Originally Posted by dustwvl
If I am understanding this correctly, the only time you would need this on is when you are displaying a 24fps image that is also in 1080i. If that is the case I do not think I will ever be doing that as the only 1080i stuff I watch is broadcast HD which would already be at 60. Am I correct?
All due respect to Bill_H, who did a great
job explaining 3:2 pulldown, just a couple small corrections:
48Hz vs. 60Hz are refresh
rates, not to be confused with FPS (frames per second).
Feature film is shot at 24 FPS; SD and HD video are both
shot at 30 FPS.
The "i" in 1080i stands for interlaced
, which means each of the 30 video FRAMES that make up one second of motion video -- (actually, there are 29.97 fps in video due to drop-frame timecode, but that's a whole other movie) -- is split into 2 fields
, comprised of one field made up of the ODD numbered scan lines, and one made of jus the EVEN numbered lines. Both fields combined together comprise one full frame of interlaced
video, hence the term.
There's no actual frame rate of 60 fps, but sometimes you'll see or hear the term "60i," which is in fact interchangeable with "30 fps." So the correlation between the terms is because 60Hz means the video image is redrawn onto the screen 60 times per second. When the screen also happens to be displaying something shot on video as opposed to film, this is a perfect fit, as you get one full redraw of the image per field of video, i.e. each and every field has a space reserved just for itself, and each gets equal time alloted to it by the display.
As for 3:2 pulldown, let me put that more in laymen's terms.... 24 does not evenly divide into 60, so in order to ensure no frames "fall through the cracks," some frames have to be given extra time on the display for the math to even itself out, so the 3:2 pulldown algorithm was created to calculate which frames to display more of in order to comply with the 60Hz refresh rate.
The "p" in 24p or 1080p stands for "progressive" scan. There are no "fields" in this format of shooting, and while film is never interlaced, much video still is, although video can now also be shot in progressive format, i.e. each frame is comprised of all scan lines, both odd and even.
By displaying video at 48Hz, 24p film frames divide perfectly into each redrawing of the image, so no pulldown algorithm is necessary.