When using Auto Gordian Knot (AGK) to convert some TV and DV recordings to XVid, I noticed that the results were very “jumpy” compared to the original. The reason for this is that AGK creates 25 progressive frames per second (FPS) video, throwing away half of the fields of 50FPS interlaced PAL video. Combine this with the fast shutter speeds that some stations use these days and you end up with an unnatural “sped up” effect as motion isn’t being blurred by a 1/25th of a second exposure.
I decided to have a go at doing a 50FPS XVid conversion. I wanted something that replicated the excellent playback results I get using DScaler when watching MPEG2 video. Whilst XVid supports interlaced video, I decided deinterlacing the video and doubling the framerate (to 50 progressive FPS ) was what I’d go for.
The way interlacing works is to have each “frame” consist of 2 sub frames (“fields”), one with the odd lines and one with the even lines. If you ignore the even/odd lines then you lose half of the vertical resolution, making horizontal lines blurry. Because the fields are “snapped” at different points in time, moving objects are not in the same position between fields.
Simple strategies for deinterlacing either give you blurry stills or blurry motion.
The solution is to use a smart deinterlacing algorithm that analyses the motion in the fields. Where there is no motion, it combines the information to get clear detail. Where there is motion, it keeps the fields separate to achieve smooth motion. Whilst AGK creates clear 25FPS video, if you process to 50FPS you end up with the best of both worlds. A great piece of software that does is “Deinterlace Smooth” and its a VirtualDub plugin.
I have a page with my notes on processing video to 50 fps XVIDs here.