film-based DVDs aren't native NTSC... they use a process called "3-2 pulldown" to take the 24 frame-per-second film progressive signal and "spread" it out to fall neatly into a 60-field-per-second NTSC signal. This is accomplished by splitting each frame in half into fields (gives you 48 fields per second), and then repeating the fields in a 3-2 pattern so they fill the "60 fields per second" slots without the need to speed up or slow down the film.
this is completely different than native NTSC video which is simply raw 60 field-per-second interlaced.
I explain this difference in my original post. The different is meaningful because the *complete and original progressive frames* can be completely and accurately recovered from film material that's been "spread" into the 480i NTSC system via 3-2 pulldown.
Hence, there's no "scaling" or "interpolating" involved in generating a 480p output from film that's been encoded in 480i form as all the original data is present and merely needs to be properly identified and recombined back into the source frames.
This is fundamentally different from making a 480p signal out of *real* 480i which nececitates interpolation since each 60th of a second represents a new instance in time, and so there's nothing to zip back together (ie, there's no progresse, frame content burried in the 480i signal of an actual NTSC video image).
Do a little googling about film and 3-2 pulldown to get a better idea.
Now, of course, many newer displays are natively progressive scan, as well as many newer DVD players; but I didn't know that they started making DVDs from progressive video, (though I admit it would now make sense to do so). Are you aware of any commercial DVD releases of progressive video? Or are you merely suggesting that it is a theoretical possibility?
Since day one, DVDs have been "flagged" so that, in theory, the fields that represent the original frame pairs are identified to make it easy for any player to simply zip them back together to produce a progressive-scan output... which one could argue is tantamount to a "progressive-scan encoded DVD". Again, this has been true since 1997.
However, there's no enforced quality control to this flagging and so many DVDs are improperly flagged (ie, have the wrong fields tagged as going together). Some progressive-san DVD players use flag info to produce the progressive output, as you might imagine, and in cases with improperly authored DVDs with bad flags the result is really bad combing.
The best progressive scan DVD players employ analytical algorithms to watch the 60-field-per-second signal generated by the MPEG2 decoder (most MPEG 2 decoders are "hard wired" to produce 480i) and when it sees the 3-2 field pattern it recognizes the signal as a "film" signal and folds the proper fields back into frames... with no loss.
This is called "3-2 pulldown reversal" and I'm sure you've heard of that in conjunction with progressive-scan processing.
With film based material that starts life as 24 frames per second, applying 3-2 pulldown reversal to recover the progressive image from an NTSC signal is a lossless process.