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Author Topic: Understanding FM/AM modulation in terms of a sound wave example  (Read 1001 times)
GRANDKODIAK
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Posts: 85




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« on: February 08, 2012, 02:15:14 PM »

I understand that an AM signal on say 770khz has a a set wavelength and varies in amplitude. As a musical note, it would be a constant note that varies in how loud it is...so a radio set to recieve at 770khz could pick up the changes in its amplitude to communicate data.

Now...

What I don't understand (if the above comparison is correct) how FM works. So say 95.5mhz radio is broadcasting, the amplitude stays the same, but the frequency shifts a certain amount back and fourth, so the NOTE would change but the volume would remain the same. How though, if you have a radio tuned to the particular frequency of 95.5mhz follow it changing above and below what its set to recieve? In other words, if my ear can only hear a D note, but the note changes from C to E, how could I hear (recieve) anything but the moment when it matched the D note durings its occilations?
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AK7V
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« Reply #1 on: February 08, 2012, 02:28:29 PM »

Your radio hears a "range" - so even if it says 95.5 MHz, it's actually hearing that frequency plus or minus some number of kHz.  So it can detect those frequency excursions.  And those excursions are in the audio frequency range, so the receiver only has to "listen" to the center (carrier) frequency plus or minus the audio frequency range it's interested in.  There are equations that combine carrier frequency and "information" frequencies (audio in most cases) and produce the output waveforms -- I don't know the equation, so I don't know if it's exactly the case that the bandwidth around the carrier is directly equal to the audio range interested in.

Very simplified explanation from a simple fellow. Wink

For AM, what gets converted into audio is the frequency of those amplitude modulations.  Here's a link I found that illustrates is clearly: http://www.angelfire.com/electronic/funwithtubes/How_AM_Works.html

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GRANDKODIAK
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« Reply #2 on: February 08, 2012, 02:37:57 PM »

thanks for the link, im a visual learner and that picture nailed it for me for how i understood AM Smiley

ps. i should have checked earlier but i completly forgot about wikipedia, there are some animated gif's and some diagrams that made it all clear, thanks again!
« Last Edit: February 08, 2012, 02:45:04 PM by GRANDKODIAK » Logged

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WB2WIK
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« Reply #3 on: February 08, 2012, 02:50:26 PM »

thanks for the link, im a visual learner and that picture nailed it for me for how i understood AM Smiley

ps. i should have checked earlier but i completly forgot about wikipedia, there are some animated gif's and some diagrams that made it all clear, thanks again!

Also this is pretty good: http://www.cybercollege.com/frtv/frtv017.htm

Note that AM and FM are exactly the same thing without modulation.  In fact, even with modulation, on a spectrum analyzer they can look the same: A carrier, with sidebands occurring based on the modulation frequency.  Real FM broadcast stations usually looks more complex because they contains special signals that provide for stereo modulation, and often subcarriers containing other information. 

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