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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => AC Power and Grounding => Topic started by: Stephen Swaffer on March 30, 2014, 04:38:45 PM
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I am having trouble with horizontal bars on a projector. The projector people tell me it is a power issue and to have "the electrician" fix it. Problem is, I am the electrician!
I am assuming it is essentially the same issue as 60 HZ hum on audio that we normally cure by using balanced circuits and connecting the shield at one end of a cable as opposed to both. Of course the issue is that the video is unbalanced and the "shield" part of the signal circuit.
I get the interference using either a computer VGA signal or composite video from a consumer DVD player with a 2 wire power cord. And it matters not whether both or if just one of the sources is physically connected.
Is the area of concern the ground or the neutral? Where does the consumer DVD get its signal "ground"??
It would be fairly easy-2 hours and 100 feet of 12-2 MC to supply the projector form the same source as the DVD-is that worth pursing? The main drawback is adding a projector load to the media booth-not something I really want to do, as it may have a domino effect forcing me add another circuit there-which could be a headache.
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I am having trouble with horizontal bars on a projector. The projector people tell me it is a power issue and to have "the electrician" fix it. Problem is, I am the electrician!
I am assuming it is essentially the same issue as 60 HZ hum on audio that we normally cure by using balanced circuits and connecting the shield at one end of a cable as opposed to both. Of course the issue is that the video is unbalanced and the "shield" part of the signal circuit.
I get the interference using either a computer VGA signal or composite video from a consumer DVD player with a 2 wire power cord. And it matters not whether both or if just one of the sources is physically connected.
Is the area of concern the ground or the neutral? Where does the consumer DVD get its signal "ground"??
It would be fairly easy-2 hours and 100 feet of 12-2 MC to supply the projector form the same source as the DVD-is that worth pursing? The main drawback is adding a projector load to the media booth-not something I really want to do, as it may have a domino effect forcing me add another circuit there-which could be a headache.
http://forums.prosoundweb.com/index.php?topic=765.0
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In the 2012 revised Bill Whitlock paper, see pages 167 to 183.
"An Overview of Audio System Grounding and Interfacing"
by
Bill Whitlock, President
Jensen Transformers, Inc.
Life Fellow, Audio Engineering Society
Life Senior Member, Institute of Electrical & Electronic Engineers
http://centralindianaaes.files.wordpress.com/2012/09/indy-aes-2012-seminar-w-notes-v1-0.pdf
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Or in the 2005 version of the paper, see pages 31 to 37.
http://www.jensen-transformers.com/an/generic%20seminar.pdf
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http://forums.prosoundweb.com/index.php?topic=765.0
Tim,
I read that topic before posting. Using ground lifts (3 prong to 2 prong "adapters") is discussed as a troubleshooting method-but no one ever says what the best permanent solution is? I could install a GFCI with no equipment ground-but really don't like the idea of having a projector in a GFCI during a program.
I guess, as much as I am trying to fix this problem, which I can do by trial and error, I am trying to understand what should be done from a design standpoint. Is an $800 "video humbucker" typically used on projector installs? I find it hard to believe that all installs use the same panel-but maybe that is enough to mitigate the problem?
The Bill Whitlock paper seems to suggest -and it makes sense- that noise can be induced on the ground wire on a long cable run-in that case simply running MC cable to a different source will not accomplish the goal of getting the projector ground and the VGA ground to the same potential.
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Tim,I find it hard to believe that all installs use the same panel-but maybe that is enough to mitigate the problem?
I can't speak to all installs, but the proper way to do it, just like with sound reinforcement, is to run all connected components of the system from the same panel/sub panel with proper ground topology. If you can't do it that way and you run into ground loops, the only other option to mitigate the issues are expensive fixes. As you've seen, video "fixes" aren't cheap. The best fix would be to convert everything feeding the projector to SDI/HDSDI and use fibre optic converters (http://www.aja.com/en/products/mini-converters/fido) to get the signal to the projector.
Disclaimer: I work for AJA.
I remember installing editing systems for a particular TV show in a facility that had editing rooms on separate floors. They wanted all editing rooms for this show tied to single machine room to share VTRs and fibre channel disk storage. They got hum bars on all systems. They were using composite SD video back then, so the only fix was to buy a bunch of those expensive huge video isolation transformers. It was an inelegant solution. But it got the job done.
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Powering the projector from the same panel MAY help but ground problems are still possible. I have had ground loop problems with a system running off one plug strip.
A long extension cord would enable you to find out.
A sure fix is the fiber optic route. Another option is one of the many VGA or HDMI (if you can use that) over CAT5. These convert the video to a balanced signal so they have less problem with interference.
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I once experienced problems when feeding a projector with an S-video cable that passed through an assistive listening induction loop. The pattern of the hum bars coincided with the sound. (Turning off the induction loop made the hum bars go away.) Part of the connection was an S-video coupler; replacing the coupler improved things somewhat. As it was only a temporary setup, I didn't get the chance to find a permanent solution.
Eventually the church did permanently install a projector in the space, but they're feeding it with a low-latency TCP/IP stream rather than a pure video signal. That decision wasn't based on the previous problems I experienced, but rather on the need to be able to control the projector from a remote location. Since the projector they selected also had the ability to accept video via a TCP/IP stream, they opted to feed it that way rather than run an additional cable. It is working well.
The projectors are Epson PowerLite 95.
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As well as following the great advice given by others, there is something else to check.
See if the mounting bracket of the projector is connected to the chassis of the projector and the shield of the VGA connection.
A continuity check with a multimeter will do.
If it is, make sure the projector is isolated from the building ground if it is attached to a truss or metal beam.
We chased a ground loop for quite a while on a job where the projector chassis was grounded to the lighting truss via the hanger clamp!!
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As well as following the great advice given by others, there is something else to check.
See if the mounting bracket of the projector is connected to the chassis of the projector and the shield of the VGA connection.
A continuity check with a multimeter will do.
If it is, make sure the projector is isolated from the building ground if it is attached to a truss or metal beam.
We chased a ground loop for quite a while on a job where the projector chassis was grounded to the lighting truss via the hanger clamp!!
Forehead slap. I am going to remember that one. WE get focused on the electrical paths and it is easy to overlook the other paths to ground. Thanks
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We chased a ground loop for quite a while on a job where the projector chassis was grounded to the lighting truss via the hanger clamp!!
I once measured 5 volts AC between the I-beams on two sides of a large factory. When they "grounded" the video cameras to the roofing truss back at the railroad dock, we got video hum bars like crazy in the guard shack's video monitors which were located many hundreds of feet away.
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Problem solved today. I learned that the problem definitely started right after our annual Christmas Play-there is a lot of activity that takes place in attic where the projector is plugged in at that time-so I went looking thinking the projector had been plugged in to a different circuit. What I found was a temporary strand of lights that had been plugged in to the projector circuit-and apparently left on 24/7 since then. I did not take time to verify, but I suspect the real culprit was the fact that there were a number of CFLs in that temporary strand (and more than likely a stay neutral/ground bond as well). The projector is scheduled to be moved soon-at that time I am thinking a dedicated single receptacle-perhaps even a TL to discourage tampering!