24p judder
Feb 1, 2009 12:00 PM, By Steve Mullen
The arrival of compact and relatively low-cost HD camcorders has opened the opportunity to employ them as b-roll cameras. Naturally, attention must be paid during camera setup and/or post to achieve an optimal visual match between these camcorders and more expensive digital cinema cameras.
When shooting 24fps and 30fps video, there is another visual characteristic that must be considered. All low temporal rate media exhibit judder when viewed. Nevertheless, there is concern that low-cost HD camcorders exhibit more judder than do film and digital cinema cameras.
In two white papers written for the BBC, Alan Roberts makes a convincing case that video camera technology differences, not simply camera operator inexperience, are responsible for excessive judder from low-cost HD camcorders. (The white papers can be found at http://tinyurl.com/9n4wb9 and http://tinyurl.com/88rv95.)
Temporal sampling judder
Film and video cameras sample motion at regular intervals. Frame rates of 24p and 30p have lower temporal sampling rates than 60p and 60i. Therefore, 24p and 30p media will often be represented by too few samples to accurately capture complex motion.
For this reason, motion captured at low frames rates will be less fluid than when captured at high field and frame rates. The lack of fluid motion is called temporal sampling judder. It is a signature film look, and it is a desirable type of judder.
Motion judder
The frame rate used for motion pictures is 24fps — a rate considered ideal for narrative motion pictures. Some feel 30fps is also acceptable.
If film was presented at 24fps, image flicker would be intolerable. To increase the presentation rate to 48Hz, film projectors use dual-blade shutters to show each frame twice. (For a relatively dim picture, a 48Hz rate only slightly exceeds the critical flicker frequency. Brighter pictures demand a higher presentation rate, hence triple-blade shutters.) Flicker from 30p video is eliminated the same way — by repeating each image twice (60i).
Figure 1. Eye tracking motion vector
Doubling the presentation rate inherently creates eye tracking artifacts. Figure 1 illustrates a horizontally moving square. When film is projected with a double-bladed shutter, a new picture is flashed 24 times per second, and each picture is flashed twice. Between each presentation, the screen goes dark. The dark period tells our eyes a presentation is complete and clears the image from the retina.
As we watch the projected image, our brain uses the series of new images to determine the square's motion vector. In a series of short movements called saccades, our eyes track the moving square. When the projector shutter opens a second time on the same frame, our gaze — following this vector — has advanced halfway to the anticipated position of the square at the next new frame's presentation.
The square, therefore, is imaged onto our retina a second time, at a position displaced along the motion vector. These repeat images, which are not where they should be based on the motion vector, degrade the perception of motion. This degradation is called motion judder. A certain amount of motion judder is accepted as part of the look of film projected in a theater.
Excessive motion judder can be prevented, for example, by panning with a moving object. Follow panning itself creates another eye tracking artifact called background strobing. However, by forcing a shallow depth of field, background detail is reduced, thereby minimizing background strobing.
Pulldown judder
When film is telecined or when 24p video is broadcast, 2:3 pulldown is added to enable the media to be carried within 60i video. (See the blue and yellow cells in Figure 2.) While temporal sampling judder remains, motion judder is replaced by pulldown judder.
Figure 2. Interlace 2:3 pulldown
Click to enlarge
The dots in the second row in Figure 2 represent an object moving from left to right. Each field, within frames A, B, C and D, should successively carry an image captured 1/60sec earlier. Fields 3 through 5 show the process of adding 2:3 pulldown.
In field 6, the odd field within video frame 3 carries motion captured 1/30sec earlier. And the even field within video frame 4 carries motion captured 1/30sec later. The nonuniformity of motion in video frame 3 (dark green cells) and video frame 4 (light green cells) mixed with the uniform motion in video frames 1, 2 and 5 creates a visual 2:3 cadence. The 2:3 cadence creates judder. For this reason, video frames 3 and 4 are called judder frames.
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