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OfflineMadtowntripper
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Explain AM vs FM to me.
    #5861934 - 07/15/06 01:50 PM (17 years, 6 months ago)

Why can I only get FM stations when I'm inside the labs at school? I like talk radio when I'm stuck in here all day, and there are very few FM talk radio stations here. Its a big building, and I understand that has something to do with it. But I dont understand what it is about AM vs. FM that makes it possible to operate inside here. I assume its a wavelength difference? Which goes through the building easier? Long or short? I'm assuming short, but thats my third or fourth assumption, so I'm going to stop.

Explain plz.


--------------------
After one comes, through contact with it's administrators, no longer to cherish greatly the law as a remedy in abuses, then the bottle becomes a sovereign means of direct action.  If you cannot throw it at least you can always drink out of it.  - Ernest Hemingway

If it is life that you feel you are missing I can tell you where to find it.  In the law courts, in business, in government.  There is nothing occurring in the streets. Nothing but a dumbshow composed of the helpless and the impotent.    -Cormac MacCarthy

He who learns must suffer. And even in our sleep pain that cannot forget falls drop by drop upon the heart, and in our own despair, against our will, comes wisdom to us by the awful grace of God.  - Aeschylus


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InvisibletrendalM
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Re: Explain AM vs FM to me. [Re: Madtowntripper]
    #5861958 - 07/15/06 01:59 PM (17 years, 6 months ago)

No it's not the wavelength, or not just the wavelength, that separates the two.

Think of a perfect sine-wave. The amplitude is the distance between the peaks and lows of the wave. The frequency is the number of times per second the wave completes one cycle (from 0 to peak, peak back to 0, 0 to low, low back to 0). The wavelength is the distance between one wave peak and the next.

Ok, so on to AM vs FM.

AM stands for "Amplitude Modulation".

FM stands for "Frequency Modulation".

If you take that perfect sine wave and broadcast it, you have what we call a "carrier wave". The frequency of the carrier wave is the frequency that you dial in on your radio.

Now if you modify that carrier wave (modifying it is called "modulation") by adding a sound-wave on to it...then you have an actual radio transmission.

The difference between AM and FM is what part of the wave gets modulated.

In AM transmissions, the wave is modulated by changing the amplitude of the wave - in essence making the wave "louder" and "quieter".

In FM transmissions, the wave is modulated by changing the frequency of the wave - in essence you are "compressing" and "expanding" the wave in step with your sound wave.


Now AM is a very noisy way to broadcast information. Any other wave of the same frequency that intersects with the AM radio wave will change the amplitude of the wave. When you change the amplitude, you change the "sound" that the wave will create inside your radio. That's why you can hear lightning and other forms of EM interference really easily with an AM radio. It's also why thick walls and hills are good at blocking AM waves - the matter "drains" some of the wave, which makes it "quieter" and lets more of the interference be heard instead of the radio broadcast itself.

FM doesn't suffer from these problems (or, at least, not nearly as badly). Because the wave is not a single frequency, no single frequency of interference can have much of an effect on the wave. It also penetrates walls much easier because of this.


--------------------
Once, men turned their thinking over to machines in the hope that this would set them free.
But that only permitted other men with machines to enslave them.


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Offlinedaba
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Re: Explain AM vs FM to me. [Re: trendal]
    #5862382 - 07/15/06 05:19 PM (17 years, 6 months ago)

www.wikipedia.com is the first place I look!


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InvisibleLe_Canard
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Registered: 05/16/03
Posts: 94,392
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Re: Explain AM vs FM to me. [Re: trendal]
    #5862397 - 07/15/06 05:25 PM (17 years, 6 months ago)

That's a great explanation!  :thumbup:


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OfflineMadtowntripper
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Re: Explain AM vs FM to me. [Re: Le_Canard]
    #5866233 - 07/16/06 05:15 PM (17 years, 6 months ago)

It is an awesome explanation.

Thanks Trendal!


--------------------
After one comes, through contact with it's administrators, no longer to cherish greatly the law as a remedy in abuses, then the bottle becomes a sovereign means of direct action.  If you cannot throw it at least you can always drink out of it.  - Ernest Hemingway

If it is life that you feel you are missing I can tell you where to find it.  In the law courts, in business, in government.  There is nothing occurring in the streets. Nothing but a dumbshow composed of the helpless and the impotent.    -Cormac MacCarthy

He who learns must suffer. And even in our sleep pain that cannot forget falls drop by drop upon the heart, and in our own despair, against our will, comes wisdom to us by the awful grace of God.  - Aeschylus


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Offlinesupra
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Registered: 10/26/03
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Re: Explain AM vs FM to me. [Re: trendal]
    #5866654 - 07/16/06 06:17 PM (17 years, 6 months ago)

so why do am stations get better reception? Or for better play of words, more distance. There isnt a single station on FM in my town I can get to the next town, but i vcan listen to the same AM station all over the state ( at least where i go).

Unless they broadcast it from multiple different places?

peace


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InvisiblePinhead
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Re: Explain AM vs FM to me. [Re: supra]
    #5866971 - 07/16/06 07:29 PM (17 years, 6 months ago)

AM signals can "skip" off the ionosphere, whereas FM signals travel along the earth's surface in a pretty much straight line.


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InvisibletrendalM
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Registered: 04/17/01
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Re: Explain AM vs FM to me. [Re: supra]
    #5867071 - 07/16/06 07:59 PM (17 years, 6 months ago)

Quote:

supra said:
so why do am stations get better reception? Or for better play of words, more distance. There isnt a single station on FM in my town I can get to the next town, but i vcan listen to the same AM station all over the state ( at least where i go).

Unless they broadcast it from multiple different places?

peace




AM radio waves are reflected by the ionosphere (the upper atmosphere) back towards the Earth, and are also reflected (though not quite as well) from the surface of the Earth.

So AM radio waves can "bounce" around inside the Earth's atmosphere for quite a long distance. In fact it's not too hard to pick up AM transmissions from almost the other side of the planet, if conditions are right (a cloudless night).

FM goes straight through the atmosphere. It isn't reflected at all, really, so FM stations only work well in "line of sight" conditions - if the FM radio tower is under the horizon, the radio waves can't bounce around to get to you (though they can "hug" the ground to some extent...but this doesn't increase the range by too much).


--------------------
Once, men turned their thinking over to machines in the hope that this would set them free.
But that only permitted other men with machines to enslave them.


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