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Interlaced vs. Progressive Video

For decades, NTSC has been the standard for US television and video, using the 29.97 frame per second interlaced video format. All standard definition TVs, video cameras and VCRs conformed to that spec, but now with HD video we hear a lot about progressive formats such as 720p and 1080p. What does this mean?

Let’s use a round number of 30 to describe the number of individual “frames” that make up each second of 29.97 interlaced video. Each frame is made up of two “fields”, with one field consisting of all the odd lines in the image, the other field having the even lines. Combining these two fields creates a single frame of video.

Since there are 30 frames every second, each frame is 1/30 of a second long. As two fields are needed to create a frame, fields must be recorded every 1/60 of a second! The two fields combined create an entire video frame every 1/30 of a second. A typical video camera shutter speed is 1/60 of a second, so the two fields are captured 1/60 of a second apart from each other.

When capturing moving subjects, some motion takes place between the capturing of the first field and the second field that make up the frame, so the images in the two fields may not match up perfectly, so fast motion may have “interlace artifacting” apparent in the picture.

Hollywood movies captured on film use 24 frames per second, no fields, each frame being grabbed in its entirety in 1/24 of a second. At 24fps, the human eye can perceive that it is not viewing reality, like seeing “flip book images” displayed rapidly one after another, so subconsciously, a little imagination is used in your head to “make it real”, filling in the blanks so to speak. This is what gives film its surreal quality, making it different than “video”. We don’t know why, but we just like the look of film!

In the video world, progressive video is the equivalent of film, capturing complete frames at once rather than mixing two interlaced fields. Many new HD cameras offer the option to shoot at 24p or 30p, which is 24 or 30 frames per second respectively, with the resulting images more closely resembling film than video. Of course, with less frames per second to work with, slow-motion may not come out as smooth, so there are certain trade-offs to get the coveted film-look. The videographer must also be careful to not pan too fast or the motion can look very jerky. It requires an entirely different way of shooting and editing and may not be for everyone.

Of course, if you are videotaping a low-budget movie with the intent of having the end result transferred to film, then acquiring the video at 24 frames can make sense since no frame rate conversion will be necessary when transferring from video to film. Fast-action sports like football may also have a better “look” when shot in progressive.

Another option for shooting Progressive HD is to use one of the new “60p” formats coming out, which offer 60fps shooting of non-interlaced video, in which case you can get smooth slow-motion. Some tapeless cameras even offer “overcranking”, which records double the frames per second so that when played back at the normal rate you can achieve super slow motion!

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Interlaced vs. Progressive Video