fps basics

[saprano]

Originally Posted by saprano

Can somebody explain to me what pulldown does and how it works.

For starters, let's explain why the 3:2 pulldown process is necessary. If you have ever seen a reel of film, you know that it is nothing more than a bunch of still pictures run at a rapid rate in order to create the appearance of fluid motion. In the movie theater, each second of film is actually 24 of those still pictures (or frames).


While film runs at 24 frames per second (fps), NTSC television signals (or video) have a run rate of 30 fps. In fact, television is also different from film in another significant way. Film projectors project the entire image all at once. TV's actually draw (or burn) each of the 525 lines of resolution on the phosphorus gray material on the front of your television. However, they cannot draw all 525 lines at once without noticeable 'flicker.' A system was created that eliminates flicker by drawing only half of the 525 lines per half second. Odd lines (lines 1-525) are drawn first in one 'field,' followed by the even lines (lines 2-524) in a second 'field.' So, you have 30 frames per second, or 60 fields per second. This system is called interlacing.

If you have paid attention so far, you might start seeing a problem. First of all there are 24 fps in film versus 30 fps in video. Secondly, video has two fields per frame whereas film does not have fields at all. That means we cannot simply transfer the film to video without issues. If we simply showed 24 fps on an NTSC display, what we would see is jerky, sped up action, much like an old "Keystone Cops" movie.

This is not a new problem, since we have been converting film to video for years. We even have a name for the process. It is called 'telecine' or 3:2 pulldown.