Watching HDTV on a Standard Definition Screen
Henry asks, "Can you Watch HDTV on a Standard Definition TV?"
Standard definition television is typically 480i in places like the U.S. and Japan where NTSC video is standard and 576i in the parts of the world where PAL is the standard. HDTV resolutions are generally either 720p or 1080i (or 1080p for HD-DVD and Blu-Ray content) in all countries. I could break this down even further and get into the various differences between the evolutionary steps of standard definition television, but this breaks down quite simply as standard definition televisions cannot display the full resolution of HD video. SD screens simply can't provide HD-quality viewing. With a little help from an HDTV receiver, you can view HD content on an SD scren, but it will likely be hampered in one or more ways.
For a visual perspective on the differences between SD and HD see the image below. At this point, content deemed standard definition generally is defined as 720x480 in countries with the NTSC standard or 720x576 in countries relying on the PAL television standard. What's thought of as 720p content is 1280x720, with 1080i or 1080p content coming in at 1920x1080.
If you were to put 4 30-inch televisions side-by-side, each with one of the 4 resolutions, the picture below is a visualization of the amount of picture information each screen would display.
You can't tune in HDTV over-the-air broadcasts with a Standard Definition television, just like you can't force your standard definition screen to magically support anything higher than 480 lines of image data.
You can watch HDTV content on a standard television screen using an HDTV receiver connected to the screen. The receiver displays the image on the television at a compatible resolution, often framing the image with letterbox black bars on the top and bottom of the screen, or in some cases, you will also have pillar box black bars on the left and right of the video image as well.
The reason you get the black bars around the image is HDTV content is typically 16:9 aspect ratio, where as standard definition content is 4:3. The black bars represent the unused portion of the screen when displaying the video at it's intended resolution. If you watch Widescreen DVD movies on a standard definition screen, you've seen this many times already.
I've tested this scenario by connecting a Comcast HD receiver to a standard definition television. I know a number of the DirecTV receivers also support a compatibility mode for standard definition screens as well. While this is far from an ideal scenario, if you want to view something only available in HD, with an SD television, there are ways to make it work.