Is 720p Necessary?
Dear Gary: I respect what Widescreen Review is trying to do for HDTV. But I can’t help wondering if, perhaps in our enthusiasm to get the big stations to adopt 720p, we are forgetting a lesson we have learned with current NTSC material? Am I wrong to assume that most movies that will make the transfer to hi-def are first telecined progressively at 24 fps (frames per second) or 24p, then the data reduced to an interlaced form for video release, and finally 2:3 pulldown added by the DVD player to make it 60 fps (or 60i)? My point is that of all the movies I watch or have in my DVD collection, I think only a handful were shot as video, and all the rest originated on film. Data from all the DVD movies that originate as film can be reconstructed to its original progressive form (24p), then converted to 60 or 72 fps for viewing. DVDs that originate as video present some problems for progressive scan players, but the ones which have DCDi™ make it very watchable. But I digress. Back to my point. Why persist in making 720p a widely adopted standard? If indeed the hi-def material is progressively scanned as 1080p at 24 fps in the first place, and if it gets to go through the same process to make it 1080i at 60 fields per second, why can’t a company like Faroudja make another wonderful deinterlacing box using the same principles as what we have for our 480i DVDs to get 1080p, at first in the analog domain and then perhaps later, directly from the digital bit stream? I can see the attraction of 1080i for broadcast and manufacturers because it is not such a bandwidth hog as 720p, and if in the end we can deinterlace and display our movies successfully (may need some downconversion as I understand that most displays won’t handle 1080p), then 720p may not be such a desperate necessity. Am I barking up the wrong tree or can this be explored further? P.S. Did you get your hands on the hi-def version on the latest T2 DVD? Is Microsoft launching its rise of “his” machines? Are you doing a piece on that, the MPEG-4 codec, etc.?
Andre Lam, Singapore
Contributing Editor Joe Kane Comments:
We’ll be covering MPEG-4 and Windows Media® 9 in future issues of WSR. Hopefully by then you’ll be on firm ground with interlaced versus progressive because it’s only going to get more complicated as we address these additional issues. We recognize how confusing all of the HD formats can be, especially when you don’t live with it every day. We’d like to start by restating that the 1080i/60 system is the data hog, not 1080p/24 or 720p/60 or 720p/24. Cable companies here in the United States initially refused to carry 1080i/60 because it was so bit-intensive. When 1080p/24 is converted to 1080i/60 there is about a 30 percent loss in vertical detail, yet 1080i/60 requires more bit space to get it to you. The detail lost in that p-to-i conversion can never be fully recovered in going back to progressive video. Why not just switch over to 1080p/24? Unfortunately there are a large number of DTV receivers that cannot properly deal with 1080p/24, let alone display devices. This is true even though their manufacturers claim to be fully compliant with ATSC Table 3, which includes 1080p/24. We openly acknowledge that detail is also lost in the conversion from 1080p/24 to 720p, but 720p/60 requires about 25 percent less bit space to transmit than 1080i/60. It is also more likely that affordable high-quality 720p display devices will be available to consumers than 1080i or 1080p. In defending that statement you probably know that we believe the majority of “1080i displays” currently on the market can’t properly display 1080i. There also appears to be a hint of confusion between the analog and digital domains. In analog a 480p/60 video signal consumes twice the bandwidth of 480i/60. In that domain 480p/60 is a “bandwidth hog” when it comes to frequency space available for transmitting a video signal. But that’s not necessarily true in the world of compressed digital. It’s highly probable that 480p/60, with its 30 percent greater vertical resolution, would occupy less compressed bit space than 480i/60. In the analog bandwidth of HD signals, 720p/60 and 1080i/60 are within three percent of each other. That is not a difference in which we would attach the word “hog” to one system over the other. It’s part of the reason why we say that if analog displays won’t do 720p they’re probably not HDTV. Details about all of these topics have been covered in several Widescreen Review articles. We believe our support of 720p is appropriate for a transmitted HD signal. We certainly hope to see 1080p being used in program distribution for HD-DVD and other “closed” delivery systems. We stand firm in wanting interlace video to go away. It is an analog crutch and has no added value in a world of digital video.
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