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Old 01-22-2010, 03:43 AM   #1
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Default VGA Cable Questions

Ok have a question about a vga cable.

We install CCTV systems, we just finished a 32 camera install.
We are powering two 42" LCD TV's from one computer with 50ft VGA cable. Quality looks good, no problems with video on both TV's. we did have a problem with computer freezing at first which is fixed now. My boss seemed to think that the video card has to work harder to send video to both TV's with longer cables.

Just wondering how the video card signal works. I figured that the video card sends a set signal to any device that plugs into it. If you have a 5ft or 200ft cable the card wont have to work harder to send a signal, the signal will just drop and you will have poor video. Am i wrong or does the video card detect a device over a longer run an works extra hard. I think i read somewhere that VGA cable peak voltage is at .7. Sorry for this stupid pointless question but it has been bothering me. thanks

2nd question. Does the video card have to work harder to power a 42" TV over a 19" monitor? i have onboard video that hooks up to all my tv's at home.


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Old 01-22-2010, 05:29 AM   #2
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Default Re: VGA Cable Questions

You're right that the VGA RGB peak voltage is 0.7V - I've heard in rare cases some displays can support a peak of 1V but .7 is the standard. Careful with your wording though, it's just the RGB pins that have a peak of 0.7, the sync pins work on TTL and therefore run up to 5V.

While I'm not 100% sure, I'd be very surprised if your average video card sent out stronger signals for monitors further away - it's just not a standard designed for working over large distances reliably. Whilst in all sensible applications it tends to work fine, perhaps just picking up a bit of interference, if you really want to push it you should be sending it over Cat5 instead (which IS a standard designed for working over large distances!)

As for your second question, video cards don't care about what size screen they power - they're just sending the information to the screen to display. What will make them work harder however is a higher resolution - so if you're feeding a monitor 800x600 this will be a lot less work than sending it 1900x1200 - regardless of size.

Hope that helps!

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Old 01-22-2010, 05:33 AM   #3
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Default Re: VGA Cable Questions

q1, no. but inteference, noise and signal loss does come into play.
-which is why there is a limit on the length of LAN cables, there will be practical lengths for video cables which are sent as signals.

q2, no. the video card on provides a signal to the video board in the TV/monitor, this video board os powered by the TV, and the screen/inverter/tube or whatever the monitor is built around is powered at the monitor.
the card/machine will work harder to power monitors with higher resolutions however. so you may see a difference in performance running a small screen at 800x600 or a large TV at 1450x1024.

but... resolution is such that now your 19" monitor is likely to be able to use the same resolutions as your 42" TV.

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