How Is HDTV Processed?
Dear Gary: First I want you to confirm that 1080i HDTV is essentially 60 fps (frames per second) at 540 lines each constructed to 60 fields per second yielding a 30 fps image with one “frame” being comprised of two interwoven 540-line images. If I understand 1080i correctly, then I can understand why upconverting 1080i to 1080p would prove difficult. It’s not like any given algorithm can combine fields 1 and 2 (taken from exactly the same moment in time) to create a perfect 1080-line progressive frame. My question is this: Is there any 1080i material that is shot in 30 fps 1080p and then converted to an interlaced, 60-fields-per-second image? And what about movies broadcast in 1080i? Can they not be reconstructed properly (or least as well as progressive DVD) to a 1080p image? While I am on the topic of 1080i movies, how are they broadcast? Three:Two (3:2) pulldown or every fourth frame repeated? Either way, would it not be easy for good 1080i-1080p algorithms to provide a 1080p picture in much the same way as current progressive DVD players do? And what will the case be for future 1080i HD-DVD? The general question here is this: Is there any easy path from 1080i to 1080p?
Jason Steg, Hickory, North Carolina
Video Technical Editor Greg Rogers Comments:
The1080i HDTV format is displayed at a rate of either 60 or 59.94 fields per second. Broadcasters normally transmit 59.94 fields per second, which is the same as the NTSC standard, because it simplifies switching between upconverted and native HDTV content. Frames consist of two interlaced fields, so the normal 1080i frame rate is 29.97 frames per second. The active (visible picture) region of each field consists of 540 (horizontal) lines with 1920 pixels on each line. This produces an active frame with 1080 scan lines. The total number of lines per frame is actually 1125, but 45 of those occur during the vertical blanking interval and have no picture information. The total number of pixels on each line is 2200, but 280 of them occur during the horizontal blanking interval and contain no picture information. In answer to your various questions, I’m not aware of any video being shot at 1080p, 30 frames per second. Normal HDTV video cameras produce 1080i, at 29.97 (or 30) frames per second. There is a 1080p, 24 frame per second standard for archiving and editing film-source video, and the ATSC standard permits broadcasting at 24p. However, an HDTV set-top box would have to convert back to 1080i for compatibility with most HDTV displays. But as far as I know, all HDTV movies are currently being broadcast at 1080i using 3:2 pulldown, just as they are with standard-definition video. It is possible to convert 1080i film-sources (with 3:2 pulldown) to 1080p using inverse-telecine processing, which I have described many times in these pages. However, the HDTV pixel rate is 5.5 times higher than standard-definition video (74.25 MHz vs 13.5 MHz), so it is still very expensive to do inverse-telecine processing at these speeds. Right now there is no inexpensive path to convert 1080i to 1080p. The Faroudja DVP5000 performs this processing for around $25,000. It is far to early to speculate on HD-DVD output formats, which may be 720p or 1080i. But it is safe to assume that movies will be encoded at 24 frames per second.
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