All very interesting, but one of the criteria that any new system would have to satisfy is that it would have to work in parallel with the existing system. Adding features to the new system that are not available in the old system would create dangerous differences between the two. Seeing fancy displays in the ATC or tower for the lucky digital users won't help deal with traffic from old AM users, and it might even confuse things enough to cause problems. A highly advanced solution would require replacing everything at once, which isn't going to happen. A simpler solution that just provides better quality audio could coexist with older systems without a problem. New systems (P25) already do this type of thing. I develop digital radio systems. Police, fire, FBI, CIA, DoD, DoE, various municipal utilities, and various branches of the military are all taking advantage of this technology. In many cases, the old analog systems must co-exist with the newer P25 systems. In some cases, more rural analog systems actually connect with a P25 network via a specialized repeated. Integration is not a problem. Last I read, an FAA study indicated they need lots and lots of money (sorry, don't remember the amount) to upgrade their infrastructure from analog to digital. The sad thing is, it does not appear Congress is going to give it to them. Worse, the same report indicates, over the next 10-years, the FAA will exceed their required conversion dollars by simply maintaining and repair their existing, archaic, analog infrastructure. In other words, the FAA needs to do something...even if they are simply updating their existing analog infrastructure. Regardless, the money does not appear to be available. Advantages of this technology include: o call queuing - meaning, PTT places you in a queue so you can get a word in, even when the controllers are very busy. BTW, this also means no more "walked on" transmissions. o call prioritization - All sorts of cool things can be done here - including, most recent exchange receives priority. Also, should IFR traffic receive higher priority over that of VFR? What about commercial traffic? Priority could be adjusted dynamically too. This means planes in distress could be assigned higher priority. So on and so on... o hang timer detection - a stuck PTT is not going to lock everyone out o caller id - imagine your tail number, altimeter, heading, and aircraft type provided to the controller on every PTT. o MUCHO better frequency utilization o Limited data services The list could go on and on...needless to say, digital has some neat features. The only con of digitial compared to analog is reception. With analog, you can hear a weak signal. It may sound like absoluete crap, but you can still hear it. With digitial, either you have a strong enough signal to hear it...and it sounds awesome...or you hear absoluetely nothing at all. Greg |
#5
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Why don't voice radio communications use FM?
Greg Copeland writes:
New systems (P25) already do this type of thing. I develop digital radio systems. Police, fire, FBI, CIA, DoD, DoE, various municipal utilities, and various branches of the military are all taking advantage of this technology. In many cases, the old analog systems must co-exist with the newer P25 systems. In some cases, more rural analog systems actually connect with a P25 network via a specialized repeated. Integration is not a problem. So why wouldn't it extend to aviation? Last I read, an FAA study indicated they need lots and lots of money (sorry, don't remember the amount) to upgrade their infrastructure from analog to digital. They need not upgrade it all at once. The sad thing is, it does not appear Congress is going to give it to them. Congress, like most of America, is hysterical about imaginary human threats these days, and has probably lost track of the much more mundane but much more serious safety risks associated with infrastructure, aircraft, and crews. Advantages of this technology include: o call queuing - meaning, PTT places you in a queue so you can get a word in, even when the controllers are very busy. BTW, this also means no more "walked on" transmissions. Do other aircraft hear the transmission when you make it, or when the controller hears it? Granted, they are only supposed to listen to the controller, but in practice they will be listening to other aircraft as well. How do you make this work in parallel with analog systems that cannot queue? o call prioritization - All sorts of cool things can be done here - including, most recent exchange receives priority. Also, should IFR traffic receive higher priority over that of VFR? What about commercial traffic? Priority could be adjusted dynamically too. This means planes in distress could be assigned higher priority. So on and so on... It's best not to jump off the deep end with gadgets. Just because something can be done doesn't mean that it should be done. o caller id - imagine your tail number, altimeter, heading, and aircraft type provided to the controller on every PTT. Where does this leave people with analog equipment? o Limited data services What kind of data services do pilots need? Are they going to be surfing the Web? The list could go on and on...needless to say, digital has some neat features. Neat features aren't necessarily desirable features. There is too much of a tendency to bloat digital systems with features that have been hastily designed, inadequately analyzed, and barely tested at all. The only con of digitial compared to analog is reception. With analog, you can hear a weak signal. It may sound like absoluete crap, but you can still hear it. With digitial, either you have a strong enough signal to hear it...and it sounds awesome...or you hear absoluetely nothing at all. If the digital threshold is set where the threshold of intelligibility would be in analog, there's no net loss. -- Transpose mxsmanic and gmail to reach me by e-mail. |
#6
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Why don't voice radio communications use FM?
On Sun, 03 Sep 2006 15:37:38 +0200, Mxsmanic wrote:
Greg Copeland writes: New systems (P25) already do this type of thing. I develop digital radio systems. Police, fire, FBI, CIA, DoD, DoE, various municipal utilities, and various branches of the military are all taking advantage of this technology. In many cases, the old analog systems must co-exist with the newer P25 systems. In some cases, more rural analog systems actually connect with a P25 network via a specialized repeated. Integration is not a problem. So why wouldn't it extend to aviation? Great question. I don't have an answer. I've been planting a seed to have the federal marketing types start sniffing around for FAA/political upstarts...but so far, for my company, it seems to fall on deaf ears. Last I read, an FAA study indicated they need lots and lots of money (sorry, don't remember the amount) to upgrade their infrastructure from analog to digital. They need not upgrade it all at once. I agree with that. I didn't read the whole report and it was a couple of years old. I believe the plan was to upgrade over a number of years...I don't recall the window. [snip] Advantages of this technology include: o call queuing - meaning, PTT places you in a queue so you can get a word in, even when the controllers are very busy. BTW, this also means no more "walked on" transmissions. Do other aircraft hear the transmission when you make it, or when the controller hears it? Granted, they are only supposed to listen to the controller, but in practice they will be listening to other aircraft as well. Sorry. I forgot most people don't know how this stuff works. You are queued when you activate your PTT but you don't actually get your "beep" (think NexTel walkie-talkie sound) back until you're granted your call. Only after you're granted your call do you speak. Otherwise, no one hears you because your radio doesn't xmit. Thusly, no more "stomped on" radio calls. Example: Pilot 1 Pilot 2 PTT PTT "beep" Pilot speaks Hears pilot 1 Release PTT "beep" Pilot Speaks Hears pilot 2 How do you make this work in parallel with analog systems that cannot queue? The repeater initiates the call on your behalf. The repeater is queued rather than the analog radio. Likewise, the reply goes to the repeater, which then re-RXs ("repeats") as analog. For this to work, the analog and digitial systems must have their own frequencies. o call prioritization - All sorts of cool things can be done here - including, most recent exchange receives priority. Also, should IFR traffic receive higher priority over that of VFR? What about commercial traffic? Priority could be adjusted dynamically too. This means planes in distress could be assigned higher priority. So on and so on... It's best not to jump off the deep end with gadgets. Just because something can be done doesn't mean that it should be done. Agreed. I was just tossing stuff out to show the types of things that can be done. A more likely scenario is to give priority to controllers. This allows controllers to pre-empt pilots when the talk group is busy. Which is, more than likely the prefered solution. Also, the concept of "emergency" call is also very useful. For example, it places you at the top of the queue. Combine "emergency" with a GPS source, plus data services, and now your squawking 7700, your GPS position is sent with your PTT, and you now have priority with the controller. o caller id - imagine your tail number, altimeter, heading, and aircraft type provided to the controller on every PTT. Where does this leave people with analog equipment? An anachronism? No worse off than they are today. Until everyone is converted, such features would simply be a perk to controllers; with the potential to increase QoS for those that digitally participate. o Limited data services What kind of data services do pilots need? Are they going to be surfing the Web? Oh, most definiately not web browsing. TAFs, METARS, in route weather, PIREPs, TFRs, ATIS, ASOS, TWEB, NAV IDs, etc... [snip] The only con of digitial compared to analog is reception. With analog, you can hear a weak signal. It may sound like absoluete crap, but you can still hear it. With digitial, either you have a strong enough signal to hear it...and it sounds awesome...or you hear absoluetely nothing at all. If the digital threshold is set where the threshold of intelligibility would be in analog, there's no net loss. Doesn't work that way. Nor, would you want it to. One of the key points of digitial radio is that everything is crystal clear. This means lots and lots of filtering takes place to pull out voice from the background. If it's intelligible, chances are, it will be considered background noise and filtered out. Greg |
#7
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Why don't voice radio communications use FM?
On 2006-09-02, Mxsmanic wrote:
The reason I ask is that improper and misunderstood radio communication is a leading cause of accidents, and so it seems that If you look at the NTSB reports, you'll find that this is not so - in fact, accidents caused by bad radio communications are so rare, they barely register as statistical noise! For each accident caused by poor communication, there are probably thousands of accidents caused by a pilot flying into weather they cannot handle. Aircraft fly on the principles of Bernoulli and Newton, not Marconi, and will fly quite happily with no radio at all, so long as the pilot remembers to look out of the window and not bang into anything. -- Yes, the Reply-To email address is valid. Oolite-Linux: an Elite tribute: http://oolite-linux.berlios.de |
#8
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Why don't voice radio communications use FM?
Dylan,
accidents caused by bad radio communications are so rare, they barely register as statistical noise! Ah, but if it weren't for AM, that noise would be so much less ;-) -- Thomas Borchert (EDDH) |
#9
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Why don't voice radio communications use FM?
FM receivers typically use a PLL (phase lock loop) circuit for
demodulation. In the presence of multiple received signals the largest amplitude one will be locked to and thus demodulated. In AM receivers all signals present at the detector above a certain noise threshhold are detected. This feature, while often noiser, allows your friendly controller to hear two guys calling simultaneously. For voice transmissions, narrow band FM can be used and equivalent bandwidth results. Sorry if this is addressed somewhere in this thread already. I didn't wade through all of it. |
#10
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Why don't voice radio communications use FM?
Mxsmanic wrote:
Perhaps this is a naive question, but: Why don't voice radio communications for aviation use FM radio instead of AM radio? FM offers better quality that AM when signal is relatively strong (or signal to noise ratio is high). As the signal strength decreases, there is a point when the quality is identical in both cases and then the quality of FM deteriorates _rapidly_, while AM is still usable. So, it might be a safety issue - using FM would effectively filter out the weakest stations that could be heard if AM was used. Just a theory ;-) Bartek |
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