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Why isn't FM video used?

Discussion in 'Electronic Basics' started by Radium, Sep 12, 2006.

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  1. Radium

    Radium Guest


    Video signals for NTSC, PAL, and SECAM television are transmitted on AM
    carriers. Why?

    AM is sensitive to static. Electrical disturbances -- such the the EM
    waves emitted florescent bulbs -- will cause distortions on any AM
    receiver. The video reception will be altered due to static.

    Why not use FM carriers to broadcast video signals? It would be


  2. Lord Garth

    Lord Garth Guest

    Better look into how the L-R signal is modulated at any FM station in the
  3. Phil Allison

    Phil Allison Guest

    "Lord Garth"

    ** Why ??

    It still frequency modulates the carrier, but above the audio band.

    ......... Phil
  4. John  Larkin

    John Larkin Guest

    It would use too much bandwidth/spectrum.

  5. BobG

    BobG Guest

    What comes down from the birds? QPSK? QAM? DPSK? Thats the same stuff
    they send down the cable right? MPEG compressed digital video and
    audio, 4 or 5 megabits a sec?
  6. In the old days, it was FM.

    All those "build your own satellite receivers" of 20 years ago had
    wideband discriminators of some kind, it was kind of amusing that
    they could have an IF of 70MHz and still have suitable selectivity.

    Some people playing with amateur radio tv have played with FM for
    the video. Though, nobody even considered it until about 15 or 20 years
    ago, after monitors were common so you'd just build up a receiver from
    scratch. When regular TVs were the only game in town, they never
    looked beyond AM for the video.

  7. Bob Eld

    Bob Eld Guest

    Simple answer: FM is wasteful of bandwidth with other things equal. The
    video signal needs about 3-1/2Mhz of bandwidh. It would be difficult to get
    this with FM unless the channel spacing was maybe 10 to 20MHz depending on
    modulation index.

    Also note that digital modulation is also AM where the phase also carries
    information. There are many modulation schemes such as QAM, etc. but they
    all use AM as the backbone. The idea is to cram as many bits into a Hertz of
    bandwidth as possible. This is done by utilizing both the amplitude and
    phase in the modulation scheme.
  8. Radium

    Radium Guest

    What if the WMV [Windows Media Video] digital compression is used and
    the color resolution is decrease sufficiently?
    Is it possible for digital modulation to use FM as a backbone? Has this
    ever been done?
  9. John Fields

    John Fields Guest

    Because we handle visual noise much better than we handle audio

    True, but the nature of the radiated interference from fluorescent
    lamps will be limited by the species of the harmonics generated and
    the selectivity of the receiver.

    120Hz and its harmonics are not likely to cause me any grief when
    I'm watching TV, and I often run about a kilowatt's worth of
    fluorescent lamps when I'm watching TV with no ill effect.

  10. Dana

    Dana Guest

    You can use one form of Modulation (digital) to modulate another form of
    modulation (fm)

    I think you have not digested what the person said about QAM above.

    Depends on what you want to do will determine what kind of backbone stystem
    you would use.
  11. Radium

    Radium Guest

    By "digital" you mean PCM?
    QAM uses two AM carriers that are 90 degrees out-of-phase with each
    Does PCM have to have an AM component? Can't PCM have an FM component
  12. Dana

    Dana Guest

    I used digital as you used digital above.
    It went over your head. You asked if you can use digital modulation on a
    frequemcy modulation system. QAM can be considered as a form of digital
    modulation. So can FSK as someone else has pointed out. FSK answers your
    question about FM being used."is qam a digital modulation"As for video and FM, FM is often used in fiber optics for video signals, and
    there are products that I know of for security cameras that transmit video
    via FM links.
    I suggest you try GOOGLE.
    PCM is a digital representation of an ANALOG signal.
    You are confusing your terms here.
  13. Eeyore

    Eeyore Guest

    It doesn't need to be.

  14. John Fields

    John Fields Guest

  15. Ron Capik

    Ron Capik Guest

    So, in your words, what are the various "components" of AM, FM, and PCM ?

    Now, what is the degree of orthogonality of those components?

  16. Bob Eld

    Bob Eld Guest

    Most digital modulation schemes use phase as a component of the modulation
    as well as amplitude. Phase Modulation and Frequency Modulation, FM are
    related. One is the differential of the other. Think of it this way: If you
    change the phase of a wave, you instantaneously compress or expand the wave
    which is a short term change in frequency. So, in a sense, Frequency
    Modulation is a part of many digital schemes. Howerver, except in
    rudimentary forms like FSK (frequency shift keying), true FM is not used.
    Again it comes down to cramming as many bits of information into each Hertz
    of bandwidth as possible and Phase Modulation has a defintie edge over FM.
    There are complicated mathematical reasons for this.

    To answer your question, it would be possible to combine AM and FM to form a
    digital constellation of bit values but I don't know of any commercial
    scheme that does this. I suspect that communication engineers have tried
    everything and picked what works best.
  17. Radium

    Radium Guest

    Beleive it or not, the French use AM for the TV's audio carrier, while
    using FM for the video carrier. Broadcast System L
  18. Radium

    Radium Guest

  19. Bob Myers

    Bob Myers Guest

    Not so; FM is used only for the chroma components, not the
    luminance (Y) signal (which is what lives on the "video" carrier),
    and only one chroma signal is sent per line (that's the whole
    SECAM thing). FM's still a bandwidth hog, and can't be used
    for the full-bandwidth Y signal in any analog TV broadcast

    Bob M.
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