Film-Tech Forum ARCHIVE: Maximum distance for VGA signals.

This is topic Maximum distance for VGA signals. in forum Digital Cinema Forum at Film-Tech Forum ARCHIVE.

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Posted by Stephen Furley (Member # 1277) on 08-17-2013, 01:02 AM:

I should know this, but I don't. What's the maximum distance that VGA signals from a computer can be sent if broken out onto five strands of good quality 75 Ohm co-ax?

Posted by Scott Norwood (Member # 30) on 08-17-2013, 08:29 AM:

Isn't this resolution-dependent?

I am sure that there is real answer to this, but the practical side is that the image will get unacceptably smeary before you lose sync.

Posted by Steve Guttag (Member # 268) on 08-17-2013, 12:27 PM:

Scott is correct. It is all about bandwidth and attenuation. How high is the frequency and what is the attenuation of the cable at what distance and at what frequency.

A typical mini-coax cable can go about 10.5m without visible degradation on XGA signals (1024x768). While you can certainly run longer and get an image...it WILL start to smear. As you move up in frequency (higher refresh rates and resolutions), the distance before smearing drops (geometrically). If you move up to a full-size RG59 or RG-6 (times 5, plus, if you want EDID to travel, you'll need to add additional conductors), you can push that out a bit...as I recall, RG-6 can carry XGA a full 100 meters. Note, only the R, G and B really need the fatter cable...the sync cables can be smaller and survive.

What is more popular is to use an Interface of some kind to boost the overall level as well as high frequency (think of it as a slit-loss correction) so that at the other end of the cable, you have essentially the same signal as if it was coming out of the device.

http://www.extron.com/product/product.aspx?id=rgb160xi&subtype=1&s=3

What most have taken to is to run everything on a CAT 5 style solution. Most are proprietary.

http://www.extron.com/product/product.aspx?id=mtpt15hda&subtype=5&s=4

Generally, the twisted pair solutions start to be more cost effective on longer runs. The cable is cheaper and easier to pull in though more fragile.

Posted by Stephen Furley (Member # 1277) on 08-17-2013, 05:10 PM:

No Cat. 5 installed, but there is some old co-ax. I don't know exactly what it is, but it's only about 25 metres, so it's worth giving it a try.

Posted by Kris Verhanneman (Member # 5562) on 08-18-2013, 04:13 AM:

Hello,

might be cheaper to add an cat 5 cable but if not you can alway have a look at scalers that give you an output on HD SDI (or SDI).
Like this one for example
Link

Posted by Bruce Hansen (Member # 281) on 08-18-2013, 05:04 PM:

The VGA to cat5 converters that I have seen suck. Stick with coax. Don't forget that VGA is ANALOG.

Posted by Steve Guttag (Member # 268) on 08-18-2013, 10:28 PM:

It all depends on what you use. The Extron ones are generally pretty good and even better if they are set up properly. To the point, you'd be hard pressed to know that the Cat 5 solution was used...even with an O'scope (because that is how I set them up for proper level and frequency response).

Posted by Stephen Furley (Member # 1277) on 08-20-2013, 04:01 AM:

Tried it, and it works ok. Five strands for the VGA plus a sixth carrying S/PDIF from an amp for a mic. Not ideal, but it's there and available, and it will do the job. It's only needed for a few minutes.