Pulldown

Simulator

Why converted footage limps

Frame rates do not divide into each other neatly. Whenever footage crosses from one rate to another, something has to be repeated, dropped or run at the wrong speed, and each of those leaves its own mark on the movement.

• Cadence

  The rhythm with which source frames are held on screen. An even cadence, one source frame per output frame, reads as smooth. Anything uneven shows up as a repeating stutter, strongest on slow steady moves like a pan.

• 3:2 pulldown

  Film runs at 24 frames a second and NTSC video at just under 30. To fit, one film frame is held for three video fields and the next for two, repeating. The pattern is regular, but the holds are unequal, so motion visibly limps.

• The PAL speedup

  For 25 fps territories, 24 fps film is simply run 4% fast. Every frame maps to one frame, so motion is perfectly smooth, but the whole film is about four minutes shorter and the audio sits almost a semitone sharp unless it is corrected.

• Dropping frames

  Going from 30 to 24 means one frame in five has nowhere to go. Dropping it is cheap and keeps the running time, but the discarded frame leaves a hitch that is very hard to miss on a pan.

• Why it is still a problem

  Cameras, phones, broadcast standards and streaming services do not agree on a frame rate. Any time footage crosses between them something has to give: the cadence, the running time or the pitch.

• What to do about it

  Shoot at the rate you will deliver at wherever possible. If you cannot, convert once, late, from the highest-rate master you have, and never convert something that has already been converted.