Good question. What does happen to a high-resolution 1080i signal on a 1024X768 plasma panel? Say you're looking at a live U.S. Open tennis broadcast and CBS's HDTV court camera has zoomed in on Anna Kournikova's ear for a second as she serves.
According to CBS engineer Bob Ross (his 8/10 post here
), the camera isn't filtered and should deliver full-resolution 1/30-sec frames with 1920X1080 active pixels. In bright sunlight the camera might capture an ultra-fine blond hair, say, 1 or 2 pixels wide near Anna's ear.
Say that Kournikova blond hair signal (KBHS
) makes it all the way to your HDTV receiver, despite MPEG-2 compression. Since there's little movement during that one-second shot, MPEG-2 isn't filtering high-resolution details such as the KBHS
But the KBHS
may not make it past the next step. According to the mid-1990s FCC's experts committee
, whose report I simplified here
(8/22 post; forum conversion scrambled chronology), your receiver's built-in filtering will whack some 20 percent off the horizontal resolution to minimize interlace artifacts.
So, the FCC's experts measured 800 X 1638 pixels for a stationary black-and-white test pattern. No doubt they used CRT displays with a 1080 horizontal-line raster. But their 1080 X 1920 test pattern, after filtering, only displayed 800 lines by 1638 pixels (see the 8/22 post above). A 1080-line CRT raster remains constant, but an interlaced video signal on such a raster usually delivers about 70% of its static vertical resolution to viewers because of the so-called Kell factor.
Say the KBHS
makes it through, taking up two of the 1536 horizontal pixels per line involved (1920 - 20%). Next, circuits in the plasma panel combine all the pixels in the two 1/60-sec fields making up each 1/30-sec frame into a single 1/60-sec progressive frame.
Then the scaler has to squeeze this image into the plasma panel's 1024X768 pixels. Numerous algorithms can be used to manipulate the HDTV pixels, but the KBHS
undoubtedly won't survive in its original form. Perhaps adjacent pixels would be blended that create the appearance of a single hair. Perhaps the KBHS
would be filtered out.
Higher-grade CRTs can display the resolutions provided with 1080i signals after receiver filtering. High-end monitors can display an unfiltered full-resolution 1080i signal. Also, D-ILA systems use a hybrid progressive/interlace technique outlined here
(my 9/17 post). Notice not everyone favors this hybrid technique. -- John