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I want to see SHF FM video signals.

Discussion in 'Electronic Basics' started by Radium, Jun 7, 2007.

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  1. Radium

    Radium Guest

    Hi:

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

    My question is, let's say I have a television set that is capable of
    receiving and demodulating FM video carrier waves. What would I see on
    the TV? I am aware that no company uses FM video. Would I see sawtooth-
    like patterns on the screen due to frequency-modulated electric fields
    present in the environment?

    I'd really like buy a TV with a FM-video receiver; I want to find out
    what FM-video disturbances in the SHF [Super High Frequency ]
    frequency-range look like. I am sick n' tired of AM video.

    AM should be used for analog audio. FM should be used for digital
    video.


    Regards,

    Radium
     

  2. Go away, Idiot!


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  3. Smarty

    Smarty Guest

    Radium,

    Some television is actually transmitted using FM modulation schemes, but not
    for commercial broadcasting. Examples of FM TV are ham/amateur ATV/SSTV,
    unlicensed 2.4 GHz surveillance links, and a number of point to point and
    studio to transmitter systems. FM signaling provides benefits in noise
    immunity and also permits better channel re-use, since it exploits one of
    the best features of FM called "the FM capture effect", wherein nearly equal
    strength signals which would otherwise interfere in AM systems will cause an
    FM receiver to "capture" only the stronger signal and ignore the weaker,
    even if differences of 1 dB of signal strength exists.

    There is no specific answer to what you would see as far as video patterns,
    and there is no reason whatsoever to expect to see sawtooth waveforms in
    particular. The demodulated signal from FM will conform to the spectral
    changes just as the demodulated signal from an AM detector would conform to
    amplitude changes, and random noise would be considered "snow" in either
    case. Unless a transmitted signal with a frequency ramp (sometimes called "a
    chirp") is present, the video would have no sawtooth. A Doppler radar, for
    example, could generate such a waveform, since some radars create chirped /
    swept signals. The video scan rate(s) would additionally need to be in the
    range of the chirp rate to create the appearance of a sawtooth.

    FM disturbances in the SHF band are likely to be man-made and not
    atmospheric, and thus only "viewable" if the "FM Video Receiver" you
    envision had a demodulator / discriminator whose bandwidth was tailored to a
    specific transmitted waveform, and even then only if sweep rates were
    suitable. Absent a man-made transmitter, the SHF environment is mostly
    thermal noise (both circuit and atmospheric) and only a radio telescope or
    other enormous aperture / antenna will see beyond the atmosphere.

    The choice of using AM versus FM is really way more complicated than "AM for
    audio" or "FM for digital video". When designing communication systems of
    any type, the engineer is faced with balancing many issues, and the channel,
    media, noise environment, interference sources, power budget, multipath,
    complexity, and cost are only a few of the considerations involved. A highly
    reliable cable modem to transmit fast digital content may indeed by phase
    modulated with an amplitude trellis; a secure and interference resistant
    link may use spread-spectrum frequency hopping AM for digital signaling; and
    FM winds up being used heavily in many voice communication systems mostly
    because the capture effect reduces co-channel interference.

    The closest I can suggest to what you might enjoy exploring would be a
    satellite dish and receiver designed for L band which will see and decode
    some broadcasting which is unprotected / unencrypted. It gets you into the
    range of SHF, has true TV signaling for public viewing, and is a hobbyist
    activity with others involved.

    Smarty






     
  4. Don Bowey

    Don Bowey Guest

    No FM shf receiver is built for FM video, because there aren't any FM video
    broadcast stations. If there was such a receiver and you tuned it to a
    standard ntsc video carrier, you would probably see nothing (no sync, no
    video), because the limiter stages would saturate and there would be no AM
    output.
    It's ok to have an opinion, but who do you suppose is won over by your
    preferences?
     
  5. What did you do to serve your country? Bomb Iraq?
     
  6. Ian Jackson

    Ian Jackson Guest

    I've got one (or at least the demodulator part of it). It's called a
    'Grundig Satellite Receiver GSR2'. It receives German TV. I use it to
    watch the German version of the Teletubbies. [Talented chaps, those
    Teletubbies, speaking German without a trace of an English accent.
    However, 'Eh - Oh' in German is still 'Eh - Oh'].

    I also sometimes watch that English 'dinner' programme which several
    German channels transmit on New Years' Eve.

    Radio amateurs also often use FM to transmit TV signals. With a little
    ingenuity (and maybe some modifications), you can actually use satellite
    TV receivers to watch these transmissions.
    No. If the signal is demodulated correctly, you see normal pictures,
    usually of very good quality (and in colour too, if you have a colour TV
    set), created by waveforms called 'video'. Potentially, on an AM TV set,
    you might be able to get some sort of poor quality picture if you could
    slope-detect the FM TV signal. However, the deviation of normal
    satellite signal is much too great, and I doubt if you would get
    anything usable (just a lot of squiggles on the screen, like you
    suggest).
    On a spectrum analyser, you see a wide FM signal, where the deviation is
    caused by the 'video disturbances'. This is normal.
    It can be (eg where the instantaneous deviation or frequency corresponds
    to the value of a digit). However, for digital signals, the boffins have
    devised all sorts of clever systems of modulation. Some of these can be
    thought of simplistically as a mixture of AM and FM, but some are really
    complicated. These are designed to be much more efficient than 'simple'
    AM or FM.

    Ian.
    --
     

  7. Why? Are you thinking of enlisting? My MOS was Broadcast Engineer.


    No, I was too old, and in the wrong branch. Dropping old TV
    transmitters from planes doesn't do much damage, anyway.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  8. On a recent visit to Spain, I was amused to see a TV programme
    featuring Jamie Oliver speaking in fluent Spanish and making a
    sausage, egg, black pudding, bacon and tomato pizza!

    "Breakfast jus' like Mama make!"

    73 de Wlat
     
  9. AM or FM are used for analogue video.

    QPSK, or higher order schemes like QAM16/64 are used for digital, with
    or without (C)OFDM.

    73
    Mike G4KFK
     
  10. So what's German for "Tinky-winky fallen in da tubby-tussard"?
     
  11. Tinky-winky ist in den fetten Vanillepudding gefallen.
     

  12. I served during Vietnam, but only on US soil. I was scheduled to go,
    to work at an AFRTS TV station, but I ended up being sent to the station
    in Alaska, at he US Army cold weather test site. That didn't stop the
    idiots from screaming and cursing at me when I flew home in 1974.


    --
    Service to my country? Been there, Done that, and I've got my DD214 to
    prove it.
    Member of DAV #85.

    Michael A. Terrell
    Central Florida
     
  13. In fairness, it wasn't really your fault. If Blair had been our Prime
    Minister at the time (perish the thought!), we would probably have
    been implicated in it as well!

    73 de G3NYY
     
  14. default

    default Guest

    In all fairness it is/was our fault. Theoretically (in a democracy)
    the people rule. We must really like the idea of war . . .
     
  15. You've been trolled by the latest king of trolls.
     

  16. PLONK
    Let me introduce you to "Radium" and other twits
    on our lists.
     
  17. Thanks Gordon, very useful. Now I need to figure out how to bring it
    into conversation :)
     
  18. Smarty

    Smarty Guest

    I generally treat all questions as sincere unless they are conspicuously
    juvenile or inflammatory. I am not aware of the trolling reputation /
    history of this particular individual, but I fully accept the possibility
    that the inquiry was bogus. It is sometimes hard to separate sincere but
    poorly stated legitimate questions from those which are deliberately
    misguided and troll. Let's see how Radium replies, if at all.

    Smarty
     
  19. Lord Garth

    Lord Garth Guest

    HDTV will solve your desire not to watch AM video carriers now.
     
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