readme.2-3pulldown

来自「Motion JPEG编解码器源代码」· 2-3PULLDOWN 代码 · 共 59 行

What is 2-3 pulldown?

Frame rate of FILM is 24fps.
PAL video is 25fps.
When we play FILM thru PAL video, 25fps.
So motion is quicker and playing time is shorter 96% (24/25).

NTSC video is 30fps (to be exact, 30/1.001fps).
If we play FILM thru NTSC video as PAL
(1 frame on FILM is 1 frame on video),
motion will be quicker and playing time be shorter 80% (24/30).
It is too quick so it will not seem natural motion,
when we play FILM thru NTSC video, 1 frame on FILM is played
2 or 3 fields on video, so motion and playing time is same as FILM.

FILM frames  0--.  1--.--.  2--.  3--.--.
             |  |  |  |  |  |  |  |  |  |
NTSC fields  0T |  1T |  2T |  3T |  4T |
                0B    1B    2B    3B    4B

In analog video world, this process is called 2-3 pulldown,
because NTSC video is repeating of 2 fields from original 1 frame
and 3 fields from original 1 frame.


What is 3:2 pulldown?

FILM is recorded in MPEG2 / DVD video for NTSC at 24fps
(to be exact, 24/1.001fps).  Some flags are also recorded on each frame.
DVD video player use top_field_first flag and repeat_first_field flag
to play 30fps (to be exact, 30/1.001fps) for NTSC monitor.

FILM frames         0     1        2     3
 top_field_first    1     1        0     0
 repeat_first_field 0     1        0     1
 FILM fields        0T    1T----.     2T    3T
                    |  0B |  1B |  2B |  3B-+--.
                    |  |  |  |  |  |  |  |  |  |
NTSC fields         0T |  1T |  2T |  3T |  4T |
                       0B    1B    2B    3B    4B

This process is called 3:2 pulldown.


What is different 2-3 pulldown and 3:2 pulldown?

Results seem same.
Difference of name is maybe where to start counting:
2 fields or 3 fields.
2-3 pulldown is processed before recording and recorded at 30fps.
3:2 pulldown is processed at playing and recorded at 24fps.
If we play with progressive (60 frames/seconds) monitor,
2-3 pulldown video need rather complex process at progressive monitor
to display beautifully.  This process isn't reliable perfectly.
When we play DVD 24fps video for 3:2 pulldown with progressive monitor,
if player and monitor connected with cable which support progressive,
player may directly generate progressive frames instead of normal
3:2 pulldown process.  This is simple and reliable perfectly.