Newsgroups: alt.engineering.electrical
Subject: Re: Noisy AC
References: <[email protected]>  <[email protected]>
[email protected] (Beachcomber) writes:

> >before proceeding with costly "fixes" i would suggest you hire an
> >experienced TV engineer to look over your system. broadcasters face this
> >dilemma on a reoccurring basis.

Been there, seen that. I have worked several times on 
TV broadcasting system beign the tech person hunting out 
those problems from the system. 

I have written some documentation to the web what I have 
learned over the years   
http://www.epanorama.net/documents/groundloop/index.html

(that document is mostly written several years ago 
and I am slowly updating it..)
 
> I'm a former TV maintenance engineer that has had some experience with
> this sort of thing.   Ground loops on a remote truck can be common for
> a large and complex system.   

Ground loop problems are common for all large and compex systems.  

> The problem might be that the safety AC
> ground on much of your equipment is electrically connected to the
> shield ground on your video coaxial cables and possibly your audio and
> control cables as well.   

That's usually the case with all normal video, audio and computer 
equipment. The defauls is normally that the video, audio and data 
grounds are connected to equipment case that is connected to mains 
ground (excpetion are doulbe insulated equipment with no ground).   

> The circulating current flowing in a ground
> loop produces a 60 cycle (in the US) hum that can show up in video
> monitors, distribution amps, waveform monitors, and audio lines.

That's true. 

The current flowing on ground can cause noise to signal lines 
in two ways: 
1. direct voltage difference between different ends of the cable shield 
   (U = I * R) , gets directly added to unbalanced signal on line 
   (audio or video)
2. circulating ground currents can create considerable mains frequency
   magentic fields that can couple to all signal lines nearby

Both of those can cause considerable noise problems. The second 
magentic field related problems are then ones harder to debug...
 
> With newer equipment, this is generally less of a problem as the power
> supplies are designed with more isolation.   

Many newer equipment have power supply that is somwhat better 
isolated than older equiment, that's true. But this better 
isolation generally helps considerably only with non grounded 
equipment (doubly insilated). With grounded equipment still 
have the signal line ground connected to mains power ground
(there are good reasons why this is the norm).

> Safety has first consideration.  

That's true. 
The system should be always built in such way that it is safe. 
From the possible electrically safe system setups use the 
configuration that is least noisy. 

> You'll want to start with your truck's electrical
> system and make sure that AC connection is grounded properly and
> according to the national electrical codes.  

That��'s true. Make sure that everythign is wired crectly 
(no ground/neutral interconnections or wired siwtech in any 
part of system) and in sensible ways (star grounding usually best 
idea).

> If you operate on
> generator power, you still need a proper ground system.  Watch out for
> things like amps, prompters and monitors connected to a building's AC
> system if the truck is on an isolated power feed.
> 
> Some devices may need to be connected to an isolation transformer to
> minimize or eliminate ground loops

That's true.

> For coaxial video feeds, there is a device called a hum bucker which
> is kind of a brute force approach to minimizing a ground loop.  Here
> is one example:  (I'm not necessarily recommending or not recommending
> this particular branded device)
> 
> http://www.american-technical-resources.com/video-guard-buck-male-female-p-188.html
 
Hum bucker transformer and video signal isolation transformers 
are both very effective tools for fighting agains mains frequency 
noise on video lines.   

> Make sure your jack fields are wired correctly by someone
> knowledgeable who knows when lifted grounds are appropriate.

Use audio signal isolation transformers where necessary. 
 
> For extremely long video (and audio) feeds... the use of fiber optics
> lines with the appropriate converters at either end are preferable to
> coaxial cables.   The price of this technology has come down over the
> past few years.



-- 
Tomi Engdahl (http://www.iki.fi/then/)
Take a look at my electronics web links and documents at 
http://www.epanorama.net/