Building a Standards Converter

[ppppenguin]

Yes of course you can use a mixture of lines from different fields. This is exactly what you do when changing field frequency. The IBA's DICE used a 2 field 5 line aperture. The BBC's ACE used 4 field and 4 lines. I used something very close to the ACE aperture on a design I did for a client some years ago. With a fixed aperture there is always a compromise between different artefacts. Some of the ACE research was done at BBC Research Labs while I was in my gap year there in 1974/5. I knew what they were doing was clever but I didn't understand it. Probably in a research report or 2 somewhere.

Later converters used adaptive apertures which also have their limitations as you can often see the aperture being switched as the speed of motion changes. Later still there was motion vector compensation, a technique now widely used in compressed video bitsteams. Even this isn't 100% perfect as in an interlaced system there is always some ambiguity between temporal and vertical information.

Unless you are really determined, I would strongly recommend you not to experiment with adaptive apertures. I've tried it in a small way with horrible results. The cure was worse than the disease. Don't even think of trying motion vectors yourself. I wouldn't touch them unless there's a some freely available intellectual property that shows you how. Really need to do the work with computer based simulation with digital files for input and output.