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Video and comms question

Discussion in 'Electronic Design' started by flint, Mar 26, 2007.

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  1. flint

    flint Guest

    Hi All,

    A question for you clever guys and gals -

    I am using a conventional radio control unit (as used for model
    planes) to activate servo motors in an experimental setup. This works
    well but I now want to add video feedback from a camera. Radio control
    is really not a preferred option for me in this appllication and to
    keep the overall system simple, a cable would actually be better. The
    separation diistance is about 200ft. The cable needs to be as small as

    Is it possible to achieve all of the communications including video
    down just a single co-ax? By this I guess I mean can either of these
    work? -

    A) A single co-ax cable carrying the video but could I somehow 'add
    in' the modulated RC control i.e. get rid of the aerial at each end
    and suppliment the transmitter and reciever with some electronic
    interface to make the direct cable connection. Would the modulated RC
    signal screw up the video or visa-versa?

    B) Could I achive the same effect but not using the RC unit. i.e.
    again by adding a modulated control signal to the video conection.

    In order to keep the cable simple, cheap and lightweight, I don't
    really want to go for a more conventional video plus RS422 twin
    screened twisted pairs setup.

    Your comments, and any recommendations, humbly recieved.


  2. You could insert data in the first few video lines of the video
    signal. See LM1881 sync stripper data sheet, for the basic circuit.
    Use a switched PNP current source to drive the coax with data at a
    suitable time. You could put say one or two bytes in a video line,
    quite easily, but with a bit more care you could get say 10 bytes per

    Because video is normally a 75R source and 75R termination you need to
    drive approx 37R to say 0.5V.
    If you require two way data you could use the odd/even flag from the
    1881 to set what end is transmitting.

    Obviously you will need a micro at either end, which are locked to the
    video, and a simple line counter to select the correct line, will
    simplify the software.

    Elantec/intersil make better video sync strippers that the LM1881,
    that may offer a more flexible approach

  3. flint

    flint Guest

    Thanks Martin.

    As you may have gathered, I am not skilled in the art (or mystery) of
    electronics (I am a mechanical guy) and will need to gather more info
    and assistance on taking this further. One issue is going to be cost
    but I guess there are some pretty cheap micros or PICs about that
    could be considered.

    The other issue is going to be how effective the data speed will be
    given that I am trying to control something dynamic (it's not a
    vehicle or airplane but the action is similar). I can't see that
    having a bidirectional link is that important although could be useful
    in the future.

    I had been hoping that I could just load the 27MHz (If that's what it
    is) RC control straight onto the video signal. How naive is that?

    Thanks again. Any further assistance always welcomed.

  4. you could feed the 27MHz into the video feed, but i can't really
    suggest an optimal method.
    Notch filters may be rquired to prevent it upsetting the video.

    But back to basics:

    How fast is the data going, I dont know much about RC protocols, and
    i'm too lazy to google

  5. John Barrett

    John Barrett Guest

    Didnt you say "video feedback" ?? so RC to the equipment, video back from
    the equipment ?? you cant piggyback the RC data on the video that way if the
    signal sources are at opposite ends of the cable :)

    Get a camera with modulated channel 2/3 output, and make sure your RC is
    running on a VERY different freqency.... you said 27mhz RC ?? thats far
    enough away from TV freqencies at 54 to 66 mhz for channels 2/3

    setup low-pass/high-pass filters at each end to isolate the RC/video signals
    ... you will probably need to attenuate the RC signal a LOT since its
    designed for over the air rather than wireline. Set the frequency split at
    about 40-45 mhz -- dont have to be horribly accurate, nor do you need a
    filter with sharp roll-off. The technique is called duplexing and ham radio
    repeaters do the same thing so they can transmit and recieve at the same
    time over a single antenna.. there are lots of online resources including
    applets that will design the filters for you.

    you could even feed DC power over the same coax along with the video/rc
    signals if you like :) an inductor at each end to block the RF from the DC
    powered circuits, and a capacitor to couple the RF to/from the coax
  6. Rich Grise

    Rich Grise Guest

    From what you've described, it's absolutely doable, with the right
    kind of support circuitry at each end. For example, you'd modulate
    your video onto some carrier and apply that at the camera end, and
    modulate your RC control onto some other carrier and apply that
    at the control end, with suitable filters and demodulators at the
    corresponding other end.

    It does sound like a project of some magnitude, however. :)

    Good Luck!
  7. Rich Grise

    Rich Grise Guest

    I just can't restrain myself:

    TV rcvr-<-[TV filter]-<-+ +-<-[TV filter]-[TV xmtr]-<-Camera
    RC xmtr->-[RC filter]->-+ +->-[RC filter]-[RC rcvr]->-Servos

    All you need is somebody to design the stuff for you, or tell you what
    you need to buy. :)

    And just FYI, RC protocol is almost audio, so no worries. :)
    (I once had to generate servo control signals for a robot, and so I
    went down to Futaba and asked the tech. It's something like a 1-20
    ms pulse at a 100 ms period, or something way slow like that. Please
    don't quote my numbers!)

    Good Luck!
  8. Some days you just can't see the wood for the trees

  9. flint

    flint Guest

    Thanks for all the suggestions everyone. I was trying to establish if
    it was possible and I think you have confirmed that.

    I shall be speaking to an electronic design guy I know to see if he
    can knock something up for me.

    Question - If control functionality such as this can be added to video
    on a co-ax, why is the norm to provide a dedicated video plus either
    RS232 or RS422 on twisted pairs?.


  10. john jardine

    john jardine Guest

    I'd be thinking maybe the twisted pair is about 1000X simpler and cheaper?.
    (wouldn't want to knock one up without a customer underpinning the week's
    work :)
  11. Rich Grise

    Rich Grise Guest

    Why not, exactly?

  12. Rich Grise

    Rich Grise Guest

    Probably because it's simpler. You could have baseband video on the
    cable, and ordinary RS*** on a pair, so you don't need any modulators
    or demodulators or filters. :)

  13. joseph2k

    joseph2k Guest

    Easy to do, if you understand what to do. There is endless combinations on
    what you can do. Bandsplit is just the start, directional couplers and
    tx/rx bridging add to the things that can be done. You can sucessfully
    send power one way, voice, data, and video both ways all at the same time.

    Just the same, i would have just used used an additional RF link for the
    video, just like the RC flyers do.
  14. John Barrett

    John Barrett Guest

    because if I understand correctly -- the video is sourced from the
    operational equipment end of the proposed cable -- and the RC is sourced
    from the control at the opposite end -- you dont have both signal sources at
    the same end to combine them

    also -- RC is a continuous signal -- would be a huge problem to pick up the
    signal from the RC transmitter and convert it to something that can be
    combined with the video signal -- would be better off sticking the RC on an
    audio subcarrier IF the RC signal and video were sourced at the same end of
    the cable -- but that doesnt appear to be the spec -- RC to equipment, video
    feedback from equipment
  15. Rich Grise

    Rich Grise Guest

    But, on what do you base your assertion that both signal sources _MUST_
    be at the same end? Did you miss my diagram?

    TV rcvr-<-[TV filter]-<-+ +-<-[TV filter]-[TV xmtr]-<-Camera
    RC xmtr->-[RC filter]->-+ +->-[RC filter]-[RC rcvr]->-Servos

    The RC signal is going from left (your end) to right (the remote end),
    and the TV signal is going from right to left. Can you come up with
    some kind of physics that proves that that's impossible?

  16. John Barrett

    John Barrett Guest

    what have I been saying from my first post ?? every one else has been
    talking about embedding the RC signal in the video
  17. jasen

    jasen Guest

    They do that with stinger missiles, using a very thin coax.
    both could work.

    ordinary video occupies a basically the frequencies from 0 to 5Mhz
    anything above that is available for signalling.

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