HD Techniques

Progressive scan produces a ‘judder’ or image flicker with rapid movement in the frame, or whilst panning quickly. What is the cause of this and how can it be remedied?

This is inherent with shooting progressive. The eye can resolve quicker than 25fps, so it registers the changes in each frame and this appears as a judder when panning quickly (hence the development of 50 fields/second interlaced video as a remedy). So when shooting progressive, this has to be taken into consideration.

A solution is to control the speed of the pans - a film technique. Some film panning speeds charts can be downloaded here.

The ‘judder’ effect can also be reduced slightly by using the camera's electronic shutter at a frequency twice that of the frame rate (ie. 25p - 1/50th shutter, 30p - 1/60th shutter). This solution is equivalent to operating with a 180 degree shutter on film cameras (if shooting 24fps, a 180 degree shutter is effectively 1/48th of a second).

The Panasonic Varicam displays its shutter setting in degrees.

The 'judder' problem doesn't occur when shooting interlaced.

What should I use for a production which involves a lot of fast action?

Interlaced is definitely more suitable than progressive. As mentioned above, shooting fast motion in progressive-scan mode leaves a ‘judder’ effect across the picture. As a result, most fast action (e.g sport) shot in hi-def is shot in interlaced mode.

What HD format is better suited if I am using special effects?

This is largely dependent on the type of effects but usually shooting 24p, 25p or 30p is better because rendering the effect on one field of progressive video is quicker than having to work on the two fields of interlaced video.