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De-interlacing is the conversion of interlaced video to progressive scan video. Ideally the result should look like it was progressive scan to begin with. Good de-interlacing switches back and forth among several methods but is still not perfect.


Weave (taking material from the enxt field to fill in the intervening scan lines) is one method used to accomplish de-interlacing, but if used exclusively, it produces mediocre results.


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Weave is the approach used for film based material where two successive video files (with the odd and the even video scan lines) are from the same frame of the original movie film. The these odd and even scan lines from these two fields are woven together such that (in theory) they could produce a perfect non-interlaced (ie. progressive) video frame. The complications come in because movie film has 24 frames per second and progressive scan video is normally displayed at 60 frames per second. This conversion results in a need to repeat the contents of the movie frame multiple times and since 60 is not a even multiple of 24 a scheme was developed where a sequence of two repeats is followed by 3 repeats. Thus in 5/60 of a second (= 1/12) two successive frames of the original movie film are displayed. In order for a de-interlacer do to a good job of taking the interlaced video input and converting it to progressive scan video output a combination of weaving the appropriate fields plus applying what is call inverse 2:3 pull down is necessary (i.e. to account for the uneven rate at which the movie frames are converted to video). Some inexpensive de-interlaces only do weave and will produce relatively poor results. Also some variations on the algorithm used to correctly detect which are the correct fields that are to be woven together as well as the actual algorithms to do the weaving can also impact the results.



Ron Jones
www.dtvmax.com
 
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