eli5 How does Pulldown work?


If it’s 30FPS, why does it look like 24 FPS?

In: 2

The refresh rate of a conventional television is actually approximately 60 “fields” per second. A field is a picture consisting of alternating horizontal rows. In one field all even rows are drawn, then all odd rows. This yields a much more realistic motion. 60/24=2.5. A pulldown is a cheap method of converting the framerate: one input field is displayed twice, then the next one is displayed 3 times. The average is 2.5. The duration of the clip has been maintained.

This introduces a slight stutter that might be visible in a smooth pan of the camera. But it is small because the original frames are distributed quite evenly. With different conversion factors the framerate would need to catch up less often by a bigger step.

The informaton content of the original video hasn’t been changed through this process. To actually get 30 or 60 frames per second, the camera would need to take more frequent pictures. A computer monitor might refresh 75 times per second, but still show low framerate video, holding a frame unchanged for several refreshes.