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VGA Cable high noise environment

Posted on 2006-05-13
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Last Modified: 2010-04-29
Hi All,

I am involved in a project that needs custom VGA cables to be made which did not pose a problem originally.

I sourced some high quality VGA cable and proceeded to wire the up with particular care. All cables are wired correctly and pin out perfectly. My problem being that some of the are causing ghosting of the image on screen. The environment they are run in in fairy noisy with alot of mains in close quaters.

Are there any extra measures I can take for filtering out noise?

Thanks
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Question by:scottgeorge
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Expert Comment

by:rfportilla
ID: 16675224
Try using magnets.  A lot of VGA cables have magnets around the cable to help with emi.  Typically looks like a round cylindrical thing a few inches away from the cable.  You tend to see this on cables with higher voltage and more prone to emi.
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by:rfportilla
ID: 16675227
It really is worth it to just purchase the cables.
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Accepted Solution

by:
jhance earned 375 total points
ID: 16675254
Ghosting is NOT caused by noise. Ghosting is caused by reflections on the line due to impedance mismatches at connectors other discontinuities in the cable.  A VGA signal is of quite high bandwidth (> 100MHz) and so once the cable is 1/4 wavelenth long or longer (1/4 wavelength @ 100MHzbeing about 0.75 meters) you must have the cable impedance match the source and destination impedance and all connectors must be of the correct type.

Noise from the environment will not show up as ghosting on the image.  It will show up as lines, hash, or interference patterns, or other things that are independent of the image on the display.

What is the length of the cables you've wired?  If more than 0.75 m you should get special cables and cable drivers.  I've used Black Box products for long VGA runs and have had excellent results.
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Expert Comment

by:Gendaru
ID: 16675266
Well, the rfportilla is half right.  They aren't magnets, but ferrites.  Magnets would just interfere with the picture more.  Though I would assume anyone willing and able to build a VGA cable themselves would think to put on ferrites.

Anyways, if not, you can buy them here:  http://www.allelectronics.com/cgi-bin/category/235/Ferrites.html
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Assisted Solution

by:arthurjb
arthurjb earned 375 total points
ID: 16675369
Lots of paartial information here it is corect that they are not magnets, they are ferrites, but they are not there for the picture quality they are there to reduce noise out of the cable to other devices, which is why you don't see them on cheap cables.

The answer from jhance is correct.  The ghosting is caused by the impediance mismatch because the cables are too long.  I

f you look around you will notice that the average vga cables are less than 6 feet and usually about 4 feet.  With cables longer than that you have to use either repeaters, or a different video transport media.  You can also try using a slower sync rate for the monitor, if the video card supports it, since lower frequencies can travel longer cables without as much degredation..

Not know the reason why you need longer cables makes it hard to give specific advice on how to take care of the problem.
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Expert Comment

by:markps_1
ID: 16675634
You have to shield your cable with alluminum foil or a coper wire wick (coper wire mesh). The shield will incredibly reduce the EMI.

  If the problem is that the cable is too long (you haven't said that but this question has been raised)  A simple and cheap solution for your problem is to use a KVM switch as a repeater.. http://www.geeks.com/details.asp?invtid=JC-102CUA&cpc=SCH&srm=0  You'll gain 6 feet with it.
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by:markps_1
ID: 16675640
An interesting thing to pay attention is to make sure the shield is attached to the armature (to the earth ground) on both tips of the cable.
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Author Comment

by:scottgeorge
ID: 16677115
Thnakyou for all of your advice. I have indeed installed ferrites on the cables to no avail as pointed out by some of you.

So my only option if I am correct in what youare saying is to regenerate the signal at the monitor end?
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Expert Comment

by:jhance
ID: 16678028
No, that is not correct!!  By the time the signal gets to the monitor end it's too late, any degradation of the signal has already taken place.  I'm not sure which responses you've been reading.
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by:jhance
ID: 16678053
See:

http://www.blackbox.com/Catalog/Detail.aspx?cid=1578,1588&mid=4961

For an example of something that might assist you.  But I'd call them first and explain you situation and application to their experts to be sure you get the right product.
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Author Comment

by:scottgeorge
ID: 16680884
Sorry I should of clarified be regenerate at the monitor end I meant at both ends. i.e. a Cat5 KVM extender or the like.
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by:markps_1
ID: 16680960
yep that was my solution and you gave me no points... "KVM switch as a repeater"
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by:markps_1
ID: 16681008
KVM stands for "Keyboard Video Mouse" as I've already mentioned you could use a cheap KVM switcher as a repeater.

Actually you don't need to do it on both ends.. just on the middle.. You would imagine that since the recomended cable is from 4 to 6 feet if you plug a switcher on the middle of your cable you could double the range.

 Regardless to say that it will only work if the switcher is powered. :o)
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Author Comment

by:scottgeorge
ID: 16681048
OK here is a late addition I have just tried. The installed cable causes a ghosted image. However if I run a 10m manufactured nVGA cable from computer tomonitor follwing close to the identical path the picture is perfect. Which would indicate that the peoblem lies within my cable rather than the cable length.
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Expert Comment

by:markps_1
ID: 16681069
So THAT lies into my second part of my answer. And I still don't understand why you gave me no points..

-- You have to shield your cable with alluminum foil or a coper wire wick (coper wire mesh). The shield will incredibly reduce the EMI. ... An interesting thing to pay attention is to make sure the shield is attached to the armature (to the earth ground) on both tips of the cable. --

 
The problem is with shielding.. A factory made cable is perfecly shielded.  
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Author Comment

by:scottgeorge
ID: 16681095
The cable is copper braid shielded with foil inside so I do not think shielding is a problem. Shield is connected to armature on both ends which is connected to ground. The cable seems to be of high quality the RGB carriers are all coaxial individually screened.

From what everyone else says however the ghosting is caused by reflections and not EMI.  Is this true?

I may have been a bit early giving out points as it seems my problem has not yet been solved to my satisfaction.
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Expert Comment

by:markps_1
ID: 16681186
Reflections are very unlikelly since your wires are individually shielded.

 Since you've partially eliminated the size of the cable, the shielding. You've got to check now your solder joints, or crimp points on the connectors. Another problem is could be that the guauge of your cable is not the recommended gauge for the size of your cable.
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by:jhance
ID: 16681432
>>Which would indicate that the peoblem lies within my cable rather than the cable length.

Indeed!  Go back and READ my first comment (which you accepted as the answer!).  The length, in and of itself, is NOT the problem.  It's quite possible to make a VERY VERY long cable which will carry the VGA signal very well.  Unfortunately, you don't have the tools, parts, or expertise to do that.

Stuff like shielding, whether the joints are crimped or soldered, cable size, interference have LITTLE to NO effect on this problem (i.e. ghosting).  In fact even heavily shielded cable, if it were the wrong type, could be worse than what you have now!!

This is a TRANSMISSION LINE problem and it's driven entirely by IMPEDANCE.  Impedance, for those that have no clue about it, is a CHARACTERISTIC of all transmission lines and any specific type of cable will have an impedance figure.  In a transmission line system (which you have here whether you like it or not) a wave travels down the wire essentially undisturbed unless it hits a discontinuity.  A discontinuity is the other end of the wire, a splice, a connector, a sharp bend, anything that changes the impedance.  When that happens a portion of the signal is replected back to the source.  If it hits another impedance mismatch on the way back part gets reflected back to the destination.  Etc....  This is the "ghosting" you see.  Reflected signal, each less in amplitude than the original bouncing back and forth on the cable messing up your picture.

How to properly assembled lines/cables prevent this?  By matching the impedance at every point from the source to the destination they reduce or eliminate reflections and you end up with an excellent image.

Why isn't this an issue with shorter lines?  Transmission line effects happen at all lengths but they don't become noticeable until the length of the cable is about 1/4 wavelength or longer.  Below that the effect is very difficult to see because the transit time of the signal is short compared to the frequency of the signal.  Above 1/4 wavelength this starts to change and the problem becomes more and more problematic.

As I mentioned earlier, a VGA signal is at least a 100MHz signal (sometimes more) and so 1/4 wavelength is about 0.75 m.  The effect is negligible at 0.75 m but grows more and more significant as the length grows.

It's quite difficult to make a good LONG VGA cable which is why I strongly suggest buying a commercially prepared solution.  As you saw, your attempt was bad but the one you purchased worked very well.

I'm sorry you've received so much poor advice on this question, but this problem is beyond the understanding of many of the "PC support" types who are participating here in this TA.  It's an engineering problem not a PC tech problem.
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Expert Comment

by:jhance
ID: 16681436
>>Reflections are very unlikelly since your wires are individually shielded.

As I already said, reflections have NOTHING to do with shielding.  They have EVERYTHING do to with IMPEDANCE.
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by:arthurjb
ID: 16682322
The answer(s) that jhance have given are very good.

But I disagree a little, the shielding if improperly grounded at each end of the cable, could effect the impedance of the cable, creating reflections.

The fact that your wires are individually shielded, may even be a partial cause.  For example if the shields are not properly grounded, then the effect could be what is refered to as a ground loop, which changes the impedance causing a mismatch.  When manually making a cable, the more shields you have, the more likely it is that one shiled could not be connected properly.

As jhance has said this is a Transmission line problem and most folks don't understand the complexities involved.  Each step in the cable making process can cause a mismatch, and the shields although they are more important in their effect on noise, can cause a mismatch, resulting in reflections.

I would suggest taking the cable that you made and inspecting each end, make sure that the shields are all connected and not just hanging loose.
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by:rfportilla
ID: 16683088
I understand that we all want credit for the answer, but I think it is better to let it go than to appear childish.  The ultimate goal is for the asker to get his/her answer resolved.  

I've pretty much dropped out of this question because I don't know quite enough about the cabling to answer this effectively.  

scottgeorge, it seems to me that if you already have a working cable then this question is nullified, no?  Or are you just looking for an answer for academic reasons now?  I'm just curious.  I'm somewhat interested in the final result now, but it just seems to be much more work to build your own cable than to just buy prefab.  (Which has always been my experience with vga cables)
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by:jhance
ID: 16683439
rfportilla,

For me at least, it's certainly not about points.  In fact, points have already been awarded here in this question.

It's about information vs. mis-information at this point.  I've tried to clarify what is going on here so that folks (including experts) will understand and why in spite of multiple recommendations about shielding and ferrite beads these things are not really a part of the problem or the solution.

The point that arthurjb made about shielding affecting line impedance is accurate but not really the problem or the solution.  Further, a shielded wire is not even necessarily needed here. UTP cable can and is used quite effectively to carry video as well as many other types of high-bandwidth signals.

What most important is that the cable, connectors, etc. all be of the proper type (i.e. designed for this use) and installed properly to get a good video signal at the far end.
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Expert Comment

by:arthurjb
ID: 16683487
>The point that arthurjb made about shielding affecting line impedance is accurate but not really the problem or the solution.

Technically yes, but the shields exist in the cable he is using, so they are part of the equation, and could be the reason for the problem.

I made the assumption (rightly or wrongly) that he bought cable that was advertised as being correct for vga cables...
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Author Comment

by:scottgeorge
ID: 16683516
Actually I have several valid answers but am also interested further in getting a definative answer as to why this happens which several of you seem able to provide.

jhance you say the the cable and connector need to be fit for purpose can you give any more details?

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