Frame Rates And Pulldowns Explained
Interlaced and progressive scan
A progressive scan is a method of updating the image by drawing all the lines of each frame in a sequence. In contrast, interlaced
video alternately scans odd and even lines. In old analog TV interlacing was commonly used as a way of doubling the refresh rate without
consuming extra bandwidth.
The following artifacts are common with interlaced video:
• edge tear (combing)
The image lands between two elds and blurs. This is commonly observed when viewing rapid lateral movement.
• aliasing (stair-stepping)
The texture of the image becomes populated with unrealistic patterns. Aliasing occurs because of differences between the original frame
rate and the destination format.
• twitter
The image shimmers, for example when showing rolling credits. This happens when the image contains thin horizontal lines that only
appear in one eld.
Frame rates of image sources
Originalanaloglms are made at 24 fps and the whole frame is projected at once. To eliminate icker and create an impression of
continuous movement, the projector blades divide the images so that the viewer sees 48 frames per second.
Interlaced video scans odd lines, then even. Two elds are blended into one image. NTSC video (60i) is 29.97 fps, or 59.94 elds per
second.
24p video is progressive but without the benet of projector blades dividing the images, so it looks jumpier on playback than lm. 24p is the
optimal format for projects that are nished on lm.
30p is optimal for projects nished on video. It has fewer strobing issues than 24p in video playback.
Notes
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