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How I would like to change the cell phone industry [was AM electromagnetic waves: 20 KHz modulation frequency on an astronomically-low carrier frequency]
"Brenda Ann" hath wroth:
"Jeff Liebermann" wrote in message .. . I'll make it really simple for you. FM is "hi-fi", while AM is noisy "no-fi". Don't you want to be cool strutting down the street with your iPhone watching HDTV with 7.1 sound? It wouldn't do to have it sound like the typical AM broadcast station. For decent quality, you gotta have FM. FM is not inherently any more 'hi-fi' than AM. FM was invented by Edwin Armstrong specifically to eliminate the noise problems of AM broadcasting. What I think you might be referring to is the huge ****ing match between Armstrong and John Carson over whether FM was any better than FM in the 1930's. The consensus is that very narrow band FM isn't that much better than AM (of equal occupied bandwidth), but wide band FM (as used in broadcast FM and TV) is far better than AM for just about everything. http://fecha.org/armstrong.htm Fidelity is a product of bandwidth, not modulation type. Correct. Actually, it's also a function of modulation linearity (distortion and intermod) and encoding method (dynamic range), but I don't wanna slither down that diversion. Pretend I didn't mention it. AM is not even so susceptible to noise as the frequency goes up, since the energy of the noise pulses goes down logarithmically as frequency goes up. If you're thinking of impulse noise, you're mostly correct. However, there are plenty of other sources of AM noise available. For example, the typical VHF aircraft radio requires substantial filtering of the magneto to avoid hash. Same with any onboard motor. If you've ever tried to install a TV (VSB is a form of AM) in a vehicle, you'll also find that ignition and motor noise can be a problem. Also, your statement isn't quite right. I think what you meant to say is that as the frequency increases, the energy produced by an impulse source, in a given bandwidth, goes down. Even that's not accurate as I have a fluorescent lamp calibrated noise source that's quite noisy well into the GHz range. AM is used for aeronautical communications very successfully for several reasons, one of which is the LACK of 'capture effect'. The FAA, FCC, and various manufactories have tried to move aircraft radios away from AM and towards FM several times in the past 30 year or so. They failed mostly due to international WRC reluctance to swap out expensive radios. It took literally forever to get GPS receivers TSO approved and about 15 years for nav/com radios to go from 50KHz to 25KHz channel spacing, and that was just the FAA. Where else can you find an industry, where progress is somewhat retarded by a regulatory agency of the federal government? I listen to a mix of VHF aircraft AM channels and FM ham and public safety channels on my scanner almost constantly. It's easy to recognize the AM stations by their uniformly crappy audio. Most domestic ground to ground airport traffic is now all FM, as is military ground to ground and ground to air. The reason is that it's difficult to find a decent AM aircraft band walkie talkie. So, they use commercial FM radios. The only AM walkie talkies are used by experimental aviation and ultralights, some of which do not have much of an electrical system that can handle the grossly inefficient AM transmitters. Also, nobody really cares about the "capture effect" as the tower usually has multiple receiver sites and can generally deal with simultaneous transmit collisions. However, they do care about the heterodynes produced by simultaneous transmissions, which obliterate both transmissions. With FM, they could use commercial receiver voting systems and largely eliminate the problem. There are still some frequencies where AM will be more susceptible to interference than FM, but FM would still suffer, for instance the segment between 1330-1400 MHz which is the natural frequency of Hydrogen (lots of that around). If my AM or FM receiver is sensitive enough to hear something in the "water hole", it would be attached to a very big dish antenna. -- Jeff Liebermann 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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