Interlaced signals require that equipment know which field is the start of a 2 field sequence. Is the image an odd or even field as this makes a big difference. This was provided with a sync pulse so that CRT's and downstream equipment could deal correctly with the interlaced signal knowing the start of a sequence of two fields. The timecode then also coincides with this sync pulse hence 59.97 fields becomes 29.97 frames. But as far as the viewer is concerned there is no temporal difference between 60i and 60P. They both provide 59.94 individual images a second. One just happens to be half the vertical resolution of the other and our eyes fill in the blanks !!!
24fps vs 60fps
- Thread starter richardwalter
- Start date
As others have said, "29.97i" and 29.97p are not the same thing. 29.97i and 59.94i are the same thing.
I illustrate it visually in the tutorial, but the common convention of two interlaced fields making up a "frame" is nonsense, and needlessly confusing. The two fields making up a "frame" are taken at different moments in time. It's like shredding two different photographs taken at different times, stitching them together, and calling the result a "frame." It doesn't work because it's clearly not one picture, it's two.
What's more, the "frame" is never shown on screen AS a "frame." One field is scanned on, and then the next one is scanned on. Thus, 60 individual fields are drawn onto the screen per second, in succession, so everything moves 60 times per second, not 30. And thus, the movement of 60i and 60p are the same. The movement of true 30p isn't.
In the tute, I do a slow-motion of what actually happens on the screen, and it becomes pretty clear why the idea of "30 interlaced frames per second" or "30i" or "29.97i" just needs to go away. It doesn't describe what happens, and it leads people to think 30p and "30i" are the same thing, among other conceptual problems.
Thomas Smet
Veteran
Interlaced was really just one of the worlds first methods of video compression. It crammed 60 frames into the bandwidth of 30 frames cutting the bandwidth in half yet still having the motion of 60 frames per second. Despite the bad visual reputation with interlaced it really is a clever marvel of engineering and in a way its sad to see it hated so much today. It was an ingenious way to create a more realistic form of motion at the time and it really does deserve that recognition. When HD first came out and we still had tube based HDTVs it looked stunning. It is the digital displays that ruined interlaced because they were never going to be a good match for each other.
Yeah I'm an old fart that loves the history of video so I guess I am able to appreciate some of those things a bit more.
Yeah I'm an old fart that loves the history of video so I guess I am able to appreciate some of those things a bit more.
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There is, unfortunately, piles and piles of bad info regarding frame rates and 24p out there.
I should also point out that it's not just the US. It's anywhere that uses the NTSC standard, which is all of North America, Japan, and a few other places.
