FM vs AM Broadcast: Advantages & Disadvantages

[chocofingers]

Hi,
Could somebody tell me what's the difference b/w FM ( frequency modulation) and AM (amplitude modulation) broadcasts? Also please mention the advantages and disadvantages of both ...

I'll be thankful :)

 

[fss]

In AM the amplitude is modulated. In FM the frequency is modulated. You should be able to use Google and/or Wikipedia to get further information.

 

[chocofingers]

@fss thnxz
well I used google but everytime I forget the info since it is not well-explained. I needed sumone to list out the major differences :'(

 

[jtbell]

What makes you think we're going to be able to do any better with a general explanation?

Asking specific questions to clarify something that you've read, is probably likely to give you better results.

Also, people are probably more likely to try to answer a specific question than to try to come up with a general explanation when they don't have any idea where your problem is.

 

[yungman]

Like Chocofingers said, you can read the detail on web. But in current radio broadcast, the difference is more the regulation than the nature of the AM and FM. For example, the AM band limit the upper limit of frequency of the channel to something like 5KHz, you can get hi-fi out of this. FM is more like 15KHz, don't quote me on the exact number. AM don't not allow stereo and that make it even worst! That is the reason you don't really listen to music on AM.

From existing bands, AM is in much lower frequency, maybe that is the reason they put all the limitation to limit the side band to avoid breading over to the neighbor channel. The upper frequency response imply the side band spread from center frequency ( eg, 810KHz for a station ) to +/-the frequency response. So the higher the frequency response, the wider the frequency the channel is and you can put less channels in the given band. Stereo funther increase the band width.

You should read into AM and FM modulation and look at the side band characteristics to get more insight too, I have not read it for a long time, I am sure I missed a lot of other stuffs.

 

[Danger]

The main difference that I've noticed in real-life is that AM bounces off of the ionosphere and thus has better range. FM is pretty much a line-of-sight thing.
It is also far easier to have stereo broadcasts on FM. That's not impossible for AM, but a lot harder to implement.

 

[K^2]

^ That has more to do with wavelengths used, rather than AM/FM difference, if I recall correctly.

 

[Danger]

^ That has more to do with wavelengths used, rather than AM/FM difference, if I recall correctly.

That is very possible, but the US FCC determined what bands were available for what purposes back in the early days of the 20 century, in which they had jurisdiction over only their own citizens. We Canuks went along with it because there was no reason not to, and to this day usually follow FCC guidelines.

 

[yungman]

With low modulation frequency like less than 1MHz and the whole AM in about 1MHz range, you are very limited on the bandwidth of each station. Where is FM, you have from something like 85MHz to 110MHz or something like that, that is over 20 times the AM band. ( please don't challenge me on the exact numbers, I am just pulling out from my behind!). But you get the drift.

AM go farther where FM is more line of sight only because of the frequency range of the band, if you put AM in 100MHz range, it will behave the same. Go kick those that set the AM and FM band's behind why they do it this way instead of the other way around!

 

[fss]

Where is FM, you have from something like 85MHz to 110MHz or something like that, that is over 20 times the AM band. ( please don't challenge me on the exact numbers, I am just pulling out from my behind!).

You can have FM (and AM, for that matter) pretty much wherever you want in the spectrum. Whether it comes in clearly at the receiver is a separate story. The FM band in the US is 88-108 MHz, with a maximum bandwidth of 400 kHz per station (although in practice this can vary up to 415 kHz or so).

well I used google but everytime I forget the info since it is not well-explained. I needed sumone to list out the major differences :'(

What was not well-explained? I already answered your question about the major difference- in AM the amplitude changes, in FM the frequency changes. Imagine a sine wave. Scrunch it together and you have FM. Stretch/compress it vertically and you have AM.

 

[Danger]

if you put AM in 100MHz range, it will behave the sam

Very true. It is a matter of frequency. That brings us back to the FCC. They have no jurisdiction outside of their own borders, but we choose to follow their rules in order to maintain harmony and safety.

 

[wheelatharm]

Tha main advantage of FM is that you don't get static from the white radio noise generated by every item using AC power. You can get a clear signal even when driving under high voltage lines, unlike for an AM signal.

Marc

 

[yungman]

Tha main advantage of FM is that you don't get static from the white radio noise generated by every item using AC power. You can get a clear signal even when driving under high voltage lines, unlike for an AM signal.

Marc

THis is still because of the band current AM resides in, not the nature of the AM. If you switch the AM to current FM band, it will not affected by the power line, but it will be more directional like the current FM.

 

[wheelatharm]

O.K. I did not know this about the overlap between AM and radio noise from AC power using devices.

I was taught though, that in general AM is more suseptable to transmitting noise as part of the signal, where FM is less because the signal is a change in frequency around a carrier frequency.
There is always some level of noise for an AM signal, and the noise will always get added to the signal for AM, whereas there is not usually "noise" foran FM signal just because few natural objects vary in frequency in a way that would add to the FM signal.

M

 

[yungman]

O.K. I did not know this about the overlap between AM and radio noise from AC power using devices.

I was taught though, that in general AM is more suseptable to transmitting noise as part of the signal, where FM is less because the signal is a change in frequency around a carrier frequency.
There is always some level of noise for an AM signal, and the noise will always get added to the signal for AM, whereas there is not usually "noise" foran FM signal just because few natural objects vary in frequency in a way that would add to the FM signal.

M

I could be wrong, being amplitude modulation, anything that cause change in amplitude will add noise if you look at it this way. I am not a radio engineer, I just studied long time ago.

 

[Danger]

Tha main advantage of FM is that you don't get static from the white radio noise generated by every item using AC power.

Oh, jeeze, but I remember my teen years when we knew that a Volkswagon was slinking about within half a mile of us because of the static on the radio...
For any kids reading this, I will point out that their ignition systems were not shielded.

 

[chocofingers]

You can have FM (and AM, for that matter) pretty much wherever you want in the spectrum. Whether it comes in clearly at the receiver is a separate story. The FM band in the US is 88-108 MHz, with a maximum bandwidth of 400 kHz per station (although in practice this can vary up to 415 kHz or so).



What was not well-explained? I already answered your question about the major difference- in AM the amplitude changes, in FM the frequency changes. Imagine a sine wave. Scrunch it together and you have FM. Stretch/compress it vertically and you have AM.

well actually I know the wave forms of FM and AM and also know the major differences in their waveforms only. Btw thankyou so much for clarifying ... :) And ur scrunching compressing rule is awesome :P

 

[chocofingers]

Like Chocofingers said, you can read the detail on web. But in current radio broadcast, the difference is more the regulation than the nature of the AM and FM. For example, the AM band limit the upper limit of frequency of the channel to something like 5KHz, you can get hi-fi out of this. FM is more like 15KHz, don't quote me on the exact number. AM don't not allow stereo and that make it even worst! That is the reason you don't really listen to music on AM.

From existing bands, AM is in much lower frequency, maybe that is the reason they put all the limitation to limit the side band to avoid breading over to the neighbor channel. The upper frequency response imply the side band spread from center frequency ( eg, 810KHz for a station ) to +/-the frequency response. So the higher the frequency response, the wider the frequency the channel is and you can put less channels in the given band. Stereo funther increase the band width.

You should read into AM and FM modulation and look at the side band characteristics to get more insight too, I have not read it for a long time, I am sure I missed a lot of other stuffs.

Now what I get that u said r the two major differences... Number 1: FM has higher frequency so greater bandwidth but line-of-sight communication ... Am I right ? What about the distortion of the signal or the noisy signal obtained in AM ... ?