Why does my 30p video show up as 60i?
I’ve seen some confusion on the online forums regarding shooting in 30p (or 24p) and then importing into a video editor.
Why does my 30p video show up as 60i in <name your editor>?
In an interleaved 60i video (normal old fashioned video), the image is scanned 60 times per second, producing one half frame at each scan (say the odd scan lines, followed by the even scan lines in the next half frame). Because movement can occur between the two half frames, you some times see interlaced jagged edges.
A better way is to take 30 still pictures per second of the entire image.
When your camera records at 30p video, it takes a single image – but splits it into two half frames and stuffs those into two 60i half frames (without any jaggies since its splitting one image in to two pieces whereas 60i creates one image from two separate pieces taken 1/60th of a second apart).
Consequently, a 30p video is stored as a 60i video. And two consecutive half frames, put together, become 30p. Your video software can’t tell the difference between between 60i and 30p.
So why do we have these strange 60i half frames? Historic reasons. The earliest TVs were not able to scan the full image top to bottom before the next image would arrive. The solution then was to draw only half the lines in each interval. Thanks to the persistence of the phosphor image of old TVs, the first lines remained glowing while the TV then scanned the alternate lines.
No one would design a TV like that today – but we’ve lived with it for many decades and it is still supported for compatibility reasons.
I can import 24p video into iMovie (or other editor) but it plays weird – why?
This depends on the camera. But it is common to store the 24p image in half frames, similar to 30p. But since 24 does not evenly divide into 60, the sequence of half frames is a little different.
In 30p, the sequence is basically 2:2:2:2:2:2 and so on where the 2 signifies 2 half frames.
In 24p, the video may be stored in various combinations such as:
2:3:3:2 or 2:3:2:3
In this way, the 24p mode uses up more half frames – consider 2 half frames and then 3 half frames. This slows down the video frames to match the 60i storage of the tape. Consequently, 24p gets mapped into a funky sequence of half frames on the 60i tape.
To a program like iMovie, this 24p footage looks just like 60i. But unfortunately, iMovie (And many other video editors) have no way of knowing that it is not really 60i footage with some frames appearing 3 times. So play back and edits produce strange artifacts.
If you want to convert this to proper 24p footage and then edit in an editor that supports 24 frames per second see these instructions.