Noisy AC

I would expect neutral to be connected to ground at the service entrance to the building. You should say where in the world you are, and what sort of building you are in, it may make a difference.

as far as i can tell "balanced AC" is strictly an audiofool term. the biggest problem i see with any means of ungrounded feeds to a live truck it that it would require that you travel with an electrician to properly ground it at each venue. GFI is not a viable option in this case (picture yourself hanging you head and saying "i'm sorry i missed the shoot on this one time event someone touched something and a breaker popped")

if you mention balanced AC to an electrician he is going to assume you are talking about load balancing: a totally different thing.

it would be more accurately be referred to as 'isolated AC'.

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.

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. 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. 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.

With newer equipment, this is generally less of a problem as the power supplies are designed with more isolation. Safety has first consideration. 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. 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.

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)

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

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.

Beachcomber

Yea I wish there were broadcast engineers in our area, but it's rare to even see an event truck hit our city, and when it does they're usually too rushed to talk.

Our system was designed as a special effect, there are over 40 cameras in our system, thus making filtering individual cameras a big task, and alot of extra equipment. All of the cameras we use take in a 12vDC supply, of which the negative terminal is tied to video ground, and they are all supplied by the same DC power supply. The DC supply is isolated, but it doesn't really matter as the video ground hits the other side which attaches to computers and patch panels, and is grounded to case. So it's very difficult to isolate this at our rack. We ground our trailer using the ground supplied by our 100 amp AC input, which is basically tied directly to the building's ground. I've noticed today that the noise (which your right, it's 60hz hum) comes and goes at different times during the day, so I'm convinced it's caused by some piece of equipment within the building, and not within our trailer. Of course I have no control over that, we need a way to isolate this noise before it hits our equipment. I'm thinking of looking into some power conditioning device I can install at the panel, I came across a few manufacturers today.

I wish I had a waveform monitor handy, I've only got a portable oscilliscope (and it' s only a 20Mhz) so I can't really see the noise. I've considered hum buckers, but I'm convinced this noise is also affecting our data lines to the cameras and am looking for a solution that filters noise at the source (our ac input feed). I'm going to look into a 100amp isolation transformer later tonight (if such a thing exists).....

Thanks!

-Michael Brown

Beachcomber wrote:

where are you?

light dimmers are often the culprit, particularly older systems.

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

(that document is mostly written several years ago and I am slowly updating it..)

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

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).

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...

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).

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.

ThatÃÃ'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).

That's true.

Hum bucker transformer and video signal isolation transformers are both very effective tools for fighting agains mains frequency noise on video lines.

Use audio signal isolation transformers where necessary.