Scaled vs. HD
Dear Gary: In Issue 50, page 22, Contributing Editor Alan Koebel wrote this in response to a letter writer on the subject of SDI connection for DVD players: “And SDI does not lead to ‘virtual HDTV image performance.’ Even when using analog components, true HD is much better than scaled standard definition regardless of how the latter is connected (provided the display device is capable of showing the difference).” In Issue 51, page 28, Bill Cruce, explaining progressive scan (as performed first by computers), wrote: “Indeed, inexpensive computers can deliver images with high resolution, beyond even that of HDTV.” Could you please explain those two statements further? Thank you
Otelio Garcia, Makati City, Philippines
mailto:otel.b.garcia@shell.com.ph
Contributing Editor Alen Koebel & Contributing Editor Bill Cruce Comments
Contributing Editor Alen Koebel Comments: Broadcast HDTV images come in two flavors: 1080i and 720p.The former has an image format of 1920 pixels x 1080 lines (which is actually sent in two sequential fields of 540 lines each, but we needn’t concern ourselves with that detail here). Images in 720p have a format of 1280 pixels by 720 lines (with all the lines sent as a single frame). DVDs store images with a pixel format of either 720 x 480 (NTSC) or 720 x 576 (PAL). Clearly, a DVD image contains far less information than an HDTV image. In other words, it has much less resolution. This fact doesn’t change if the DVD image is scaled up to occupy the same number of pixels as an HDTV image. Such a process can’t create new information—more detail. All it can do is take the information that is there and spread it over a greater number of pixels to reduce the visibility of the pixels or scan lines. Given the huge disparity in the number of pixels in a DVD image compared to an HDTV image, an HDTV transfer (assuming a movie transferred from film) would have to be extremely poor for it to look no better than a DVD image, even when the DVD image was transmitted to the display over a pristine digital connection. The only other situation that would make HDTV look no better than DVD is if the display device itself has insufficient resolution. A perfect example would be a DLP projector with a native pixel format of 848 x 600. While some such projectors accept both 1080i and 720p HDTV, they display the 16:9 images using only 848 pixels by 480 lines. That isn’t nearly enough total pixels to properly display an HDTV image. In my experience, a fixed-pixel projector needs at least an XGA panel—resulting in an effective display format of 1024 x 576 for 16:9 images—to make the difference between images from DVD and HDTV clearly noticeable. Contributing Editor Bill Cruce Comments: Computer displays all use progressive scanning, which produces a smoother, higher resolution image than interlaced scanning. A common computer format for a 4:3 monitor is 1280 x 1024 pixels, which displays 16:9 HDTV images as 1280 x 768. This describes the number of pixels in the visible area of the picture. The “1280” tells you the number of pixels in each horizontal line, while the “768” tells you the number of vertical pixels (or scan lines). This is virtually the same number of pixels and scan lines found in a 720p (1280 x 720) HDTV display. The computer I am writing this on has a video graphics card that goes up to 1600 x 1280, and displays 16:9 images as 1600 x 960. This is not quite as many pixels as the 1080i HDTV format (1920 x 1080), but it’s progressive. The 1080i format is interlaced and each 1080i field is only 1920 x 540. So the 1080i format has 35 percent more pixels for static images (where the image is defined by the entire frame), but the computer image has 48 percent more pixels for moving images (which is defined by the field for interlaced formats). Yet another computer format is 1920 x 1440, which produces 1920 x 1080 progressive 16:9 images. This is twice the resolution of 1080i for moving pictures. And there are still higher computer formats available. However, it is also generally recognized that very few display devices can actually display 1920 non-overlapping pixels per line, and few if any HDTV broadcasts have details that use all of that resolution. Likewise, even though my computer can generate 1600 x 1280, my 19-inch monitor can’t faithfully display that resolution, though there are computer monitors that can.
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