Advanced Imaging


Advanced Imaging Magazine

Updated: January 12th, 2011 10:01 AM CDT

What's a TV Buyer to Do?

Is the answer 720p, 1080i, 1080p? It depends upon what you want
HDTV definitely results in a better image, but what resolution should you buy when considering 720p, 1080i or 1080p?

By Barry Hochfelder

How about a few definitions? You have 1080p and 1080i and 720p. The number is lines of resolution, the letter p is for progressive scan and the letter i for interlaced scan. With a progressive scan, the image is captured, transmitted and displayed line by line from top to bottom. The interlaced pattern scans every second line, from top left corner to the bottom right corner, and then repeats the process starting from the second row to fill in the gaps. An interlaced scan uses two fields to create a frame; one contains all of the odd lines in the image, the other all the even lines. Each pass takes 1/60th of a second, meaning a full frame is conveyed in 1/30th of a second.

Interlace was the technique of choice for improving picture quality by removing flicker from the time it was invented in 1932—and first demonstrated two years later—by an RCA engineer. It was created because cathode ray tubes were becoming brighter, increasing the level of flicker caused by progressive scanning. It was the top choice in television until the 1970s when the needs of computer monitors resulted in the reintroduction of progressive scan. Interlace still is used for most standard definition TVs and 1080i HDTV, but not for LCD, DLP or plasma displays.

Progressive does provide higher vertical resolution than interlaced with the same frame rate; it doesn't have the visual artifacts of interlaced video of the same line rate so it doesn't require intentional blurring of video to reduce interline twitter. It also offers better results for scaling to higher resolutions than equivalent interlaced video, such as upconverting 480p to display on a 1080p HDTV.

So what's the problem with 1080p? "It's really a misconception," says Gary Pitre, Eastern Regional Sales Manager—Imaging Systems Division, at Toshiba America Information Systems. "In talking to industry professionals, there's nothing in the foreseeable future that will be sent out in 1080p. The bandwidth isn't there. It's a lot of data."

The digital TV standard in the United States, Canada and Mexico is the ATSC (Advanced Television Systems Committee). ATSC supports resolution up to 1080 by 1920 pixels in an interlaced format at frame rates of 24, 25 and 30 frames per second. The ATSC also approved the 720p format that delivers 720 by 1280 pixels every 1/60th of a second in full frames, which doubles 1080i's 30 frames per second.

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