Film-Tech Forum ARCHIVE: VGA baluns

This is topic VGA baluns in forum Film-Yak at Film-Tech Forum ARCHIVE.

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Posted by Brad Miller (Member # 2) on 05-21-2006, 04:34 AM:

Anyone ever used one of these?

Essentially you put a "pc side" balun at the computer and a "monitor side" balun at the monitor and use shielded cat5 cable instead of costly VGA cable.

I have used these one on one (one computer driving one monitor) without any problems, but have found that if I take a VGA cable out of the computer and into an 8 port splitter, then hook up the "pc side" baluns to the outputs of the splitter that none of the monitors work.

I am assuming this is a level issue. Does anyone have any experience with this, or could recommend a splitter with more gain? The one I tried this with was good for 250 feet or something like that (using regular VGA cables, of course).

Posted by Ian Price (Member # 14) on 05-21-2006, 09:33 AM:

The AV Staging and Rental market has been using these for a few years now. The cable is lighter, cheaper and can handle long runs. Last month at NAB there were dozens of small venders just selling Video via Cat 5 boxes. Most had there box hooked up to a monitor with a 1,000' spool of cable.

Posted by Steve Guttag (Member # 268) on 05-21-2006, 10:40 AM:

Brad, is it an active box or passive? If it is passive then it is a stupid device...all one is doing at that point is trading balanced for unbalanced...which will help with noise and that is it.

To prove to yourself what is going on, take a good test pattern (with text and such that will come up in real life)...connect your monitor directly...then use the baluns and 350 feet of twisted pair...now look at the signal....you will have color misalignment and significant signal loss.

A proper twisted pair solution doesn't use Cat 5 but a skew free cable made by Extron, Belden and I'm sure others. Cat 5 (or the other Cat cables) twist their pairs asymetrically for noise and chatter reasons...in video you need the cables to be nearly the same length to avoid your colors not matching up (put up a cross hatch on a long Cat 5 cable to see what I mean).

Additionally, any time one is going to send XGA signal at 60Hz over 50-feet (with a mini-coax cable), they really need an active solution or a notable drop in picture quality will result. It has to do with the bandwidth of the signal versus the losses associated with the cable (capacitivly, as well as resistively). Alternately, one could use RG-6 cable which is about as big as a garden hose when 5 cables are bundled and get up to 300 or so feet without any active solution.

We sell Extron's line of products mostly, though there are others out there too. Extron has many UTP solutions, depending on your application. However, they will all be active boxes and they will make the signal at the other end of as much as 1000-feet of UTP cable look nearly identical to a monitor that is connected directly to the puter.

Extron does have a nifty set of Twisted pair products that could have application in theatres...one buys a single transmitter and then a receiver for each display...the UTP cable is then strung in daisy-chain fashion from one unit to the next rather than a DA system. One sets up the transmitter for the full length of the run and then each receiver is set for perfect level and response so each theatre has identical and perfect appearence.

Extron Loop-Through UTP products

Steve

Post edited to correct mistatement about cable length with VGA above and to add UTP Extron product reference.

[ 05-21-2006, 09:18 PM: Message edited by: Steve Guttag ]

Posted by Tristan Lane (Member # 1169) on 05-21-2006, 02:44 PM:

Another manufacturer that I've had good experience with

Magenta Reasearch

They make both passive and active devices. Magenta Research

Posted by Bruce Hansen (Member # 281) on 05-21-2006, 06:36 PM:

Brad,

I believe this is just a passive device. The only reason I can think of that this would not work, is if the grounding (returns) on the output of the VGA splitter is a little strange. See if you can borrow another VGA splitter, and try it.

Dumb question: have you tried hooking a monitor directly to one of the outputs of the VGA splitter, to see if the splitter itself is working?