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#11
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Why don't voice radio communications use FM?
That's just not true. For a given voice signal, I can squeeze the same
amount of fidelity into an FM channel that I can into an AM channel. The current actual transmitted bandwidth of a VHF AM signal is about 4 kHz.. Standard deviation on a VHF FM signal is 3.5 kHz.. Bessel and Armstrong to the rescue once more {;-) BTW, the current European channel spacing is 8.3 kHz.. Now THAT's going to be a challenge for us AMers to meet. Jim "Larry Dighera" wrote in message ... AM frequencies are currently 25 kHz wide. FM would require more bandwidth. Regardless, where would you place these newly allocated frequencies? |
#12
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Why don't voice radio communications use FM?
"Larry Dighera" wrote in message .. . AM frequencies are currently 25 kHz wide. FM would require more bandwidth. Regardless, where would you place these newly allocated frequencies? On Sat, 2 Sep 2006 09:10:19 -0700, "RST Engineering" wrote in : That's just not true. For a given voice signal, I can squeeze the same amount of fidelity into an FM channel that I can into an AM channel. That's the first time I've heard that. The current actual transmitted bandwidth of a VHF AM signal is about 4 kHz.. Does that mean the highest audio frequency transmitted it 2kHz? Standard deviation on a VHF FM signal is 3.5 kHz.. Bessel and Armstrong to the rescue once more {;-) BTW, the current European channel spacing is 8.3 kHz.. Now THAT's going to be a challenge for us AMers to meet. And, I suspect, it would be completely impossible for FM to fit within 8.3 kHz channel spacing with the same fidelity? |
#13
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Why don't voice radio communications use FM?
In article ,
"RST Engineering" wrote: BTW, the current European channel spacing is 8.3 kHz.. Now THAT's going to be a challenge for us AMers to meet. huh? Putting FM into 8.33 kHz spacing? or did you mean something else? -- Bob Noel Looking for a sig the lawyers will hate |
#14
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Why don't voice radio communications use FM?
James Robinson wrote:
Mxsmanic wrote: Perhaps this is a naive question, but: Why don't voice radio communications for aviation use FM radio instead of AM radio? I understand it is because of a characteristic of FM called "capture effect" that blanks out weaker transmissions when two radios transmit at the same time. The listener would have no idea that a second, weaker transmission was being made. No actually, it's just historical. Early av radio used AM, and for that reason we still do. |
#15
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Why don't voice radio communications use FM?
That's just not true. For a given voice signal, I can squeeze the same amount of fidelity into an FM channel that I can into an AM channel. That's the first time I've heard that. The first time I heard it was when VHF FM at 2 meters became popular in the early 1960s. The first time I had it explained using Bessel functions was as a first year graduate student in the late 1960s. The first time I had a chance to design with it was my first FCC type acceptance gauntlet in the mid 1970s. Take a look at a ham 2 meter rig sometime. Channels are 5 kHz. wide. The current actual transmitted bandwidth of a VHF AM signal is about 4 kHz.. Does that mean the highest audio frequency transmitted it 2kHz? No, sorry, I should have been absolutely technically precise. The current actual transmitted bandwidth of a VHF AM signal is plus/minus 4 kHz.. In practice, with symmetric modulation ("good" AM or FM) you generally give the bandwidth as the distance from carrier to one sideband and not sideband to sideband. The highest audio frequency that we try to achieve is about 3 to 3.5 kHz, with rapid rolloff above 2.5 kHz. -- generally 12 to 18 dB/octave cornered on 2.5 kHz.. Yes, there will be some higher order stuff leaking through; the idea is to contain as much of it as you can in the filter before it hits the modulator. Standard deviation on a VHF FM signal is 3.5 kHz.. Bessel and Armstrong to the rescue once more {;-) BTW, the current European channel spacing is 8.3 kHz.. Now THAT's going to be a challenge for us AMers to meet. And, I suspect, it would be completely impossible for FM to fit within 8.3 kHz channel spacing with the same fidelity? Easier for FM than AM, but it is a moot point. FM will PROBABLY never happen on the VHF COM band. Jim |
#16
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Why don't voice radio communications use FM?
"Ron Natalie" wrote in message m... No actually, it's just historical. Early av radio used AM, and for that reason we still do. Didn't all early radio use AM? |
#17
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Why don't voice radio communications use FM?
On Sat, 2 Sep 2006 12:02:26 -0400, "John Gaquin"
wrote: "Mxsmanic" wrote in message ....that improper and misunderstood radio communication is a leading cause of accidents, Cite, please. ... I can barely understand what I hear on the radio. I suspect the reasons for this relate more to the environmental effects and quality of the speakers, etc., than to the nature of AM transmissions. The previous comment re capture effect of FM is valid. i.e. the strongest signal wins. This is desireable for broadcast radio but not aviation. With FM the signal remains much clearer until the point where it suddenly becomes unreadable when itl becomes weak. With AM is that readability gradually reduces as the signal gets weaker. If you open the squelch you can often still read AM when FM would be unreadable. The audio bandwidth for acceptable communication is 3KHz. When modulating an AM transmitter you have two sidebands. One up to -3KHz the other up to +3KHz so transmitted bandwidth is 6KHz. With an FM transmitter the bandwidth will still be 6KHz plus the deviation of the system. In addition the sidebands theoretically extend to infinity but they become rapidly weaker. To get the best signal to noise ratio with FM you need higher deviation. If you try increase the number of FM frequencies you need to reduce the deviation. That in turn would reduce its effectivness. As for the original comments I would suggest there's something wrong if AM is not clear. Could be poor hearing, inadequate headset, turning up the volume causing overload of either headset or receiver audio. Ignition or alternator interference distorting the received signal, poor transmitter, poor microphone, poor microphone technique. Sorry but the problem is NOT AM! |
#18
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Why don't voice radio communications use FM?
"Larry Dighera" wrote in message
... "Larry Dighera" wrote in message .. . AM frequencies are currently 25 kHz wide. FM would require more bandwidth. Regardless, where would you place these newly allocated frequencies? On Sat, 2 Sep 2006 09:10:19 -0700, "RST Engineering" wrote in : That's just not true. For a given voice signal, I can squeeze the same amount of fidelity into an FM channel that I can into an AM channel. That's the first time I've heard that. The current actual transmitted bandwidth of a VHF AM signal is about 4 kHz.. Does that mean the highest audio frequency transmitted it 2kHz? Standard deviation on a VHF FM signal is 3.5 kHz.. Bessel and Armstrong to the rescue once more {;-) BTW, the current European channel spacing is 8.3 kHz.. Now THAT's going to be a challenge for us AMers to meet. I believe they were just implementing that when I left avionics work 20 years ago. The main reason for the relatively wide spacing was poor frequency stability. The real problems with any changeover would/will be the large amount of existing infrastructure in place and the need for radically "better" adjacent channel rejection. And you don't dare to "improve" the adjacent frequency rejection of the receivers that much untill you are really sure that the transmitters in service can meet the new standard ... and so forth ... And, I suspect, it would be completely impossible for FM to fit within 8.3 kHz channel spacing with the same fidelity? Wouldn't be much of a problem, IIRC the hams have been doing it forever. I just don't know of and good reason to choose one modulation scheme over the other, and certainly not to change from one to the other! |
#19
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Why don't voice radio communications use FM?
Steven P. McNicoll writes:
Wouldn't that reduce the available frequencies? For a given audio bandwidth, FM tends to require somewhat more radio bandwidth, as I recall, but the audio bandwidth of aviation radio is already so limited that I don't think this would be an issue. The gain in clarity would outweigh any loss of audio fidelity, assuming that the same channel widths were used. If frequencies were reallocated (instead of allocating new ones), that would be different. That would also obsolete older equipment much more quickly, which might not be acceptable. But there must be some space somewhere that could be added to the frequencies, or perhaps some band so rarely used that it could be reassigned. -- Transpose mxsmanic and gmail to reach me by e-mail. |
#20
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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? I realize there's substantial inertia in the installed base of AM equipment, but surely one could allocate some new frequencies to FM and use them in parallel for some years to ease the transition. If one were to mandate a replacement technology, it would be far far more effective to use the packet-based mechanisms that digital cellular phone technology and 802.11 wireless Ethernet (aka WiFi) rely on. Both these technologies turn over the job of transmission collision resolution to chip logic and take humans out of the loop. And it is possible to put audio over WiFi using Voice over IP (VoIP) technology. Such a system would be incredibly flexible. If one had, say, ten planes in the air and they all started to talk to ATC at once, a packet-based system would make it possible to do any of the following: 1) Clearly deliver only one of the voice signals to the controller and provide a visual display that indicated 9 other planes had attempted to speak also. It could even provide audio or visual feedback to the other 9 pilots that their transmissions were not delivered - or it could automatically sequence the delivery of the transmissions to the controller if the transmissions were not too lengthy. 2) If multiple controllers were available, the audio from several of the planes could be routed to multiple controllers with no impact on audio fidelity as far as the controllers or pilots are concerned. 3) Once you go packetized audio, you can put all sorts of useful stuff in the packets for presentation to the other end - such as aircraft number, the location and velocity vector from the aircraft's GPS or altimeter/DG/airspeed indicator, and so on. A pilot could key the mike and make a request without needing to ID themselves or their position - that information would be extracted from the audio packet's header and automatically presented on either a simple display to the controller or mapped to a fancy map display. The technical issues have been pretty much solved and commoditized in both the WiFi VoIP and digital cellular realms. It is my humble opinion that the radio technology currently being used for aviation communications is now less reliable and useful than even that used in home WiFi networks. Maybe someday the FAA and/or ICAO will consider replacing analog radios with a more capable digital system.... |
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