Broadcast Engineering - February 2009 - (Page 27)

Production cliPs Digital hanDbook Figure 5. Anti-judder correction applied (blue) 24p pulldown judder When film is telecined or when 24p video is broadcast as 60i video, pulldown is added. (See the blue and orange cells in Figure 6.) While temporal sampling judder remains, motion judder is replaced by pulldown judder. The dots in the second row 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. Figure 6. Interlace 2:3 pulldown The 2:3 cadence created judder. For this reason, video frames 3 and 4 are called judder frames. Most Blu-ray players provide a 1080/p24 output that many HDTVs can accept. The goal of these features is to present 24fps media free from 2:3 judder. (detail enhancement) control that adjusts the amount of boost applied to the signal. In Figure 4, the green and blue curves represent, respectively, normal (midpoint) and maximum sharpness. The boost expands the area under the curve — thereby increasing overall image sharpness — and lifts the higher frequencies thereby preventing loss of fine detail. Unfortunately, even at a normal setting, the horizontal frequency response curve has a moderately large peak within the frequency range that creates judder. (See the gray zone in Figure 4.) More sophisticated camcorders have separate controls for detail enhancement and aperture correction. While the detail control alters edge sharpness, the aperture control alters the amount of fine detail. These controls enable a camera operator to balance edge detail and fine detail to minimize judder. Figure 5 illustrates the judder band (orange) plus three representative response curves: film (purple), a highend video camera (black) and the response of this camera with negative detail enhancement (blue). Negative detail correction, as offered by Sony HDCAM and CineAlta camcorders, reduces the perception of judder. Until low-cost HD camcorders incorporate the ability to dial-in negative detail without reducing the amount of fine detail, a camera operator can try to eliminate excessive judder by setting sharpness midway between minimum and normal (red in Figure 4). (Setting sharpness at the minimum, as is often done in an effort to create a film look, simply strips video of fine detail, as shown by the red curves in Figures 4 and 5.) Another judder reducing solution is to use a slightly slower shutter speed that increases motion blur. Likewise, a camera operator can control camera motion while the director controls the movement of objects within the frame. BE Steve Mullen is owner of Digital Video Consulting, which provides consulting services and publishes a series of books on digital video technology. February 2009 | broadcastengineeringworld.com 27 http://www.broadcastengineeringworld.com

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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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