Thursday, April 26, 2007

A Quick Primer on Digital Video Cables

I just purchased a beautiful 22" LG LCD Monitor. The prices have come down tremendously and I thought it was long overdue to get one. The fact that I can now work from two screens (my laptop's native one and this one as well) has improved my productivity a great deal.

On the back of the monitor is the standard 15 pin VGA port that has long been on computers. But now there's a new one as well called "DVI", short for Digital Visual Interface. I don't have this on my computer but a friend of mine does, so I got to wondering what differences one might expect to see with it.

As I often do in such matters, I turned to my good friend & colleague, Geoff Meredith, who is the guru of gurus for anything related to computer hardware or software. As one past programmer in my company once said about him, "He answers questions you're thinking of before you even ask them!" Here's what Geoff had to say about digital video cables:

It's quite possible that the VGA cable is good enough. The issue is that VGA is analog and DVI is digital. The DVI cable will give you perfect signal reproduction and VGA will be degraded to some extent but if the VGA cable is good and the environment is not too electrically noisy, you are not likely to see a difference. Also, as the frequencies go up, as they do with increased resolution, the chance that VGA signal will degrade goes up. So as your monitor size gets bigger, DVI becomes more important.

The degradation that I've seen in VGA signals seems to come from capacitance issues that affect harmonics of the signal. For instance this rounds off the edges of a square wave. In the analog world this give you ghost signals that you see as slight shadows or ghost lines at high contrast vertical edges such as next to the letter "I" when it's black on white (or even more, white on black). If you can see ghosts of letters slightly offset, then you are seeing VGA cable degradation. If not, you are likely getting as good a signal as you are likely to get.

While there is a theoretical possibility of colour reproduction degradation with VGA, the colour reproduction on LCD panels are so bad that you wouldn't notice any issue introduced by a VGA cable.

As an example, my 22" is running on DVI and my 19" is running VGA. I can't see a difference in image sharpness between the two although the colour reproduction is radically different. I had meant to run my 19" on DVI but at the time that I bought it, my video card did not have dual DVI outputs and such a card was quite expensive so I decided to wait. It turns out that I really didn't need it.

As for VGA cable quality, one indicator is the thickness of the cable. The thicker the better. Also the shorter the better. As an example of the extremes, we use a 50' VGA cable to connect from our media center to our projector. It's a very thick, and seemingly high quality cable and it does a great job on the 800x600 signal that our projector needs. It will be interesting to see when we move up to 1280x1024 if that cable will still perform well. We paid a ridiculously low price of $28 from NCIX for this cable. Others I was looking at wanted about $150.

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