Judder is a rather distracting effect which is typified by some elements of a scene appearing to stutter or jerk. There are actually two causes of judder. One is a product of the 24 frames per second (fps) rate at which films are recorded. The second is caused by the 3:2 pulldown conversion process which artificially boosts the frame rate of a 24 fps film source using frame repetition to be shown on the typical 60 fps television.

24p Judder

As mentioned, movies for the cinema shot on 35mm or 70mm film capture 24 individual images (fps) which are projected at 48 fps to reduce flicker. To achieve 48 frames per second each frame is projected twice.
The cause of judder is that 24 fps is inadequate for cleanly capturing the scene in front of it when panned. Unfortunately, it is an industry standard created partly to support an audio track for “talkies” way back in the late 1920′s, partly due to slow film exposure times and partly to save money on expensive film. The solid state digital storage era has it advantages!

Judder would more noticeable at the theater if it wasn’t for the filming techniques used to minimize judder. Take note of how often background scenery is out of focus – it conveniently obscures any judder.

3:2 Pulldown or Telecine Judder

The second form of judder is telecine judder which is introduced by a 3:2 pulldown which uses frame repetition to boost 24 fps to 30 fps for display on a NTSC TV (60i). NTSC (National Television Standards Committee) video is interlaced with each frame being constructed from two consecutive fields of video containing 50% of the 480 horizontal scan lines containing picture information. NTSC video displays at 29.97 fps or 59.94 fields per second.

The 3:2 pull-down works well apart from the fact that two out of every five video fields come from different frames and when there has been image movement between frames (eg. camera is panning) there is a mismatch (smearing) of detail between the sequential fields making a frame. The human eyes persistence of vision takes care of smoothing this anomaly apart from some situations like slow panning or spinning objects. Frame mismatch makes films look subtlety less smooth on home video equipment as opposed to the cinema screen and this different type of judder is referred to as telecine judder. Other television broadcast systems such as PAL suffer the same judder problems.

It’s worth noting that pulldown tends to somewhat lessen 24p judder and is more noticeable the worse the judder is.
Telecine judder is not applicable to frame repetition pulldown such as 5:5 which is 5x frame repetition (eg. 24 fps to 96 fps). In this case you are still left with the judder from shooting film at 24 fps.

Anti-Judder Technology

Judder is dealt with by the application of video processing to apply frame interpolation and smooth motion between frames. Unfortunately, video processing often produces such a smooth result many videophiles find it unnatural would rather switch it off and watch the native 24p with a frame repeat pulldown and accept any non perfection of 24p as part of the experience. Another reason for preferring the unprocessed 24p can be the introduction of video artifacts by the video smoothing process.