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Author Topic: What is the difference between AM and FM radio? Why can FM be in stereo?  (Read 8746 times)

Offline chris

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Can anyone offer a simple explanation to this question, which I received recently?

Chris


 

Offline ukmicky

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I Would liked to have anwsered this one myself but it would have got confusing for you and me :)

But its to do with how the signal from the radio station piggy backs its ride,one way is more efficent than the other

AMPLITUDE MODULATION  radio works by changing the amplitude of the carrier wave and i think only one signal can be pulled off it at a time and sufferes from interference as the quality of the sound is dependent on the strength of the signal

FREQUENCY MODULATION radio works by changing the frequency of the carrier wave but more than one signal can be pulled of it a time allowing for stereo and suffers less from interference as the quality of the sound isnt dependent on the strenght of the signal

Something like that, if i was you i would wait for  Soul surfer to anwser your question :)


Try the link below
http://www.cybercollege.com/frtv/frtv017.htm
« Last Edit: 16/12/2006 13:38:53 by ukmicky »
 

lyner

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The AM signal is simpler  to understand. If you look at the waveform on an oscilloscope you see a  strip across the  centre of the screen which is the radio frequency carrier wave  and that varies in width (amplitude) to match the original audio (or, indeed, video) signal.  This is Amplitude Modulation. Simple AM occupies twice the bandwidth at RF (spectrum space) that the original (baseband) audio signal took up.  You can fit a lot of audio channels into the RF broadcast bands. AM is susceptible to noise and interference in  simple proportion to the level of the interference; just listen at night to the medium wave bands.
In FM, a carrier has its frequency changed  (deviated) by the baseband signal. An ocsilloscope trace will just show a constant width (amplitude) strip of RF carrier. The amount of this deviation can be as much as you like; the overall bandwidth can be many times the bandwidth of the original audio signal. By spreading this signal over a wider bandwidth, you make it much less susceptible to noise and interference (the so-called  fm advantage); hence, fm broadcasts sound cleaner. This is not for free - you can't get as many channels in your spectrum space.

To broadcast stereo on AM, you would, basically need two separate channels - using twice the bandwidth.Both channels would be equally noisy, too.
To fit two stereo audio channels on FM, you use a clever 'coding' method which transmits the mono information as a single sound channel and the stereo information (which is the 'difference' between the left and right channels). This difference  signal is at a higher (inaudible) frequency and also frequency modulates the RF carrier. BUT you don't get anything for nothing.  The stereo signal is much more susceptible to interference etc and you lose a lot of the  FM advantage you got originally. Intelligent receivers recognise this and you can  often see the stereo light go out as you drive into an area of poor reception. Then the receiver ignores the noisy FM information and gives you the cleaner but mono signal.
Broadcast TV signals use AM, usually, but Satellite TV (analogue) uses FM with quite a low deviation because of limited spectrum space. One of the main reasons is that FM transmitters are, in fact, cheaper to build, kilowatt - for - kilowatt and much more suitable for satellites.
 

Offline daveshorts

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Sound is a variation in air pressure at a frequency that our ears are sensitive to. A radio link encodes this pressure and then reconstitutes it at the other end with a loudspeaker. The way it is encoded is different between AM and FM.
 In Amplitude modulation this pressure is represented as the amplitute - height of a radio wave.
 In Frequency Modulation the pressure is represented as a slight change in frequency from the original
 

Offline eric l

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I used to have a friend who was a radio-amateur, you know, one of those guys who communicate by radio to other amateurs all over the world.  I remember they used SSB (singe side band modulation).  He did explain me how that worked, but I was never really sure I understood.  Anyone on this forum who knows more, or can compare with AM and FM ? I lost contact with the guy, and may be SSB is not even a hot item among radio-amateurs anymore, but still...
 

Offline syhprum

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Single sideband, when a transmitter is amplitude modulated with say a single tone say 1KHz what is in fact transmitted is the basic carrier say 3MHz and two sidebands of 3MHz plus and minus 1 KHz.
as far as transmitting information goes only one of these sidebands is required and the carrier can be generated by the receiver to enable the information to be recovered.
At the transmitter either crystal filters or subtle phasing systems are used at an early stage where the power level is low to remove the carrier and either the USB or the LSB and then the remaining sideband is amplified to the required transmittiter power.
The receiver can then employ a bandwidth less than half of what would be required for AM reception with consequent improvement in sensitivity
 

Offline chris

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Thank you, everyone, for the quick response to this.

As a follow up to lyner's reply, if FM represents a deviation in the frequency of the wave, how do you tune into it?

Chris
 

Offline daveshorts

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the pressure is represented as a difference from the frequency you have tuned to. So if the signal is being transmitted at 100MHz then a high pressure will be at 100.01MHz and a low one at 99.99MHz. Another channel may be centred on 200MHz.
 

Offline Heliotrope

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Thank you, everyone, for the quick response to this.

As a follow up to lyner's reply, if FM represents a deviation in the frequency of the wave, how do you tune into it?

Chris

The receiver isn't tuned to a spot frequency.
It has a bandwidth. That bandwidth is carefully chosen by the design team to be narrow enough to only allow one station into the receiver at a time and be wide enough so that the maximum frequency deviation in the carrier caused by modulating it, from from 20Hz to 20kHz, is allowed in.
All these parameters are decided in advance by the people who control the frequency spectrum. They decide how good the quality of an FM broadcast is allowed to be. If they decided that each channel could be much wider then we could have 7:1 surround sound instead of FM stereo coming out of our brand new B&O systems.

This is why digital radio is going to be the only way forward. You can encode many, many more times the information digitally than you can using a standard FM signal for the same bandwidth. Obviously they try and penny pinch and stuff as many channels as they can using as high a compression as they can get away with into the space available across the bandwidth of spectrum they're allowed.

Anyway, the frequency response and thus bandwidth of the receiver is designed to be flat or almost flat across it's 3dB points. So that you don't get parts of the signal with a greater deviation being mixed out with a lower final volume to the ear.

Inside the receiver the varying carrier is amplified from the very low level coming down the antenna. This signal is then stuffed into a mixer circuit which removes the carrier and leaves you with just the difference signal ie. the variation or the deviation.
Now this is then amplified again a bit and then passed through a demodulator. This is another circuit which gives a voltage out that depends on the frequency of a wave going into it.
So out of that you get a sine wave varying in voltage ie. amplitude, at the same time/rate as the original signal was varying in frequency.
This voltage out is then amplified again and stuffed into a speaker.
And bingo, you have sound coming out of your radio.

I've skipped over a load of things here by the way but going all the way back to basic FM is taxing my brain this early on a Sunday morning  ;D
 

Offline syhprum

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A minor correction To post 7, although terrestial TV uses AM for the video component since the old 405 line standard was abandoned the audio signal is FM.
Re post 2050, when a carrier signal is frequency modulated although a more complex sideband structure is generated a central carrier frequency persists, this is what the Automatic frequency control of the receiver can lock onto although modern receivers dispence with this AFC and the channel is selected with the aid of a crystal controlled frequency synthesiser     
 

Offline Heliotrope

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What are "post 7" and "post 2050" ?
All I can see are the message IDs.
 

Offline syhprum

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Broadcast TV signals use AM, usually, but Satellite TV (analogue) uses FM with quite a low deviation because of limited spectrum space.
Thank you, everyone, for the quick response to this.

As a follow up to lyner's reply, if FM represents a deviation in the frequency of the wave, how do you tune into it?

Chris
These are the two to whom I was replying
 

Offline Heliotrope

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I gathered that.
I was wondering where the numbers were coming from.
Ah ! Hang on....
They're the nunber of posts that person has made.
I thought they were a unique identifier of a post but there's already the message ID thing so I wondered what was happening.
No worries.

 

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