Why 1080p Is A Magic Number (And Letter)

Scan through coverage of the recent Consumer Electronics Show in Las Vegas?a city I loathe with all my heart and soul, but never mind that?and you?ll see the mysterious recurrence of four numerals and a letter. By the first day of the show, several manufacturers were boasting that their next-generation video displays would support the 1080p signal format.

The list included not only established brand names like JVC, LG, Samsung, Sharp, Toshiba, and Vidikron but also up-and-coming names like Optoma, Syntax, and Westinghouse?and I?ve undoubtedly missed a few. Evidently all these companies believe 1080p is the next big thing in HDTV. But what exactly is 1080p and why is it worth bragging about?

Let?s cut to the chase: It?s the highest of high-definition signal formats, with 1080 vertical pixels by 1920 horizontal pixels, and uses progressive scanning. There are no 1080p signal sources yet?at least, none available to consumers?but it?s still handy as a picture-enhancing upconversion format. If you know what I?m talking about, you might as well skip the rest of the column and click on ?discuss this article.? For the rest of you, I?ll generate some more words, as I so often do.

The DTV standard used in the U.S., Canada, and Mexico is named ATSC, after the Advanced Television Systems Committee, the industry-led group that originated it. ATSC allows resolution as high as 1080 by 1920 pixels, but only in an interlaced format. That means the picture is scanned in two passes, or ?fields,? each lasting 1/60th of a second. The first pass leaves blank spaces between lines, which are filled in by the second pass. Because each field takes 1/60th of a second, and there are two of them, 1080i actually needs 1/30th of a second to convey a full frame?it is a 30-frames-per-second medium. Fixed-pixel displays don?t need to use scan lines but approximate the same thing in dots.

The upside of interlaced scanning is that it conserves bandwidth while still providing more than two million pixels onscreen (1080 times 1920 equals 2,073,600). The ATSC adopted interlacing for the 1080-line format specifically to fit over-the-air HDTV into the same 6MHz bandwidth as an analog TV channel (though 1080i also travels via cable and satellite). The downside of interlacing is that it induces motion artifacts and other forms of distortion. The best example would be a horse race. With interlacing, the horses? legs become a blur.

But 1080i isn?t the only signal format being used by broadcasters, cable, and satellite. The ATSC also approved a 720p format that delivers 720 by 1280 pixels every 1/60th of a second in full frames?without interlacing?effectively doubling 1080i?s 30 frames per second to 60 fps. That?s called progressive scanning and it?s basically how your PC monitor works. Although 720 times 1280 adds up to fewer than a million total pixels, or 921,600 to be precise, it still qualifies as HDTV as defined by the ATSC, the Consumer Electronics Association, and HDTV makers.

Don?t underestimate the visual power of 720p. It?s a major player in LCD flat panels and DLP projectors (though the flood of 1080p announcements at CES has started to change that). In fact, 720p actually delivers about the same number of pixels as 1080i in the same amount of time. Remember, 1080i can only deliver half its pixel count in 1/60th of a second?half of 2,073,600 is 1,036,800. That?s not much more than 720p?s 921,600.

What 720p excels at is temporal resolution?in our hypothetical horse race, you can see the horses? legs move. What 1080i excels at is spatial resolution?when the picture is relatively static, you get more sharpness, assuming the signal source is good enough.

The broadcasters have lined up behind one format or the other. CBS, NBC, and PBS use 1080i, while ABC and Fox prefer 720p. As a practical matter, whatever is fed into your HDTV will be converted to its native resolution.

So if you watch a Fox football game, using a 1080i tube-based rear-projector, the network?s 720p signal will be converted to 1080i. If you watch Letterman on CBS, using a 720p LCD flat panel, the 1080i signal will be converted to 720p. These conversions aren?t necessarily bad?I?ve spent the past several weeks watching PBS?s 1080i signal being converted to 768 by 1366 pixels by a 32-inch Hitachi LCD TV and it looked fantastic.

Where does 1080p fit into this? In terms of the video pipeline, nowhere. The signal format is a player in production, archiving, and other pro uses but you can?t get it via broadcast, cable, satellite, or DVD. Whether either of the two rival high-def DVD formats will ever use it is unclear at this point. So why would anyone want a 1080p display?

Upconversion is the answer. For instance, your CDs use a 16-bit format, but a good receiver will process them at up to 24 bits, shaping and smoothing out the data in various ways. HDTVs that upconvert input signals to a native resolution of 1080p do the same thing?and the main beneficiary will be that other member of the 1080-line family, 1080i.

Remember, 1080i delivers only a half-frame every 1/60th of a second, while 1080p can deliver a full frame in that amount of time. That gives the video processor some headroom that it can use to manipulate the signal. This is exactly what a progressive-scan DVD player does, except that it converts a 480i signal to 480p. If you can see the difference between an interlaced-scan DVD player vs. a progressive-scan model, you may see the difference between 1080i and 1080p.

I don?t want to hype 1080p too much. First, we won?t get the full benefit of it until it moves from an upconversion format to a consumer video distribution format?probably not any time soon. Second, the real limitation now is not the number of pixels or lines of resolution or even the scanning format?it?s the MPEG-2 video compression used in both HDTV and DVD.

The real-world significance of the move to 1080p is that 720p HD-LCD flat panels?currently priced beyond what many of us can afford?should drop in price. After all, if manufacturers think they can tool up their production lines to reliably deliver more than two million pixels without too many dropouts, then fewer than a million should be a piece of cake. If 1080p is the new state of the art, then all those 720p-capable 32-inch LCD TVs selling for $2000-4000 should sell for less than half that by, say, oh, Christmas 2005. A 32-inch LCD set for less than a grand? I?m not promising anything?actually, I may be jumping the gun?but that?s something to look forward to!

So we?ll take what we can get: 1080i and 720p upconverted to 1080p, and cheaper 720p displays. Hey, even with its current set of compromises, HDTV looks great?and anything?s better than analog television.


Mark Fleischmann is the author of Practical Home Theater (http://www.quietriverpress.com/).

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