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am signal vs. fm
- Thread starter 225inthegreatlakes
- Start date
Hmmm. AM stands for "Amplitude Modulation". It's the first method that was developed to "piggyback" audio content onto a plain "carrier-wave" radio signal. Before that, you just keyed that carrier wave on and off in Morse Code. AM manipulates the power level (amplitude) of the carrier wave in step with the audio waveform picked up by your mike. This is why you see the wattmeter kick around when you modulate an AM transmitter.
FM stands for "Frequency Modulation". It was invented by Edwin Armstrong when his boss David Sarnoff asked him to "please get rid of the static". An FM signal will, in fact cancel out most static noises, so long as the signal level to your receiver is strong enough. A weak FM signal will have static noises in it, but an AM signal the same strength will have more of that. When your audio signal is fed into a FM transmitter, the carrier's amplitude (power) stays constant. The frequency of the carrier is what gets wiggled in step with your voice audio. The receiver listens only to those changes in frequency. Since static noises are changes in amplitude, they get sliced off inside the receiver before they can be converted back into audio.
For normal voice modulation, AM and FM are almost equally effective. A strong FM signal will sound cleaner than AM, as a rule. One difference appears when the channel is crowded. The interference from two FM signals can be more annoying than when two AM signals arrive in your receiver together.
In practice, you find FM used most often for local-only radio systems. Skip fading will distort a FM signal, same as it does to AM. SSB stands up to that, and stays intelligible better than either AM or FM, when the skip is fading in and out.
73
FM stands for "Frequency Modulation". It was invented by Edwin Armstrong when his boss David Sarnoff asked him to "please get rid of the static". An FM signal will, in fact cancel out most static noises, so long as the signal level to your receiver is strong enough. A weak FM signal will have static noises in it, but an AM signal the same strength will have more of that. When your audio signal is fed into a FM transmitter, the carrier's amplitude (power) stays constant. The frequency of the carrier is what gets wiggled in step with your voice audio. The receiver listens only to those changes in frequency. Since static noises are changes in amplitude, they get sliced off inside the receiver before they can be converted back into audio.
For normal voice modulation, AM and FM are almost equally effective. A strong FM signal will sound cleaner than AM, as a rule. One difference appears when the channel is crowded. The interference from two FM signals can be more annoying than when two AM signals arrive in your receiver together.
In practice, you find FM used most often for local-only radio systems. Skip fading will distort a FM signal, same as it does to AM. SSB stands up to that, and stays intelligible better than either AM or FM, when the skip is fading in and out.
73
Nomad, I'm not sure I agree totally with your history. Yes, Sarnoff is credited with doing the bulk of the real development work, but FM was not invented by him. It was discovered, and briefly experimented with, very early on (even before the triode was invented if my memory is correct), but discarded as being too inefficient and difficult to work with, until Sarnoff "picked it up and dusted it off" (to quote an electronics history author from the 1950's who was writing about Sarnoff's work with FM).
Hmmmph. Not sure what book you're remembering, but it sounds like Edwin Armstrong gets screwed, again.
Sarnoff was the boss, the visionary. The guy who signed Edwin Armstrong's paycheck at RCA Laboratories. Armstrong did the heavy lifting, working out the practical details during the 30's. He had already invented the regenerative detector, the superregenerative detector and the superheterodyne receiver.
Sarnoff did not, as a rule, get his hands dirty. My grandfather worked for Sarnoff, thought the guy was a hero. Thought Armstrong was a weak-willed chump for tossing himself out a window. Just because he'd been in patent court with RCA for ten or twelve years. His widow kept the lawsuits alive, and eventually won, and RCA lost on all counts. Years after the guy was dead, of course. At that moment the credit for FM went to Armstrong, at least in legal terms, as well as the practical reality.
I'm kinda curious about who the revisionist historian is that gave Sarnoff the credit, though.
73
Sarnoff was the boss, the visionary. The guy who signed Edwin Armstrong's paycheck at RCA Laboratories. Armstrong did the heavy lifting, working out the practical details during the 30's. He had already invented the regenerative detector, the superregenerative detector and the superheterodyne receiver.
Sarnoff did not, as a rule, get his hands dirty. My grandfather worked for Sarnoff, thought the guy was a hero. Thought Armstrong was a weak-willed chump for tossing himself out a window. Just because he'd been in patent court with RCA for ten or twelve years. His widow kept the lawsuits alive, and eventually won, and RCA lost on all counts. Years after the guy was dead, of course. At that moment the credit for FM went to Armstrong, at least in legal terms, as well as the practical reality.
I'm kinda curious about who the revisionist historian is that gave Sarnoff the credit, though.
73
OK, I dug around and found the book to which I was referring. Its called "Electronics For Everyone", written by Monroe Upton, copyright 1954, 1957, 1959 by the Devin-Adair Company. I was wrong about Sarnoff, it was Major E. H. Armstrong that I was thinking of. To quote the author, "Major 'Pygmalion' Armstrong fished her (FM) out of the gutter, gave her some pretty clothes, and taught her the art of make-up....", and he goes on like that for a couple more lines.
According to the same author, I was right about FM being toyed with even before DeForest's audion was invented.
According to the same author, I was right about FM being toyed with even before DeForest's audion was invented.
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