When a video display is fed an interlaced video signal, each frame of video is split into two fields, each of which contains a set of alternating lines of horizontal resolution running across the screen. Standard-definition NTSC and PAL are both interlaced video formats, while some high-definition video formats, and all video displayed on a computer screen, are progressive-scanned video formats. With progressive scanning, these lines are drawn one at a time, from the top of the screen to the bottom.
When you record interlaced footage with a camcorder, each video frame is split into two fields, each containing half of the total lines of resolution in the frame. The first field is recorded, then the second, one after the other, so both fields constitute one frame. When you play the video back, the monitor displays each recorded frame in succession, first drawing one field, then the other.
Field order refers to the order in which each pair of video fields is recorded. Because video fields are recorded sequentially, it’s as if each 29.97 fps clip is really playing at 60 “frames” per second.
There are two options for field order:
Upper (Field 2 is dominant, so the second field is drawn first.)
Lower (Field 1 is dominant, so the first field is drawn first.)
Generally, Upper is used by 640 x 480 systems, while Lower is most common in professional 720 x 486 and DV 720 x 480 systems.
It’s important to render digital video with the field order required by your playback system. Because motion continues from one field to the next, it’s crucial that each field plays in the correct order.