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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 => Pro AV Forum => Topic started by: Kellen Tyburski on January 06, 2012, 11:39:57 am

Title: Video Humbucker
Post by: Kellen Tyburski on January 06, 2012, 11:39:57 am
Hey all...

Couple questions about video Humbuckers... from what I know, these are typically (always?) a transformer @ a 1:1 ratio...
What is it about the 1:1 winding ratio that actually eliminates the hum?

I have had a scenario or two where we have had noticeable video hum in our ballroom, when using lighting on dimmers in conjunction with audio and video. If the problem occurs, where is the best place to "insert" the humbucker? (between output of switcher & projector, between video component & switcher, etc)

In addition, anyone know of a good place where i can read more about the definition of ground loops, how to avoid/eliminate them. This is one of my 2012 resolutions - instead of "wingin' it" I need get comfortable with this concept.

Thanks!
 

attached is the one i have been using... we own 2 of the same
Title: Re: Video Humbucker
Post by: David Buckley on January 08, 2012, 05:48:40 am
What is it about the 1:1 winding ratio that actually eliminates the hum?

Nothing, the winding ratio does nothing to eliminate the hum.  Having 1:1 just means you get the same out as you put in.  What eliminates the hum is (a) the presence of the transformer, and (b) there is no connection between the ground of the input and the ground of the output.


In addition, anyone know of a good place where i can read more about the definition of ground loops, how to avoid/eliminate them.

In video, you get a ground loop when there is a current flowing through the screen of the coax.  This is caused by different parts of the video system being plugged into outlets that have different earth potentials.  The coax outer is commoned to electrical ground.

First thing to do is to have a single common earth point for all the equipment, so the eaiest way to do that is to have everything running off a single outlet from the wall, and have a single distribution board (of whatever needed size) afterthat.

I have had a scenario or two where we have had noticeable video hum in our ballroom, If the problem occurs, where is the best place to "insert" the humbucker? (between output of switcher & projector, between video component & switcher, etc)

Now you;ve seen why hum loops happen, its usually fairly easy to spot whre to put the humbucker.  Often projectors are run from different power sources from the switchers and stuff, so betwixt switcher and projector is often the place a 'bucker i needed.
Title: Re: Video Humbucker
Post by: Kellen Tyburski on January 08, 2012, 11:52:30 pm
Thanks for the help....helps a lot..  When you say the "screen of the coax"... what are you refering to, I have never heard this term..
Title: Video Humbucker
Post by: TonyWilliams on January 09, 2012, 01:15:19 am
I believe the shield of the coax is what he is referring to.


- Tony Williams
Sent from my iPhone using Tapatalk
Title: Re: Video Humbucker
Post by: Geoff Doane on January 11, 2012, 01:42:57 pm
  When you say the "screen of the coax"... what are you refering to, I have never heard this term..

Without checking, I would have guessed that David was in the UK by his terminology.  It turns out he's in New Zealand, which from my North American perspective, is almost the same thing  :).

It also seems like the perfect time to relate this anecdote, which was told to me by a now retired colleague in the television business.

Many years ago, Ampex and Philips had some sort of joint venture going, probably a Philips switcher and Ampex VTRs.  They were putting together a booth at the NAB in Las Vegas, and were having all sorts of problems with hum in the video when the two systems were interconnected.  The Philips engineers were all running around muttering about "earth", and the Ampex guys were talking about "ground", and the frustration level was high.

Finally one of the Philips guys asks, "What is the difference between earth and ground, anyway?" 

"About 10 f*cking volts!", was the answer.

GTD
Title: Re: Video Humbucker
Post by: David Buckley on January 11, 2012, 08:52:20 pm
Yeah, I've heard that anecdote before.

"Screen" and "Shield" I guess depends on whether the cable manufacturer sells you "screened" cable or "shielded" cable.

This youtube video (http://www.youtube.com/watch?v=Eg-zT3DZN40) explains it far better than I could :)
 
Title: Re: Video Humbucker
Post by: Keith Broughton on January 15, 2012, 08:07:10 am
Quote
First thing to do is to have a single common earth point for all the equipment, so the eaiest way to do that is to have everything running off a single outlet from the wall, and have a single distribution board (of whatever needed size) afterthat.
Also, watch when you connect a projector to a lighting truss that the C clamp is isolated from the metal of the truss.
This can cause the same "ground loop" problems as having different outlets.