Strange AV connection problem

Discussion in 'Home Theater Projects' started by Yohan Pamudji, Nov 12, 2003.

  1. Yohan Pamudji

    Yohan Pamudji Second Unit

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    I recently setup a projector mount, and ran 100 ft. of shielded Cat5 between the DVD player and the projector. On both ends of the Cat5 are video baluns to convert Cat5 to composite video. Btw, this is an auditorium setup. So when I plug up the video it works fine, but the moment I plug up the sound from the DVD player to our mixer board the picture garbles up and the projector loses the signal altogether. I've isolated the problem to the connection to the mixer board, because I can hook up the sound to the input of a keyboard (musical, not computer) and it works fine, but if I then connect the keyboard to the mixer board the picture disappears again.

    I assume this is interference, and I've read that with shielded Cat5 (STP) I only need to ground one end of the cable. Is this correct? I have both ends grounded now (connected to shielded RJ45 connectors), so could this be the problem?
     
  2. Wayne A. Pflughaupt

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    I think I’d suspect the DVD player. Or perhaps there is some signal loss in the video conversion at both ends of the Cat-5. Also verify that the cabling on both sides of the baluns are good.

    By the way, what’s with the Cat-5? If you’re sending a composite video feed, I’ve seen them go distances of over 100 ft. using regular shielded cable with no problems.

    Regards,
    Wayne A. Pflughaupt
     
  3. Yohan Pamudji

    Yohan Pamudji Second Unit

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    I hooked up a different DVD player and it did the same thing. This doesn't necessarily rule out the DVD player(s), but it makes it less likely.

    100 ft. with regular shielded cable? RCA plugs on either end? I thought that was too long?

    The reason I chose Cat5 is because we also have a VGA signal that we need to get to the projector, also 100 ft. away. It's also not working and worse (no picture ever, just flickering), but that's another story. Anyway, 100 ft. is too far for a VGA run, so Cat5 seemed like a good idea.

    Another possibility for problem source: my shielded Cat5 isn't grounded. Is there an easy way to ground it? I've tried grounding one end only, but that didn't help.
     
  4. KurtBJC

    KurtBJC Agent

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    You ought to be able to conquer the VGA run problem using full-sized coax and breakouts--I've known people to do 150 feet of VGA that way--running STP and baluns is really asking for trouble, because the impedance tolerance on CAT5 is very poor for video use. It's just not possible to control twisted-pair impedance as well as coaxial. The big issue in running VGA over 100 feet is simply loss--and that's because people generally run it in miniature cable.

    My suspicion on your composite run would be, as you suggested, interference. There again, you'd be better off running coax over that distance, because of impedance tolerance. Grounding the shield at both ends will be better than only one, but the shields on shielded CAT5 aren't very heavy, so there again, coax is going to be the better performer. Bear in mind, too, that you're getting a lot of loss by running the signal in such small-gauge wire, which probably increases the susceptibility to interference. If your signal is just barely at the threshold level required to get a decent image, it doesn't take as much noise to make it go kaplooie.
     
  5. Yohan Pamudji

    Yohan Pamudji Second Unit

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    Thanks for the suggestions, guys. I've decided to scrap the whole Cat5 idea and go with straight runs of VGA and composite. Lesson learned the hard way.
     

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