AM modulation vs FM modulation?

Discussion in 'Wireless & RF Design' started by vead, Aug 11, 2016.

  1. vead

    Thread Starter Active Member

    Nov 24, 2011
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    Hello
    I need help to understand some basic in communication. FM and AM are the two kind of modulation used in broadcasting. Voice, video or data send over the long distance by modulation process. AM modulation is use to send video signal. While the AM modulation is use to send voice signal. I don't understand reason why we don't send voice signal by AM modulation? Why we don't send video signal by FM modulation?
    Thank you for your help
     
  2. Dodgydave

    Distinguished Member

    Jun 22, 2012
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    Are you thinking of analogue TV signals, where the AM is video, and the FM is audio??
     
  3. blocco a spirale

    AAC Fanatic!

    Jun 18, 2008
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    We do.
    We do.
     
  4. vead

    Thread Starter Active Member

    Nov 24, 2011
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    Actually I have read in book that's why I am asking. You mean both types of signals can be send through AM or FM modulation. I think, AM modulation is suitable for video signal and FM modulation is suitable for voice signal. But still don't understand the basic reason
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    You can do anything you want. Some things are practical and legal, other things are not. Ask yourself about the bandwidth of an AM Broadcast Band channel and compare it to the bandwidth of an FM Broadcast Band channel. Compare both of those to the bandwidth of an Analog Television broadcast channel. Notice the substantial difference in those three numbers. They are as wide as they need to be to carry the information. Note that an analog TV channel would take six times the bandwidth of the ENTIRE AM broadcast band.
     
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  6. crutschow

    Expert

    Mar 14, 2008
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    In analog TV they used FM for the audio, to get better resistance to atmospheric noise, and vestigial sideband AM for the video because it takes the minimum RF bandwidth for the desired video bandwidth.

    But AM and FM are just different types of carrier modulation and each can be used for any type of modulating signal (video, audio, digital, etc.).
     
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  7. vead

    Thread Starter Active Member

    Nov 24, 2011
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    Broadcasting range in india
    AM broadcasting range 540Khz - 1700khz
    FM broadcasting range 88Mhz - 108Mhz
    From the frequency band, it can be seen that AM have small bandwidth as compared to FM. Now it's clear for that we can use any type of modulation but still confused. Why the type of modulation depends on bandwidth? I don't understand the relationship between bandwidth and type of modulation?
     
  8. KeepItSimpleStupid

    Well-Known Member

    Mar 4, 2014
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    Look at the channel size.

    FM can have a frequency of UP TO 15 kHz. AM isn't designed for that. it's more like the old analog telephone, restricted for voice transmissions.

    FM also has sub-carriers and Stereo was ADDED after the fact, The transmission is L+R and also L-F based on the, I believe the amplitude modulated pilot signal. Thus you get 2*L and 2*R by adding and subtracting the two signals.

    There is also reduced bandwidth sub-carriers in the FM signal - Look up SCA.
     
  9. BR-549

    Well-Known Member

    Sep 22, 2013
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    What frequencies does India transmit video on?
     
  10. crutschow

    Expert

    Mar 14, 2008
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    FM modulation can be narrow band (carrier frequency deviation comparable to the modulation frequency) or wide band (carrier frequency deviation higher than the modulation frequency).
    The narrow band FM can have a bandwidth not much larger than AM for the same modulation frequencies, but it loses some of the noise rejection properties of wide-band FM.
    Wide-band FM is used for FM broadcasting to give good S/N ratio and high-fidelity bandwidth.
     
  11. vead

    Thread Starter Active Member

    Nov 24, 2011
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    AM broadcasting range 540Khz - 1700khz
     
  12. Morvan

    New Member

    Jun 24, 2014
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    Hi. FM is much more "clear" then AM, then, if you want to receive a signal almost free of atmospheric interference, you will choose to modulate in FM, even you can do in boths ways, hence, be in AM or in FM it is just a form to surround information. Advantage of FM: clearness. Disavantage: signal reach. I can transmit in AM and anyone could, with a small amout of Watts, grab signal in Brazil, Russia, etc, mainly in Short Wave. In FM, earth curvature will impact severely reach. Because of this we need renew signal (or broadcast it via sattelites!) at, each, say, 80Km.
     
  13. BR-549

    Well-Known Member

    Sep 22, 2013
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    "AM broadcasting range 540Khz - 1700khz"

    And how many video signals do you find in that AM broadcast band?
     
  14. vead

    Thread Starter Active Member

    Nov 24, 2011
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    I did Google search but not getting exact answer,
    From quick searching on Google it's found that 10 to 14 channel digital cable. I suspect that it may be not, that you are asking
     
    Last edited: Aug 11, 2016
  15. BR-549

    Well-Known Member

    Sep 22, 2013
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    Modulation is a process of impressing a pattern, over time, on a continuous signal.

    A continuous signal has three properties. Amplitude, frequency and phase. By varying one or more of these properties, we can impress a pattern on the signal. The human voice takes a relatively small pattern to impress on the signal. A video pattern is very, very large.

    When we put a pattern on a signal, it makes the signal get fat, it swells. A small pattern only swells the signal a little, but a large pattern makes the signal very fat.

    A voice pattern is 10 kHz wide (fat). A video pattern is 5 MHz wide. That is super fat.

    That's why tv channels start in the VHF band.
     
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