24p Judder 24p judder and film cadence (3:2 pulldown, 5:5 pulldown)

What 24p judder is

Film runs at 24 frames per second, so each frame should last 41.7 ms. Displays do not refresh at 24 Hz, and the mismatch is judder: Forasoft defines it as "visible motion jerkiness caused by mismatched frame rates between source and display," most visible on slow camera pans across static scenes. TV Technology lists a separate cause that is built into film: capturing motion at 24 samples per second renders complex movement less fluid, which it treats as part of the film look. What this entry covers is the extra error introduced by the display.

How 3:2 pulldown works

On a 60 Hz display, 24 does not divide into 60 (60 / 24 = 2.5). The classic fix is 3:2 pulldown. Wikipedia describes the 2-3-2-3 cycle: frame A fills 2 fields, B fills 3, C fills 2 and D fills 3, so four film frames fill ten fields, or five video frames. Film is also slowed by 1/1000 to 23.976 fps, because 23.976 / 29.97 equals 4/5. In refresh terms, a frame held for 2 refreshes lasts 33.3 ms and a frame held for 3 refreshes lasts 50 ms, so alternate frames are on screen 50% longer than their neighbors. Wikipedia calls two of every five frames "dirty," and notes a 3-3-2-2 variant leaves only one but adds "a slight bit more judder."

The fix is to use a refresh rate that is a multiple of 24. Forasoft says premium TVs offer true 24p or cinema modes that run the panel at 48, 72 or 120 Hz. At 120 Hz, 5:5 pulldown shows every frame five times: 5 x 8.33 ms = 41.7 ms, with equal spacing. macmyths likewise says that at 120 Hz each 24 fps frame can be repeated five times evenly, with no 3:2 pulldown needed. TV Technology adds that cinema projectors historically used dual-blade shutters to flash each frame twice, a 48 Hz flicker rate.

Other multiples of 24 work the same way. A 48 Hz mode shows each frame for two refreshes of 20.8 ms, a 72 Hz mode for three refreshes of 13.9 ms, and a 120 Hz mode for five of 8.3 ms, so every pattern lands on 41.7 ms. The 3:2 case alternates 33.3 ms and 50 ms, a swing of 16.7 ms, which is one full 60 Hz refresh. Forasoft's rule for producers is to ship 24p as 24p and 60p as 60p and not convert, and Wikipedia adds that the alternate 3-3-2-2 cadence leaves only one dirty frame per cycle at the price of a bit more judder.

Why OLED makes it look worse

On an OLED each 41.7 ms frame is a clean, static image followed by a sharp jump, and slow pans are where it shows. macmyths puts it this way: "OLED response time does not create missing frames, but it can make the gaps in a 24 fps movie more noticeable." Repeating a frame five times at 120 Hz evens out the timing but, as macmyths notes, adds no motion. Even a perfect 120 Hz cadence shows the same image for 41.7 ms.

TV Technology identifies the contributing factors on the production side: judder perception depends largely on the sharpness of moving object edges, and adding 2:3 pulldown swaps motion judder for pulldown judder. The same article lists mitigations on set, such as a slightly slower shutter or a shallow depth of field. Viewers have fewer options at home. macmyths adds that slow pans look worst because the image does not move continuously with the eye.

Fixes: interpolation, BFI and metadata

There are four options, each with a trade. Motion interpolation adds synthetic frames, which fixes pans but produces the soap opera effect at strong settings. LG's TruMotion offers a de-judder slider for this purpose, and macmyths suggests keeping interpolation off or low to preserve the cadence. Black frame insertion shortens the persistence blur but "does not create intermediate motion detail," in macmyths' words, and can flicker or dim the picture. Native-cadence playback (the TV or player set to output 24 Hz or 120 Hz) fixes the uneven timing only. Metadata approaches are the newest: Dolby's Authentic Motion in Dolby Vision 2 is "creative driven" and applies per shot, and HDR10+ Technologies lists Intelligent Motion Smoothing in HDR10+ Advanced, metadata that lets a display adjust how much smoothing it applies per scene.

Where this shows up

You meet judder on any source that arrives at the wrong rate. Wikipedia notes 24 fps is converted to 59.94 fps on modern 60 Hz progressive displays, so any chain that outputs 60 Hz brings the 3:2 cadence with it. A match-frame-rate setting on a player or streaming box is the usual way to avoid that. Dolby names the Max tier on Hisense's UX, UR9, UR8 and U7 as the first Authentic Motion hardware, and Samsung's Micro RGB R95H and R85H list HDR10+ ADVANCED support. Neither Dolby nor HDR10+ Technologies has published how large the judder reduction is. For the broader motion picture, see motion resolution (MPRT).

Common confusions

"120 Hz fixes judder" is half true: it fixes cadence, not hold time. "Judder" and "stutter" are used interchangeably online, but the cadence error is a timing problem and the hold-time effect is a perception problem. The 23.976 fps rate is a legacy of the 1/1000 slowdown described above, not a fault. Rather than guess, test with a slow pan scene and toggle your TV's Filmmaker Mode and de-judder settings one at a time.