View Full Version : What can be done about proper grounding?
21st June 2005, 10:51 PM
I had a nightmare gig this past weekend. I was rigging a club that had 2 AV systems, one for each room.
* Room 1 = AV (amps, video distributor, CD player, etc.)
* Room 2 = Stand alone PA system
I tried to run my video into Room 1's video distributor, and my audio into Room 2's PA.
Here's what would happen:
* Just audio into PA: no problem
* Audio + Video: Projector would lose signal or get distorted, PA would emit 60 cycle hum.
(Sorry if this sounds like one of those horrible math problems from middle school right about now.)
I finally beat it by plugging everything directly into room 1, and blowing off room 2. :(
I'm pretty sure this was a grounding issue, and with my limited knowledge of their system, I could not fix it in time.
Do people have similar horror stories, or recommendations for how to fight grounding issues in the future?
21st June 2005, 11:05 PM
this is why they say to always use different phases (breakers) for each, A & V.... my rule of thumb is one rig (VJ or DJ) per breaker or power source...
please feel free to correct all of my layman's (mis)understanding of industrial electronics
21st June 2005, 11:20 PM
You can use ground loop isolators for audio and video humbuckers for video. Your results may vary depending on the quallity of the device. Or you can just bend the ground plug so you are running your gear ungrounded (not normally recommended, but will work in a pinch)
21st June 2005, 11:22 PM
From what I could tell, my A + V source (video camera, video mixer, laptop) and the PA was plugged into one outlet, while the projector and AV distribution box was plugged into another....pretty puzzling.
When I got there, the manager mentioned something like "The owner doesnt like to have AV running to both rooms..." Now I think I know why. :(
22nd June 2005, 01:45 AM
On the audio side, it's good to carry a pair of these transformers (http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=68656&is=REG) for each channel of audio. You will need a length of XLR microphone cable to join them and an XLR-XLR adaptor cable with the shield connected to pin 1 at one end only. These transformers are good for running audio for long distances like 1000 feet.
These video isolators (http://www.jensen-transformers.com/datashts/vbh1ss.pdf) and these (http://www.jensen-transformers.com/iso_vid.html) aren't cheap, but they are a useful piece of kit.
Tracing gound problems is a mix of science, art, visualization and a dose of trial and error. If you can get AV connectors with screw connections, you can sometimes lift the grounds and put them back as needed. A knowledge of how your gear is internally grounded (signal vs power) is useful if you can get it. Laptops shouldn't carry the ground through from the mains power plug, but Shuttles and other computers may connect it to the audio and video grounds, you can test with an ohm meter (with the unit unplugged from power!)
Sounds like the power in one of the rooms had a floating ground with some 60 cycle mains voltage leaking into it - your signal ground leads became the grounding path, which is a good way to get shocked. Good to carry one of these outlet testers (http://www.testequipmentdepot.com/AEMC/ElectricalTestTools/OutletTesters/OT1.htm) and know what it means. You could carry an oscilloscope, but that's a little extreme, on the other hand, you could run the audio to it xy and point a camera at it for some trippy visuals like this. (http://perso.wanadoo.fr/vb-audio/us/products/oscillo/oscillo.htm)
23rd June 2005, 10:55 PM
Hmmm...what a pain, eh????
Thanks for your help!
24th June 2005, 12:58 AM
As a general rule of thumb - I always run lighting/projectors seperate power line to PA. I had a techi explain to me that the high burst compression from the PA can interfere with the lighting etc. Also - run 3 phase strength cable to everything. Don't overload your power boards (I've seen some scary shit in the past).
24th June 2005, 03:58 AM
I did a gig in a venue that had no grounding at all, solved the problem by hammering a long metal rod in the ground; the type used to reinforce concrete, welded a big fat wire running to our main power board.
24th June 2005, 09:09 AM
you can always use a power transformer (you know that big heavy square thing) as a safe alternative to cutting the earth pin off. It always pays to be prepared for earthing problems, there will always be a caterer with a dodgy oven that throws things off. As for audio you can pick up a car stereo RCA line isolater for cheap and that will help with your audio issues, also remember anything that compresses/decompresses with optical will also kill loops.
Yamaha FZ6 (http://www.cyclechaos.com/wiki/Yamaha_FZ6)
24th June 2005, 11:11 PM
Anyone who wants to really research this problem can do a search on "pin 1 problem"+"Neil Muncy" as well as "AES48". Neil Muncy, an old school (before digital) audio genius wrote a paper in 1996 on hum, noise and grounding in audio. The Audio Engineering Society then created a grounding recommendation AES48. Many of the same principles apply to video.
11th August 2005, 04:31 PM
i think the solution in a pinch would have been to make sure the room 2 projector was on the same circuit as the video distributer and all your gear in room 1.
anyone know how to turn off the smilies?
12th August 2005, 04:53 AM
fairly cheap (from around $150) video and audio humbuckers. totaly essential for any complex AV setups.
12th August 2005, 01:10 PM
Now that I'm doing AV live shows, I got an Ebtech HumX adaptor for my laptop, and that killed the slight hum I used to sometimes hear when I plugged audio out of my headphone jack. Works great.
I'm going to research those video hum eliminators now...THANKS!
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