How Projectors Interpret Formats

How a 4:3 XGA Projector Handles an Incoming 16:9 Signal

Over the next several years, the current NTSC television standard will be replaced with the HDTV standard. As alluded to above, a wide-screen video signal (with a 16:9 ratio horiz:vert) cannot be mapped onto a projector's internal panel whose dimensions are in a 4:3 ratio. Nor will the incoming signal's resolution (in terms of lines) exactly match the projector's "fixed" resolution. The following two scenarios help explain how these situations are resolved:

Scenario 1:

Suppose you have a projector containing an XGA panel (1024 pixels wide x 768 pixels/lines of resolution, a 4:3 ratio horiz:vert), with an incoming HDTV video signal (1280 pixels wide x 720 pixels/lines of resolution, a 16:9 ratio). When the incoming image is mapped onto the projector's internal panel, it needs to first be "shrunken" by 20% so that the original 1280-pixel-wide HDTV image will fit onto the 1024-pixel-wide XGA panel. The proportional 20% shrinkage in the vertical dimension will result in an image that no longer contains 768 lines of resolution, but rather 575. This necessary loss of resolution occurs in spite of the fact that an XGA panel has the capacity for 768 lines of resolution, and an HDTV signal consists of "only" 720 lines. But the final picture, when projected onto the big screen, will still look great -- far superior to standard TV's 480 lines.

Also note that because the resulting 575-line image is less than the projector's internal panel's 768-line capacity, only 80% of the panel's vertical dimension will be covered with image, and the rest will be blank. Therefore, when centered and projected onto a 4:3-ratio screen, the resulting picture will have the so-called "letterbox" effect, with dark horizontal bars visible both above and below the image*.

* Some projectors will project black in the blank area, while others will actually turn off the pixels for darker black area.

Scenario 2:

Take same projector with an XGA panel, and an incoming DVD video signal. DVD uses the NTSC standard (640 pixels wide x 480 pixels/lines of resolution, a 4:3 ratio) so that discs can be fully viewed on standard television sets. However, most DVDs are also "enhanced for 16:9," meaning that they contain an additional encoded 16:9 image consisting of the same number of lines of resolution (480) but extending across a full 854 pixels horizontally. Therefore there is no loss of resolution when viewed on an XGA home-theater projector.

When this 16:9 image is mapped onto the XGA projector's internal panel (which can accommodate a line as wide as 1024 pixels), the full image can be accommodated without first having to be "shrunken." (In fact, in this case, it will be "stretched" by about 10% so that the 854-pixel_wide signal will fill the 1024-pixel-wide panel. The corresponding 10% stretch in the vertical dimension increases the image to about 575 lines high, still less than the panel's 768-line capacity, thus resulting in a "letterbox" effect.) Since there is no loss of resolution, the projected picture will be of stunning quality.

Will an SVGA Projector Do as Well as an XGA?

What about those very inexpensive SVGA projectors? Well, if you want to enjoy HDTV, don't even consider one. Because of its much lower-resolution 800 x 600 internal panel (as compared to an XGA projector's 1024 x 768) the final picture will contain even less resolution than a standard NTSC TV signal! This is because, when an incoming 720p or 1080i HDTV wide-screen image is mapped onto the projector's panel, it needs to first be "shrunken" by 37.5% (if 720p) or 41.7% (if 1080i), so that the original 1280-pixel-wide HDTV image (if 720p) or 1920-pixel-wide image (if 1080i) will fit onto the 800-pixel-wide SVGA panel. The proportional percentage shrinkage in the vertical dimension results, in either case, in an image that no longer contains 720 or 1080 lines of resolution, but rather 450, which is less than standard TV's 480 lines!