Quote:
Originally Posted by
pk442 
As I understand, a DVD's player that is set toprogressive (as well as Bluray players) take the 24fps of the original film on the the disc and does 3:2 pulldown to make 60i.
Actually, on conventional DVD, the conversion from 24 fps to 60i is done before it is put onto disk. When played back on a non-progressive player to a TV that does not do deinterlacing (does not do reverse telecine), then what you are seeing is 60 fields per second, interlaced. It is better in this case to talk about "fields" per second instead of "frames" per second.
Technically, yes, when you have all of the odd lines scanned (1st field) , then all of the even lines scanned (2nd field) that will give you one full frame at 30 frames per second, but only if there is no motion in the picture. When you have motion, no 2 fields are the same. This is what you almost always have with material that was originally done with a video camera as opposed to film. Played back on a "progessive" player, this type of material gets converted to a true 60 frames per second picture. If there is no motion between the 2 fields, the first field will just be held for 1/60 of a second and then scanned along with the second field progressively, one line at a time. If there is motion between the two fields, the deinterlacer will do motion adaptation and interpolate what the progressive lines should be.
With 24 frames per second movie material converted to 60 fields per second video, you do have some fields repeated. Frame 1 of the film will be repeated for 3 fields, then the next frame will be repeated for 2 fields, then the next frame for 3 fields, and so on. This is where the term 3:2 pulldown originates because of the 3-2-3-2-3-2 pattern. Sometimes it is done as 2-3-2-3-2-3. This is an imperfect way of doing it. It slightly throws off the "cadence" of the original 24 frames per second movie material. When this type of material is viewed on a progressive player, the deinterlacer will sense that the original material is film based and will reverse the telcine process. Some frames will be perfect (where 2 indentical fields get repeated) and some will have to have interpolation applied. Again, this is not perfect, but it will give a subtle improvement to the picture with less noticeable scan lines. Most frames will have twice the vertical resolution (a full 480 as opposed to just 240). It can't be perfect because you are still trying to force what was originally done at 24 frames per second into 30 frames per second.
Now, fast forward to the present to Blu-ray discs and players. With Blu-ray, most movies are encoded in their native 24 frames per second format. If the Blu-ray player senses that the TV can accept native 24p material, then it will pass it in it's native format. If the TV cannot, then the player must do the telecine conversion. LCD TV's that can handle this do so by refreshing at 120Hz. Each frame of the movie can just be repeated 5 times since 5 X 24 = 120. Plasma sets will switch either to 48Hz, 72Hz, or 96Hz mode and repeat each frame either 2X, 3X, or 4X.
If the material on the Blu-ray disc is native video format at 60p (60 real frames per second - not fields per second), then the TV just stays in 60Hz mode and displays each frame one time. (120Hz LCDs would display each frame 2X in this case, since LCD panels cannot change refresh rates on the fly. If the panel was designed for 120Hz, it will always operate at 120Hz.)
So, the advantage of the native 24p output is that the movie is shown with its original cadence. There will not be any additional "judder" added by the imperfect telecine process.
You are correct, the real rates are 59.94 and 29.97fps, but for simplicity, it is customary to use 60 and 30.
I hope this helps instead of confuses more. I've been doing a lot of research on this lately, mostly around this whole issue of 120Hz on LCD sets.
Robert
