How to handle interlaced video?

1261830504_008fd3c733Currently I am working in some projects, where there is broadcast TV material involved. As most people working with Video-Coding know the former TV video standart is interlaced. It doesn´t matter if the Video comes from pal or ntsc – the material is recorded and stored as 25 fps in pal (nearly 30 fps ntsc) – but each frame consists of 2 half frames. Where the 1st half frame has the lines (on the y axis) 1,3,5,7 etc and the 2nd half frame consists of the lines 2,4,6,8 etc. Phase alternating line = PAL.
So what have we got here? Calculating with square pixels PAL is a 768 × 576 resolution. But we never have the information of the full image because the two half frames (each 384 x 576) are recorded at a slightly different time. Thats why you can´t just add the two half frames to each other and get a full picture. If the camera takes a panshot of a landscape the two half frames don´t fit to each other. It´s like a puzzle. Thats why there are so many deinterlace filters out there.
What these filters try to accomplish is to generate a full image by guessing /blending / calculating the information between the 2 half frames. This works astonishing well with some filters and after the deinterlacing we´ve got a 25 fps full frame 768 x 756 movie.
Working with computers it is much more easy to handle progressive (full frame) material than half frame interlaced material. So if it comes to storage, there is a trend to store the progressive material digital on hard drives.
But heres the clou. It doesn´t matter at what quality the progressive material is stored. We lost some information. Motion information. Do you remember – when we started we had 50 fps material … now we got 25 fps material.
When it comes to fast motions (explosions, pans, anything) fps is the key to quality.
Thats why we´re using a bob deinterlace filter with double framerate settings. In this setup our deinterlacing filter is still calculating a full frame from the actual half frame. But the filter does this with every half frame so we don´t loose motion information.
Some older computers have a problem playing 50 fps Material, but this is going to change in the next years. When it comes to long time storage of material we wan´t to loose as less information as possible.

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