Grounding Mobile Production Studio

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UniversumMedia
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Joined: Tue Jun 21, 2016 8:30 am

Grounding Mobile Production Studio

Post by UniversumMedia » Tue Jun 21, 2016 8:37 am

What is the best way to ground the racks and patch panel in a mobile production trailer. I ran into some problems this past weekend with an audio ground loop causing issues with sdi video signals due to them being un-isolated in the same patch panel. I am going to switch all the un-isolated bnc through pieces to the isolated version but still want to get a good grounding set up. Any thoughts would be appreciated.

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PID_Stop
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Re: Grounding Mobile Production Studio

Post by PID_Stop » Wed Jun 22, 2016 1:29 pm

Generally speaking, the curb panel where outside feeds connect should use BNC connectors with isolating bushings, and the XLR connectors for the audio should not be bonding the shield to the panel. This forces each line to get its ground from the device to which it connects, and if you are using gear with differential inputs, much of the common-mode pickup you describe will go away. A typical approach to rack grounding is to install a copper ground busbar in each rack, and connect the busbars to a common ground point. Each piece of equipment gets its chassis connected to the busbar with a piece of braid.

There shouldn't be a point where any two incoming or outgoing lines have their shields connected, other than the equipment to which the lines actually terminate. That alone ought to help the bad interaction between your SDI and audio feeds.

If you're getting AC hum, that's a pretty good indicator that you need to look at why there's such a high potential difference between your truck's ground and the ground elsewhere. Also, some trucks (including several of ours) have multi-tap transformers for the AC shore power, and one side-effect is that the neutral inside the truck doesn't necessarily have any relationship to ground... which is not so good for some equipment. In some venues, we have had to tie the neutral to ground in order to bring ground loops under control.

If you're having audio getting into the video, one cause I've seen is from incoming balanced audio where one side of the line has been shorted to ground. If you don't have a solid ground to begin with, this can induce quite a bit of crosstalk into other audio or video lines.

We keep a few audio transformers handy that we can use to lift the ground and kill ground loops... much safer than defeating the safety ground on the equipment power cords.

-- Jeff

UniversumMedia
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Joined: Tue Jun 21, 2016 8:30 am

Re: Grounding Mobile Production Studio

Post by UniversumMedia » Thu Jun 23, 2016 6:54 am

Thanks alot for the information. The problems only come from tying in with facilities that are already full of issues. i suspect changing to isolated bnc's will fix most of the major issues and i can deal with an audio hum easy enough

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