by Marty Sems
First Glimpse Magazine, April 2008

What makes HDTVs better than the tried-and-true, inexpensive SDTVs (standard-definition TVs) we’ve used for decades? Sure, your typical HDTV has a widescreen for a better movie-viewing experience. It has an ATSC (digital) TV tuner instead of a soon-to-be-obsolete NTSC (analog) tuner. And, depending on the HDTV’s display technology, the set is probably much thinner than the old tube TV it replaced.

All these attributes are attractive, but the thing that really makes HDTV superior to SDTV is the “HD,” which stands for high-definition. An SDTV can natively display video at DVD resolution, meaning with a picture made up of 720 horizontal pixels and 480 vertical pixels, or 720 x 480 resolution. (The vertical height of a CRT TV’s picture is technically measured by the number of its horizontal lines, such as 480 in the United States. For comparison purposes, however, you can think of the number of lines in a CRT as the number of vertical pixels in an LCD or plasma TV.) A widescreen SDTV may have an 852 x 480 image, which many DVD players can fill by digitally stretching the video from 720 x 480 to a wider 852 x 480 or by letterboxing the video.

HDTVs use more pixels to display video than do SDTVs (counting tube TVs’ horizontal lines as vertical pixels). There currently are two HDTV video standards available to consumers through broadcast, cable, and satellite channels, as well as through BD (Blu-ray Disc) and HD DVD movies. A video resolution of 1,280 x 720 pixels is called 720, while the more detailed 1080 standard offers 1,920 x 1,080 pixels. Both standards are more or less named for their pictures’ vertical height in pixels.

The more pixels, the finer the detail the video can show. For instance, in a big, medieval battle scene, you’ll be able to pick out individual foot soldiers in the BD or HD DVD version, whereas they may be indistinct in the DVD edition.

More after the jump!
Of course, your HDTV’s actual picture quality depends a lot on the video you feed it. If you tune in to a 1080 cable channel or play a 1080 movie on a BD or HD DVD player, your 1080 HDTV will show you the video as it was intended to be seen–that is, a pixel-for-pixel reproduction of it.

Play the same 1080 video on a 720 HDTV, and the set will downscale the video to fit in its 1,280 x 720 (or 1,366 x 768) pixels. There will be a corresponding minor or major drop in image quality, depending on the TV; namely, you’ll notice images aren’t as sharp because there are fewer pixels to render the image than were used to create the video. Some 720 sets claim support for 1080 video, but this means downscaling is involved. Downscaling means using a single pixel to show the information of several. For example, a 320 x 240 portable DVD player will squash the video into a lower-resolution image, usually with rather jagged object edges. Check an HDTV’s pixel resolution to determine whether it can natively (without scaling) display 1080 video–that is, it has 1,920 x 1,080 resolution.

Likewise, if you pipe a 720 x 480 DVD into your 720 or 1080 HDTV, either the disc player or the TV will upconvert the video to use the greater number of pixels in the screen. During upconversion, a video processor spreads each pixel in the source video over five or six pixels on the screen. The result could look blocky or reasonably good, depending on how skillful the player or HDTV is at the upconversion process.

I vs. P

There’s one more major thing you should know. You’ll see HDTV resolution standards expressed with a lower-case i or p at the end, such as 720p, 1080i, and 1080p. These letters don’t tell you anything about the resolution of the video, even though they tag along with the resolution numbers.

Instead, i and p tell you how the video is presented on the screen. Interlaced (i) video displays half of each frame of video at a time, with the odd rows alternating with the even rows. For example, an HDTV would show a 1080i broadcast channel as the odd rows of the first frame, then the even rows, then the odd rows of frame two, and so on, faster than the eye can follow. (This is the traditional way TVs have displayed video all of these years. If you used HDTV naming conventions to describe standard TV, it would be called 480i due to its 480-line picture height and interlaced display method.)

On the other hand, progressive-scan (p) video, such as 1080p, shows you an entire frame of video all at once, then the next frame, and so on. All else being equal, progressive video changes only half as often as interlaced video, but most recent 1080p sets increase the refresh rate to 60Hz or 120Hz to compensate.

Interlaced video doesn’t cost as much to display (in terms of both video processor price and bandwidth) as progressive-scan video. However, the edges of fast-moving objects in interlaced video may seem to have staggered edges, like the teeth of a comb. Progressive video is more visually appealing but may seem to flicker on certain TVs and judder with certain types of video, such as footage of fenceposts whizzing by a moving car. To combat these problems, HDTV manufacturers incorporate features such as 120Hz screen refresh rates and anti-judder technologies such as Sony’s MotionFlow and Samsung’s Auto Motion Plus 120Hz.

Bandwidth

The higher the resolution, the more data in the video signal. Also, the higher the refresh rate, such as 120Hz versus 60Hz, the more data. Therefore, 720p video requires more bandwidth (data-carrying capacity) from its cables, equipment, and over-the-air broadcast spectrum than regular 480i programming.

Likewise, 1080p video at 120Hz takes up even more bandwidth. If your HDTV is capable of this mode, use high-quality HDMI cables and AV equipment, such as those certified as compliant with HDMI version 1.3, which can handle more bandwidth than earlier versions.

Resolution chart provided is of the public domain.