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Thursday, January 11, 2007

High-Definition Video For Enthusiasts

New cameras and software are making HD video capture and editing available to anyone

High Definition
As its name suggests, high-definition video offers much higher resolution than standard-definition video. While digital still photographers refer to resolution in terms of megapixels, videographers refer to it in terms of lines. That's because digital still images are composed of millions of tiny picture elements (pixels), while video images are scanned onto the screen as a series of lines, from top to bottom.

Standard-definition digital TV in the U.S. has a resolution of 480 lines (generally written as "480i") because the images are interlaced—more on that shortly. High-definition digital is thus anything greater than 480 lines, but HD generally is taken to mean resolutions of 720 lines and 1080 lines.

Since digital video images have specific aspect ratios (width by height), each video line-resolution specification is accompanied by a corresponding number of horizontal pixels. Standard digital TV is 720 x 480, meaning each of the 480 image lines is 720 pixels wide. In still-image terms, that's 480 x 720 (345,600 pixels) or 0.34 of a megapixel—not much as still images go, but equal to the resolution of a standard (non-HDTV) digital television set.

High-definition's 720 lines are 1280 pixels wide, for a still-image resolution of 720 x 1280, which equals 921,600 pixels, or 0.92 of a megapixel—again, not much by still-image standards, but 2.67x the resolution of standard digital TV. HD's 1080-line images are 1920 pixels wide on screen, for a still-camera resolution of 2,073,600 pixels, or 2.07 megapixels—6x that of standard digital video. Some HDV camcorders use the 720-line spec, others 1080. Most HDV camcorders can shoot SD video, too. Of course, to enjoy the full benefits of HD's higher resolution, you must show it on a high-definition TV or computer monitor.

Just for comparison, standard (nondigital) TV has an aspect ratio of 4:3 (640 x 480) in the U.S. Digital's 16:9 ratio gives it the wide-screen feel of theater movies. Standard 35mm still images (and those produced by most digital SLRs) have a 3:2 aspect ratio, while still images shot with Four Thirds System D-SLRs have a 4:3 aspect ratio, like standard TV.

Note that video resolution is simply lines (or pixels x lines), not lines per inch or pixels per inch. A 720 x 480 image contains 720 pixels across the screen. If the screen is 10 inches wide, there are 72 pixels per inch. If the screen is 100 inches wide, there are 7.2 pixels per inch. With today's giant-screen TVs, standard-definition image quality starts to look bad. HD video looks better on the big screen.

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