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question on video impedence...

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Alan Teo
question on video impedence...
on Jul 8, 2005 at 9:11:25 pm

Hi... I just started doing multi-cam events coverage recently. I happened to be involved in the equipment set-up during the last show. While connecting the BNC cable to the JVC Video Monitor, I saw there's a switch which allows me to select between OPEN or 75 OHMS.

What is the switch meant for? and when do I need to put it to OPEN or 75 OHMS???


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Charlie King
Re: question on video impedence...
on Jul 8, 2005 at 9:21:38 pm

This is for termination. If you don't terminate the overall video is doubled in amplitude. Yet if you double terminate it is pretty much cut in half. These not necessarily exact measurements but close but I think you can get the point. If you add another monitor into the line such as a daisy chain, you want to leave it open then terminate at the last item in the path.

I hope that makes sense.

Charlie



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Alan Teo
Re: question on video impedence...
on Jul 8, 2005 at 9:24:25 pm

So in other words, if I don't terminate at the last item, the video quality will drop???


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Charlie King
Re: question on video impedence...
on Jul 8, 2005 at 9:26:57 pm

If you don't terminate at the last item. The video will be very bright and clipped.


Charlie


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Alan Teo
Re: question on video impedence...
on Jul 8, 2005 at 9:27:48 pm

Thanks alot. I got it. :)


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Charlie King
Re: question on video impedence...
on Jul 8, 2005 at 9:30:27 pm

Keep in mind if you should have a terminator in line before the last item you could be double terminated and the video will be dark.

Hope this helps

Charlie


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Chuck Reti
Re: question on video impedence...
on Jul 9, 2005 at 1:29:15 am

If you have a very long line, unterminated, you may also get reflections (ghosts) back down the line.

--
Chuck Reti
Video Editor
Detroit MI


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tony salgado
Re: question on video impedence...
on Jul 9, 2005 at 3:49:37 pm



Alan,


Think of termination like a facet in which you must either turn on the water to let it flow out (loop to another monitor) or turn off to stop the flow. The last monitor in the loop is the one you must terminate.

Generally it is not advisable to loop more than once because you will start to have video signal roll off issues. I only loop monitors which are being used for content and framing not image quality viewing.


Tony Salgado


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Mike Cohen
Re: question on video impedence...
on Jul 22, 2005 at 7:56:09 pm

A few years ago I walked into a room where some of my videos were being projected. When I got there the clients were flipping out because the video was all washed out. The inept AV tech was fumbling with the projector's color settings while the videos were playing, much to everyone's dismay as it obviously wasn't fixing the problem.
When the tech had his back turned (can't mess with their setup or make suggestions in this convention center venue) I looked at the setup, moved a terminator from an unused bnc input to the correct one, and problem solved.
Despite fixing the problem, the tech was ripped - he said I wasn't allowed to touch anything and he could shut the room down yadda yadda yadda. That's gratitude for ya.

Moral of story - termination very important.

Mike


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