Flickery video

[MarshallPope]

We have recently installed a projection system in my church, and I have noticed a problem. The image looks fine without audio, but as soon as I plug the audio cable into the computer from the mixer, we get a wavy, flickery motion in the image. I have also noticed that the wave is more pronounced when a light is being dimmed. Obviously, our electrical situation is not great, as we have had work done by people who had no experience in this type of production-esque situation. Our lights are controlled by a bank of rheostats at the booth. All of the power for the lighting and sound and video comes from the same panel, and there are two circuits in the booth. I didn't think to pay attention to whether or not the board and computer were on the same circuit. Could that make a difference? Is there a way to isolate the audio that would make a difference? I'm using a 1/8" trs to 2X 1/4" ts cable.

Any suggestions on things to try? More questions? I'm sure that I've forgotten to explain something vital.

Thanks as always.

 

[derekleffew]

... we get a wavy, flickery motion in the image. I have also noticed that the wave is more pronounced when a light is being dimmed. ...

Wiki: Hum bars ? Possible/probable solution: wiki: Humbucker.

 

[MarshallPope]

It looks like that's the problem. Unfortunately, I'm not coming across and budget-friendly VGA solutions. Now that i know what to call it, though, I was able to find someone on another forum with a similar problem, and someone suggested just using an isolator on the audio line. Any thoughts on that?

 

[ruinexplorer]

Yes, you have a ground loop. Since the lighting is also affecting your video signal, just isolating the sound power will not solve your problem. Are you able to switch to a digital signal such as DVI or HDMI? Depending on the length of run, you may have to translate this signal via Ethernet cable or fiber optics for longer runs.

 

[MarshallPope]

I suppose we could switch to DVI or HDMI. Currently, we are running VGA over ethernet cable. Pardon my ignorance, but how would a digital signal differ?

Thinking about it further, I have a few other observations:
-There were still hum bars when I unplugged the laptop from power while still connected to the board.
-The laptop was plugged into booth circuit 2, while the sound equipment is plugged into booth circuit 1, and the amps/processers are plugged into another circuit from the same panel.
-The projector is plugged into a circuit coming from a different panel.

I don't know if any of that may or may not matter, but I figure all the information I can come up with could possibly help.


Just instinctually, I feel that simply isolating the sound feed would solve matters, seeing as that is the connection that is screwing things up. Of course, there is a good chance that I could be completely wrong on this. I'm hesitant to make any major investments, as we have yet to purchase the computer that will live in the booth and are still using my macbook for the time being.

 

[porkchop]

You can also look for a ground loop isolator if you want to use a more technical term. A quick Google search shows that $140 seems to be the going rate

 

[SHARYNF]

Running a VGA over Cat5 cable is still analog signal, it just uses the twisting in the cable for Shielding. Problem possibly is that you are picking up ac noise on the line. I would guess that it is the difference between circuit 1 and 2. Try to run the Laptop on battery.

There has been a lot of discussion re laptops and grounds etc. In my experience SOME of the laptop power supplies don't implement the ground system correctly. You might TRY just for a test to not connect the ground on the Laptop power cable. I know there will be a ton of people immediately cautioning warning etc, but just get one of those converter adapter and SEE if the problem goes away, at least then you will know the source



In general I also would say get your self a proper feed from the Computer to the sound system, using the headphone jack also can cause ground issues. Peavey makes an relatively inexpensive usb audio adapter

Peavey USB-P USB Playback Thingy 248-8674


Sharyn

 

[Nelson]

We used to have a ground loop issue when laptop computers were connected to VGA, audio, and an electrical outlet all at the same time. Remove anyone of the three and the problem went away. It made a bit of a hum but didn't seem to affect video much if at all. Reducing the low frequencies on the EQ was sufficient to bring the hum down to a tolerable level. Now we connect laptop computers wirelessly so I never tried to find the real cause of the ground loop. I've never had a ground loop problem with the wireless transmitter.

 

[MarshallPope]

Ok, a related question. Does anyone have a suggestion for an affordable (Preferably under a couple of hundred $), decent quality 3-way isolated VGA splitter? We will be adding a couple more projectors in the near future, so I figure we might as well kill both projected birds with one stone. I've looked around and have been unable to find anything that I would trust that meets the criteria.

 

[FMEng]

Adding a VGA splitter (DA) won't solve the video hum bar or the noise in the audio. The simplest solution to fix all is to break the ground in the audio circuit. An audio transformer or a computer interface box between the computer and the sound console will do the trick. Whirlwind, Sescom, Ebtech, ART, and Jensen Transformers all make suitable products.

 

[Chris15]

My gut is telling me that having the projector fed power from goodness knows where is not helping the situation either...
But transformer isolate the audio signal as FMEng suggested (and set any ground switch to lift...) and there's every chance you'll have resolved things...

 

[HJS]

We had precisely the same problem. It is a ground loop issue and it drove us crazy trying to track it down. We finally solved it by removing the ground pin from the power cords on everything in the chain - laptop, video processor and switching, HD displays and LCD projector. The cost for the fix is zero and the result has been perfect. If you want to try it out first, use plug adapters - the little rubber ones with three holes on the top and two blades on the bottom we used to use at home before houses had grounded outlets - if it works for you, just rip out the ground plug on the power cords with a pair of pliers and you're done. It is, however, important to isolate everything in the chain - if one piece in the line is still grounded throught its power cord the fix won't work. But if all the grounds are removed those wavy, rolling, flickering lines in the video will also be gone. It's a lot simpler than adding isolation transformers or trying to track down a ground loop problem in a building.

 

[Chris15]

NO NO NO NO NO

You are endangering people's lives by removing the essential safety Earth connections.
You are also assuming all liability for such a circumstance - the manufacturer will leave you high and dry, your insurer will leave you high and dry.
In short, there is NEVER an acceptable reason to lift the mains Earth. EVER.

 

[DrPinto]

I personally would not remove the ground pin from any power cord, but I do have a question about the laptop. If the ground was lifted from the laptop power supply by using a 3 prong adapter, is that really a bad thing? The power cord feeds a plastic encased power supply with a transformer. Some computer power supplies don't even have a 3 prong plug on them. How is lifting the ground on this dangerous?

I'm not suggesting that anyone do this, but I just want to know the reasoning behind it.

 

[jstroming]

You are endangering people's lives by removing the essential safety Earth connections.
You are also assuming all liability for such a circumstance - the manufacturer will leave you high and dry, your insurer will leave you high and dry.
In short, there is NEVER an acceptable reason to lift the mains Earth. EVER.

This is very common in Asia and South America (two areas where I have a decent amount of show experience). The typical response I get when asking local A/V and lighting suppliers why half of their equipment has no grounds is "it's optional". From that point on I try to avoid touching things near the stage made of metal. HAHA. The electrical codes of many third world countries (if there even are any in some countries) are sparse and rarely enforced.

It is our company's policy to rent ALL production equipment when we do shows in foreign countries, with the only exception being Canada. This helps to limit our liability in international law, which can be a tricky mess. In new markets we always try to deal directly with the venue, and have them amend the contract to include all production requirements. This establishes a clear line of responsibilities....producer (us) establishes production guidelines for show, venue pulls together local suppliers to meet staging, rigging, lighting, audio, video and other production requirements, and then those vendors are responsible for fulfilling their end of the production according to local laws. The venue also acts as a main point of contact for us, which means only one person to deal with (and thus only one translator needed).

Yes, it sucks to look away when you see blatant violations of safety. Like climbing on a truss 25ft in the air with no harness, leaving the ground cam unplugged from the main power distro, and running e-tape'd THHN everywhere instead of rubber-jacketed cable (all I've seen quite a few times). Unless there is imminent danger to any of MY road crew or patrons, you have to look the other way in some of these places. We had a local crew in asia who used 1/4" bolts and fender washers to hold together 20.5" plated box truss with about 3000lbs across a 50' span. HAHA I might have let it slide if it wasn't all chinese hardware!!

 

[Chris15]

Just because it's common doesn't make it right...

My old boss was doing some repairs in the workshop in China and was sitting on a roadcase doing them. He was getting a tingle off the metal cases and got the meter out. 80 or 90 volts between these "earthed" cases and the roadcase (which would have been at actual ground potential).
He went searching and found the Earth had actually been wired all the way back to the main switchboard whre it promptly exited and connected to thin air...

Again it's common, don't get me wrong there, but there are also very different cultures of liability compared to our western expectations. OHSA and the like tend to get involved when people die in idustrial situatuions as an injury caused by this sort of thing stateside would be classified...

 

[SHARYNF]

I personally would not remove the ground pin from any power cord, but I do have a question about the laptop. If the ground was lifted from the laptop power supply by using a 3 prong adapter, is that really a bad thing? The power cord feeds a plastic encased power supply with a transformer. Some computer power supplies don't even have a 3 prong plug on them. How is lifting the ground on this dangerous?

I'm not suggesting that anyone do this, but I just want to know the reasoning behind it.

m

IMO and I am SURE there will be lots of disagreements on this, removing the ground ONLY on the laptop power supply has a very very slight chance of danger. IN fact a while back when I did a lot of checking on this I found that a number of the laptop power supplies for dell for instance were shipped to different locations in the world some with three prong ground and others with just 2 prong. The problem is TYPICALLY IF you have a problem with noise in the laptop audio or view it usually means that the power supply is not constructed properly and there is a connection between the ac ground and the dc ground in the powersupply which is causing the problem. SO your options are .....

Sharyn

 

[DrPinto]

IMO and I am SURE there will be lots of disagreements on this, removing the ground ONLY on the laptop power supply has a very very slight chance of danger. IN fact a while back when I did a lot of checking on this I found that a number of the laptop power supplies for dell for instance were shipped to different locations in the world some with three prong ground and others with just 2 prong. The problem is TYPICALLY IF you have a problem with noise in the laptop audio or view it usually means that the power supply is not constructed properly and there is a connection between the ac ground and the dc ground in the powersupply which is causing the problem. SO your options are .....

Sharyn

That makes sense.

 

[ruinexplorer]

I suppose we could switch to DVI or HDMI. Currently, we are running VGA over ethernet cable. Pardon my ignorance, but how would a digital signal differ?

The main reason that the digital signal is preferable over the analog signal is that digital is generally a progressive image. The hum bars are generally caused by a difference in frequency of the vertical lines being drawn in the interlaced image. This can be caused by several factors including ground loops, RF interference, or the proximity to capacitive frequencies caused by dimming systems. All of this is carried on the shielding of the cable. By switching to a progressive digital signal, you effectively eliminate the hum by eliminating how the picture is created.

Thinking about it further, I have a few other observations:
-There were still hum bars when I unplugged the laptop from power while still connected to the board.
-The laptop was plugged into booth circuit 2, while the sound equipment is plugged into booth circuit 1, and the amps/processers are plugged into another circuit from the same panel.
-The projector is plugged into a circuit coming from a different panel.

I don't know if any of that may or may not matter, but I figure all the information I can come up with could possibly help.


Just instinctually, I feel that simply isolating the sound feed would solve matters, seeing as that is the connection that is screwing things up. Of course, there is a good chance that I could be completely wrong on this. I'm hesitant to make any major investments, as we have yet to purchase the computer that will live in the booth and are still using my macbook for the time being.

This is directly related to this thread on the laptop power supply causing noise in the audio system. Yes, we get dirty power from the laptop power supplies, they were not intended to eliminate the external problems that we face in the entertainment industry. You have the option of getting a better power supply or using a hum elimination device to correct this problem, but don't lift the ground on the power supply, please.

 

[ruinexplorer]

m

IMO and I am SURE there will be lots of disagreements on this, removing the ground ONLY on the laptop power supply has a very very slight chance of danger. IN fact a while back when I did a lot of checking on this I found that a number of the laptop power supplies for dell for instance were shipped to different locations in the world some with three prong ground and others with just 2 prong. The problem is TYPICALLY IF you have a problem with noise in the laptop audio or view it usually means that the power supply is not constructed properly and there is a connection between the ac ground and the dc ground in the powersupply which is causing the problem. SO your options are .....

Sharyn

Finding another way around, do not defeat this safety device. Sure, you may only have a slight risk, but if someone gets seriously injured or equipment is damaged, then you are at fault. What are you going to do, say you're sorry? No, there are ways around this. Yes, there are ungrounded power supplies for laptops, those are designed differently than the grounded power supplies (there's a great explanation about that in the audio thread). You can try and see if that will change your outcome by switching power supplies (even to see if it was poor workmanship), but in my experience, that has not always solved the problem. I find that desktops are better than laptops (especially consumer grade laptops) at inherently eliminating system noise, though not perfect.