sidebar: Reversing Field Dominance
As mentioned in this article, some editing systems are Field 1 Dominant – they make their cuts before “field 1” on a videotape – while others are Field 2 Dominant, making their cuts before field 2. Occasionally, a tape cut on a Field 2 Dominant system is later transferred to, edited on, or digitized by a Field 1 Dominant system.
The result of this is can be that the way frames are formed has been offset in time by one field. Cuts now straddle frames: a single frame will contain one field of the prior scene, and one field of the next scene. Another problem occurs if you shot the video in progressive scan mode, meaning each frame should not be interlaced – but now the captured footage seems to be interlaced on every frame. This is because it has been shifted in time by one half-frame (field), resulting in each original frame now being split across two frames.
It is possible to unwind this problem and get back to the original, correct, whole frames. We personally use After Effects for this job.
Import your footage, and make sure you separate fields correctly for your footage (usually lower field first for D1 or DV NTSC). Next, create a composition at twice the frame rate of the footage (i.e. 59.94 fps for NTSC) – this will give you an increment in your timeline for every field, not just every frame. Drag your clip into this composition, and then slide it along the timeline to start just one increment of time (i.e. field) later. This un-does the slip in time that occurred between editing systems, placing the correct fields back together into the same frame.
The downside of this move is that when After Effects goes to render the lower field of the new frame, it is actually using an interpolated version of the upper field of the previous original frame. To correct this, move your footage up or down in the composition by a single pixel. By doing so, you are offsetting the fields to end back up on their original lines, albeit in a new frame. Render as you would normally: 29.97 fps, with the same field order as the source footage. If you like, trim your render to skip the first frame; unless you started somewhere other than the start of the clip, your first frame will have a field missing as a result of your offset.
The resulting movie will now have its edits happen on whole frames, and any progressive scan footage will have its whole frames put back together. It will also be shifted up or down a pixel, but since this is buried beyond the Action Safe area of the frame – you won’t see it.
