about AM and FM - but FM is limited to a small distance why?

[jravi]

about AM and FM

i know AM is in Khz and FM is in Mhz . but FM is limited to a small distance transmission compared AM. why?

 

[Bhanumurthy]

Re: about AM and FM

Because of Power and Bandwidth tradeoff. The BW required for FM would be very large - hence low power requirement - hence short distance transmission.The BW req.to transmit AM wuld be less compared to FM - hence high power requirement - hence long distance transmission

 

[Audioguru]

about AM and FM

Bandwidth has nothing to do with it because FM can be wide-band or narrow-band.

Frequency has nothing to do with AM or FM. The very old AM broadcast band is at low frequencies that reflect off the ionosphere at night so can travel far. High frequencies do not bounce, they are line of sight so the distance is limited.
You can have very high frequency AM and have low frequency FM.

 

[Bhanumurthy]

Re: about AM and FM

I partially accept u'r answer.However, can u please elaborate on the BW-Power tradeoff and its significance in FM especially.

 

[electronics_kumar]

Re: about AM and FM

also during inaugural days of modulation, AM was first invented then only others . so, available frequency was first alloted to AM then remaining was alloted to PM or FM

 

[u04f061]

Re: about AM and FM

would anybody like to elaborate the effects of buildings and transmission lines on AM or FM?

I have seen that AMs are disturbed when we stay near transmission line but FM is not. what is the reason?

 

[desperado1]

about AM and FM

why FM is used to for transmission of audio in Televisions and not AM..................

 

[Audioguru]

Re: about AM and FM

why FM is used to for transmission of audio in Televisions and not AM..................

There already were FM radios in the 1930's when TV was invented and the sound quality was much better than AM. So they decided to use FM for TV audio.

 

[Kral]

Re: about AM and FM

jravi,
The reason for the difference in range is the frequency bands that are allocated to AM and FM. In the US, AM brodcast is allocated the spectrum from 540 KHz to 1700 KHz. At night, signals in this band can be received at distances of hundreds of miles. However, AM signals in some short wave bands, can have world-wide coverage due to refraction of the Heaviside layer in the atmosphere. The US FM spectrum is allocated the frequencies from 88.1 MHz to 107.9 MHZ. The range of signals in this band is slightly greater than line-of-site. The range has nothing to do with the type of modulation. Of course, with a given type of modulation, greater power results in greater range.
Regards,
Kral

 

[desperado1]

about AM and FM

Is there any technical logic behind using FM as a modulation type in televisions ?

 

[Audioguru]

Re: about AM and FM

Is there any technical logic behind using FM as a modulation type in televisions ?

I already answered your question.
But I guess you don't know why FM is much better than AM.
1) Most forms of interference are AM (amplitude changes) such as static and buzzing.
FM ignores AM.
2) A small amount of interference is loud on AM.
FM has a capture ratio so it ignores an interfering signal that is slightly less level even if it has exactly the same carrier frequency.

 

[xulfee]

about AM and FM

bcoz frequency n wavelength r reciprocal,as much the frequency it covers small distance

 

[Bhanumurthy]

Re: about AM and FM

I completely disagree with audioguru for the reason he gave with respect to the use of FM in TV audio.There is an interesting reason why the picture is amplitude modulated and sound is freq.modulated in TV and not vice versa.
Itz because, when we freq.modulate a carrier directly(Direct FM), its phase varies indirectly.Our ears are insensitive to these phase variations and hence we dont find any problem while listening to the audio in TV.
If, the picture is freq.modulated, its phase varies indirectly(as said earlier)and our eyes are very sensitive to these phase variations whereby one sees a picture distorted if it is freq.modulated.
Hence, sound is freq.modulated and picture is amplitude modulated in TV to overcome these problems of indirect phase variations.

 

[Audioguru]

Re: about AM and FM

I completely disagree with audioguru for the reason he gave with respect to the use of FM in TV audio.There is an interesting reason why the picture is amplitude modulated and sound is freq.modulated in TV and not vice versa.
Itz because, when we freq.modulate a carrier directly(Direct FM), its phase varies indirectly.Our ears are insensitive to these phase variations and hence we dont find any problem while listening to the audio in TV.
If, the picture is freq.modulated, its phase varies indirectly(as said earlier)and our eyes are very sensitive to these phase variations whereby one sees a picture distorted if it is freq.modulated.
Hence, sound is freq.modulated and picture is amplitude modulated in TV to overcome these problems of indirect phase variations.

I disagree with you.
FM changes the phase of the carrier, not the phase of the modulation.
The phase of the audio is not changed during FM modulation. If it was then FM stereo would not work. A very small change in phase destroys multiplexed stereo separation.

TV audio has a fairly narrow bandwidth so FM is easy.
But video has a very wide bandwidth. Wide bandwidth FM for video in 1940 would have been impossible so they used AM.

 

[kengloong]

Re: about AM and FM

As I understand, the higher the frequency used, the wavelength will be short... That's why AM got wide coverage if compare with FM.

Am I right?

Please correct me if I'm wrong...

 

[LvW]

Re: about AM and FM

As I understand, the higher the frequency used, the wavelength will be short.

Yes, of course (invers proportional)

.. That's why AM got wide coverage if compare with FM.
Am I right?

No, the correct answer was given already by KRAL on Feb. 10th.