Pulldown, or telecine, is the frame-rate conversion that fits content shot at one rate onto a display running at another, and done wrong it is the classic source of judder. The canonical case is 24-frames-per-second film on a 60 Hz display: each frame would need to occupy 60 divided by 24 = 2.5 refreshes, which is impossible, so the system alternates - holding one frame for 3 refreshes (50 ms) and the next for 2 (33 ms) in a repeating 3-2-3-2 cadence called 3:2 pulldown. That uneven cadence makes equal-speed motion arrive at the eye unevenly, which the viewer reads as judder. The name comes from the analog NTSC process, where 24 fps film was first slowed by one part in a thousand to 23.976 fps, then spread four film frames across five interlaced video frames (ten fields) in a 3-2-3-2 field cadence. The PAL alternative avoids the cadence by running 24 fps film 4% faster to 25 fps, trading judder for a shorter runtime and a slight pitch rise.
