Broadcast Engineering - February 2009 - (Page 24)

Production cliPs Digital hanDbook 24p and 25p judder learn solutions to solve the video judder problem. By Steve Mullen he arrival of compact, low- temporal sampling judder. It is a sigcost HD camcorders has nature film look and is a desirable type opened the opportunity of judder. to employ them as b-roll cameras. Naturally, attention must be Motion judder paid during camera setup and/or post The frame rate used for motion to achieve an optimal visual match pictures is 24fps — a rate considered between these camcorders and more ideal for narrative motion pictures. expensive digital cinema cameras. Of course, in Region 50 countries, When shooting 24fps and 25fps vid- 25fps is accepted. eo, there is another visual characteristic to consider. All low temporal rate media exhibit judder when viewed. However, there is concern that lowcost HD camcorders exhibit more judder than film and digital cinema cameras. In two white papers written for the BBC, Alan Roberts makes a Figure 1. Eye tracking motion vector 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:// Figure 2. Modulation transfer function (MTF) tinyurl.com/88rv95.) Temporal sampling judder Film and video cameras sample motion at regular intervals. Frame rates of 24p and 25p have lower temporal sampling rates than 60p and 60i. Therefore, 24p and 25p media will often be represented by too few samples to accurately capture complex motion. For this reason, motion captured at low frame rates will be less fluid than when captured at high field and frame rates. The lack of fluid motion is called 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 tripleblade shutters.) Flicker from 25p video is eliminated the same way — by repeating each image twice (50i). t 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. 24 broadcastengineeringworld.com | February 2009 http://tinyurl.com/9n4wb9 http://tinyurl.com/9n4wb9 http://www.tinyurl.com/88rv95 http://www.tinyurl.com/88rv95 http://www.broadcastengineeringworld.com

Table of Contents Feed for the Digital Edition of Broadcast Engineering - February 2009

Broadcast Engineering - February 2009 Contents Viewing the Slowdown in 3-D Examine Workflows Tapeless Technology Digital Audio 24p and 25p Judder Video Routing: A Look at What's Next Managing AFD The Right Connections Solid State Logic's AWS 900+ SE and More... Advertisers Index

Broadcast Engineering - February 2009

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