Interlacing? Fields? ...what are you talking about?!

...and why are my stills jiggling like jello?

If you shoot home movies or anything else on video tape long enough you will, eventually, encounter some undesirable artifacts or effects that result from field interlacing. Most video cameras (with the exception of those that shoot in progressive frame mode), expose each frame of a video 2 times (changing you camcorder's shutter speed will not change this). The camcorder, essentially, takes 2 snap-shots at different moments in time, then weaves (interlaces) the results together to form a single frame. This would have virtually no effect on the resulting image unless something in the image moved in the 1/50 or 1/60 of a second that elapsed between the two snapshots. When something moves, the two snapshots contain images of two, different scenes, and, when they are woven (interlace) together, the resulting frame can be anything from slightly distorted, to a complete disaster.

I'm going to use a short clip of a crane in flight to illustrate my points. The still frame shown here is a single frame from the clip. This video was shot with a moderately fast shutter speed, so the mess you are seeing is the result of inerlacing (weaving) two sharp images, snapped 1/60 of a second apart (NTSC video standard; PAL would be 1/50 of a second) into a single frame. You can see in this frame that the bird's wings appear in two distinctly different positions, the background is a mess because the camera is panning to follow the bird, the the bird's body is all but lost.

If I take a still frame from this video and I display it on an NTSC (or PAL) monitor (e.g. a television), you'll see something that looks like this flickering image. The NTSC alternates between the fields. When you display just one frame, it appears to alternate back and forth causing a fast, but very noticable jitter as the image goes forward and back in time. The effect isn't nearly as disturbing when playing the movie, but it still doesn't look ideal and it looks pretty darn bad on a computer. Click this to view the original footage with both fields combined in each frame. Pause the movie at any point to see how each frame contains two images.

There is really only one way to reconcile this sort of video, and that is to separate the two fields. If you want to play the video at normal speed, you will have to discard one field from each frame. They simply don't belong together. In Quicktime Pro, you can do this by selecting "single field" in the "high quality" section of the video track properties. You could also make a frame from each field to achieve a beautiful, slow motion clip that uses all of the fields, but contains only one field per frame. This results in twice as many frames, which means your clip plays at 1/2 speed. I used Adobe Premiere to create this slow motion clip which is an obvious improvement where this clip is concerned.

In order to deinterlace a frame and eliminate the jitter that is associated with using a single frame from an interlaced source as a still image in a video, the deinterlacing software must discard one of the fields in the video frame. This results in a loss of video resolution. Some software, including Adobe Premiere, tries to interpolate the lines that are inserted in place of the discarded lines. This means that the software actually looks at the adjacent pixels and computes a new pixel value, rather than just copying one raster line to the next. This process results in a noticably better quality, deinterlaced frame.

I hope this helps you understand what is happening, although I may not have offered a solution to your particular problem. Depending on what software you use, you may, or may not have a way to optimize your high-motion video or your stills. In most cases, you will just have to discard one field and make the best of what is left. That will, in every case, eliminate the jitters and give you something that doesn't give you a headache. If you have this problem often, buy a Canon GL-1 or XL-1(s) and set it up to shoot in progressive frame mode. Now, I must get back to my bird.