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Video overlay box

Discussion in 'Hobby Electronics' started by Bill, Nov 29, 2004.

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  1. Bill

    Bill Guest


    At my church we've just purshased a video overlay box to overlay song words
    from a computer onto a video signal which carries live video during a church
    service. however currently the setup is a little AV switcher to switch
    between a couple of cameras.
    However the overlayed computer signal starts to scroll when the AV switcher
    is switched and then stops after it's done a complete scroll. sometimes the
    scrolling is different. AFAIK this is caused by the different video signals
    from the cameras not being in sync.
    What would be an easy fix for this without having to purchase a video mixer
    or is a video mixer the only solution since it syncs all the video signals

    This is the overlay box we are using:


  2. Phil Allison

    Phil Allison Guest


    ** Correct.

    ** You need to synch the two cameras together - some cameras have
    provision for external synch.

    A video mixer would likely provide such a synch signal.

    .............. Phil
  3. Russ

    Russ Guest

    Depending on what kind of cameras you have, you may be able to supply a
    master sync signal from the video mixer to the camera, or use the composite
    output of one camera to feed the sync inputs of the other cameras. However,
    most non-pro cameras don't have the option for external sync.

    Any device, whether it is a switcher or mixer, needs the inputs to be in
    sync. You can get video switchers that have individual time base
    correctors/frame sychronisers on each video input, but the one's I've seen
    are designed for SDI and cost a small fortune.

    Another cheaper option may be a downstream time base corrector that you'd
    put between the output of the switcher and the overlay box.

    I'd suggest you give these guys a call - they probably have something that
    will help you:

  4. Mikegw

    Mikegw Guest

    Not too sure, but rather than going a to b, have a null in the middle so
    rather than a roll you get a frame or two of black screen by switching a to
    null to b?

    Just a random thought...

  5. Phil Allison

    Phil Allison Guest


    ** How *very* Quantum Mechanical of you ......

    Heisenberg would be impressed.

    ............ Phil
  6. Mikegw

    Mikegw Guest

    He would have to find me first.

    I recon you know more on this than me. would this work or am I wasting

  7. Phil Allison

    Phil Allison Guest


    ** The overlay box:
    takes the camera's video signal in, locks onto it and configures the PC
    video data to suit exactly - but not instantly. When the video signal
    changes phase as a new camera is switched in - the PC has to do a
    reconfig to suit again.

    What possible help to this would a black out be ???

    The other posts here have the options that work.

    .............. Phil
  8. Russ

    Russ Guest

    It kinda sounds half convincing, but unfortunatelty, just because a frame is
    black, doesn't mean it hasn't got sync, colour burst, front/back porch and
    all the rest that goes into a video frame. A "black" frame just means is
    that during the picture information part of the signal, the video level is
    down at the black level. So switiching to an unused input (with no signal at
    all) before the desired input would actually make things worse. In the
    absence of a proper video signal, the overlay box would probably get angry
    as it would have nothing to sync to and stop outputting. Or explode.

    Video signals are pretty simple waveforms (thankfully for me) if you do a
    little Googling it only takes probably 15 minutes to understand how they
    work (OK, maybe 5 more to get your head around the idea of fields). Any info
    you find on NTSC is very similar to PAL, but for slightly different voltages
    and a thing called "pedestal" that NTSC has.

  9. KLR

    KLR Guest

    about the only cheap and nasty ways out of this would be to adjust the
    vertical hold on the TV sets/monitors (assuming they even have a
    vertical hold adjustment these days) to an extreme so as to get the
    scrolling over with as quickly as possible - or make up a blanking
    circuit that kills the video content for a couple of seconds to a
    blank screen (but not the sync) when the source is changed to give the
    picture time to lock unseen. making up one of those video effects
    kits that have been in EA or SC over recent years that allow a " fade
    to from black" options etc might be a more elegant way to achieve this
    - and cover up the unwanted scrolling.

    I hear that in WES news (flyer that has new items not yet in the main
    catalogue) that there was an audio video mixing desk for about $800.

    However if you have your video signal is going into the overlay box (
    I would imagine it would have to in order to be able to sync itself to
    the video content) then the rolling is probably an internal matter of
    the overlay box having to re-lock itself into the suddenly changing
    input signal to it. Possibly momentarily dropping the power to the
    overlay box (to reset it) while signals are being changed may possibly
    help ? its a real long shot - but it may help ?

    I hear that in WES news (flyer that has new items not yet in the main
    catalogue) that there was an audio video mixing desk for about $800.

    I would make sure it isnt going to make problems for the video overlay
    unit though. (dont know if it will)
  10. Fred Ferd.

    Fred Ferd. Guest

    may have been the wrong way to go.

    may have been better to use a 4 input video capture card , so the PC gets
    the camera of your choice,
    writes the overlay on, and then outputs your video.

    That way the output never changes its sync, and the display doesnt scrolling
    doesnt happen.

    The problem is the overlay machine is a time base corrector, it prevents the
    sync on the input any large % of a frame per frame. when the signal is off
    sync, it outputs junk that you see as scroll.
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