SVGA, XGA, SXGA and UXGA are terms that describe common resolutions used by computers and projectors. The table below shows you the number of pixels that are displayed in each common resolution and the resolutions of a 16:9 image on a 4:3 native aspect ratio projectors and a 4:3 image on a native 16:9 projector.

Input Signal Resolutions?

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You may have noticed that our projector specifications state 1080i, 720p etc under Compatibility - HDTV but what are they? Separating the number from the letter "1080" refers to the vertical resolution 1080 pixels and the "i" refers to the signal being Interlaced ("p" is for progressive scan). Other signals are: 

How does this apply to projectors?

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The Difference between SVGA and XGA

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As we said earlier most SVGA resolution LCD and DLP projectors have a physical matrix on their displays consisting of 800 pixels across and 600 down. That means there are 600 horizontal lines. XGA projectors have a physical display matrix of 1,024 pixels across and 768 down-768 horizontal lines. A quick calculation shows that XGA machines use 64% more pixels to display an image than do SVGA machines.

Anyone who gives it two seconds of thought will say,

"Hay, if the video signal has only 480 lines, then SVGA resolution has more than enough lines to display the video information. So stepping up to XGA is a waste of money—you can't squeeze any more info out of the signal."

Wrong! There are four issues that bear upon image quality that need to be taken into account: scaling, screen size/viewing distance, pixel visibility, and colour definition.

1. Scaling

If a projector displayed a 480-line video image in 480 lines, there would be no scaling. You would see a relatively pristine picture because you are viewing it without any unnatural scaling alterations attempting to stretch 480 lines of information into a 600-line or 768-line display.

By scaling a 480-line image up to 600 lines, the picture gets fuzzed somewhat since 480 lines of information cannot be stretched to fit across 600 lines as cleanly as it looks when displayed line-for-line. If the projector has a bad internal scalar, the picture will look terrible. But on most of the newest projectors the scalars are much better than they used to be. The net result is that the image on a good SVGA machine is reasonably clean but softer than it would be if displayed without scaling.

On an XGA machine, the 480 lines are scaled into 768. With the increased lines of resolution and 64% increase in pixel density, the scaling errors are smaller. There are more pixels available to approximate the original unscaled image. So the image is fuzzed less than it is on an SVGA machine. The result is that, side-by-side, an XGA resolution machine will generally deliver a sharper picture than will the SVGA.

Now please note, this comparison only holds for like technologies. For example an SVGA LCD unit will be less sharp than an XGA LCD. Or similarly, an SVGA DLP machine will be softer than its XGA DLP counterpart. Since LCD by its nature tends to be sharper than DLP for any given resolution, mixing technology types will confuse a comparison.

So. The bottom line is that XGA is capable of producing a sharper image than is SVGA, but not because it magically gets more out of the 480-line video signal. It doesn't. Rather, it is because the higher resolution XGA scaling softens the image less than does SVGA.

 

2. Screen Size and Viewing  Distance

Your screen size and how far you sit from it is intimately related to image quality in this discussion. Assume you have an 8-foot wide screen and you set up two projectors side by side. Arrange them so they both have 4-foot wide images side by side on the screen and feed them both the same signal with S-video from a DVD player.

Now step back to view your demo from about 10 feet. Guess what? You won't see any difference in sharpness between them. The differences between them at that image size, and viewed from that distance, are too small for your eye to resolve.

Now move these two projectors back so each of them fills the 8-foot screen. Then alternate the projected images by covering one lens then the other. From the same distance of ten feet you will see that the picture from the XGA unit is quite obviously sharper.

So. Part of the "image quality" question surrounding SVGA vs. XGA has to do with your screen size relative to your viewing distance. If you intend to watch movies at a distance of 1.5 times the screen width, you will definitely see a big improvement with XGA over SVGA. If you view at a distance of 2.0 times screen width, the XGA will still have an advantage in sharpness. If you view at a distance of 2.5 times the screen width you won't notice any difference at all worth paying for.

3. Pixel Visibility

In video the visibility of pixels can interfere with your enjoyment of the image. In all cases XGA with its 64% pixel density advantage will have less visible pixels than the SVGA counterpart. A 100" image on a VGA projector has 64 pixels per inch, SVGA has 100 and XGA gives 164 pixels/inch. This difference greatly reduces the screen door effect on LCD projectors.

Once again, screen size and viewing distance are relevant factors. Let's replay the demo we just discussed. At a distance of 10 feet, you won't see any pixel structure On both the pixel structure is too small for your eye to resolve. But when each projector is blown up to the full 8-foot width of the screen, you will find that the SVGA unit has much more visible pixilation than does the XGA.

Example: on a modest projection screen of 2 metres width, from an SVGA projector each pixel is going to be a quarter of a centimetre wide, whereas with an XGA projector the image is going to be under a fifth of a centimetre wide, and over 60% more pixels are displayed. This means the image is going to be sharper and less 'blocky' when projecting with an XGA projector.

4. Colour definition

For any given image size XGA machines give you 64% more pixels. That means there is more capability to define shadings and nuances of colour. For any given image size if you focus on colour quality alone in a side by side demo, you will see colour tends to look a bit more refined or elegant on an XGA unit than it does on SVGA.

Conclusion

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XGA resolution projectors are usually capable of delivering sharper images with less pixilation and better colour for any given screen size than their SVGA counterparts. The notion that since video has only 480 lines, you only need SVGA to display everything in the signal is a simplistic and erroneous way of thinking about the issue.

SVGA offer a cost advantage over XGA so if price is a major issue consider a used projector with XGA resolution rather than a new SVGA the difference is huge!

Resolution and Computers

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