Is SVGA resolution enough for optimum video?
April 12, 2001,
There is a popular myth espoused by many projector salespeople that since an NTSC or DVD video signal is 480 lines, then an SVGA-resolution (600 lines) projector is plenty adequate to resolve all of the information in the video signal. "No point in buying an XGA machine," they say, "it's overkill."
Those who promote this myth are sorely mistaken. But it is a prevalent belief, so it's an issue that needs to be clarified. So here we go…
The Difference between SVGA and XGA
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. On the other hand, XGA resolution projectors have a physical display matrix of 1,024 pixels across and 768 down—or 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, "Gee, 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." Unfortunately, two seconds of thought is not enough. There are four issues that bear upon image quality that need to be taken into account: scaling, screen size/viewing distance, pixel visibility, and color definition.
Issue #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 scaler, the picture will look terrible. But on most of the newest projectors the scalers 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 orignal 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.
Issue #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, say the InFocus LP340 (SVGA DLP) and the InFocus LP350 (XGA DLP) 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 the LP340 and LP350. 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.
Issue #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.
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 either the LP340 or the LP350 when they are displayed side by side with 4-foot wide images. 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 pixelation than does the XGA.
Another note of caution—don't mix technologies when doing a comparative test like this. DLP at any resolution has less pixelation than its LCD counterpart at the same resolution.
Issue #4: Color 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 color. For any given image size if you focus on color quality alone in a side by side demo, you will see color tends to look a bit more refined or elegant on an XGA unit than it does on SVGA.
Don't mix technologies for this comparison either. You can see this difference in DLP if you compare something like the LP340 with the LP350. You can see the same effect when comparing the NEC VT440 (SVGA LCD) vs. its XGA counterpart, the NEC VT540.
XGA resolution projectors are usually capable of delivering sharper images with less pixelation and better color 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.
The vital issue is screen size relative to viewing distance. If you have a relatively small screen and you are viewing it from a distance of much over twice its width, there is not much reason to go for the XGA. But if you view from a distance of twice the screen width or less, your incremental investment in an XGA machine will show up visibly on the screen.
Having said all of this, keep in mind that SVGA machines are much less expensive than XGAs. For those on a budget, video quality even in SVGA resolution has advanced to the point where some projectors can produce very exciting images for the money. In particular if you want to spend well under $3,000, the InFocus LP340, also being sold as the Boxlight CD-455m, is an outstanding performer for the investment.
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