Interlacing and Telecine
From Zanecorpwiki
When video is interlaced, it means that it takes to cycles to update each frame in two steps, doing each odd row of pixels, then each even row of pixels. Hence interlaced.
Say you have 4 full frames (non-interlaced); A, B, C, and D. To turn that into interlaced video, you'd end up with 8 fields: A-odd, A-even, B-odd, B-even, etc. If you slowed down the video enough, you'd see that half the time the image was a little out of whack in fast moving areas of the screen. The odd rows of pixels update before the even rows.
When transcoding video, it's important to know whether the interlacing is natural or artificial. Natural interlacing (found in PAL DVDs and some NTSC DVDs from European sources, among others) were (essentially) interlaced to begin with. Reconstructing a full frame picture is accomplished by simply putting the odd and even fields together again. This isn't exactly perfect, and the result is not quite as good as if the source had been progressive (non-interlaced) to begin with.
For Americans, the more common problem is dealing with artificially interlaced sources. This happens when film, shot at 24 frames per second, is put in NTSC (US TV) fromat, which is 29.97 frames per second. In order to make this happen, some frames of the film are spread around such that certain fields (an odd or even row) get used in two different frames. The process is called 3:2 pulldown (telecine), and results in some frames being artificially interlaced.
Naively creating full frames from these results in making the interlacing visible to the human eye because you'll see the 'extra frames' in the resulting full frame images. Refer to De-interlacing Video for more on the process of de-interlacing.
The weirdness in all his is caused by the 29.97 (30000/1001.0 to be exact) fps used in American analog broadcasts. I don't know where this number came from, but I suspect it has something to do with the signal characteristics when broadcasting in UHF or something. That's a complete guess, though.
Note that in the telecining process, one ends up slowing down the stream ever so slightly since a true 3:2 process would yield 30 fps. This slowdown is reflected in full frame reconstruction of the original source (which was 24 fps). That's why a reconstructed film plays at 23.97 (24000/10001.0) fps rather than at 24.
