Silhouette-2022 : Appendix F - Video Fields
Appendix F - Video Fields
When working with footage that contains video fields, you have the option of either deinterlacing or removing 3:2 Pulldown. What are video fields and 3:2 Pulldown, you ask? Well, a review of the mechanics of video frames and fields is in order, so listen up.
The resolution of video images is 525 lines for NTSC and 625 lines for PAL. NTSC video runs at 30 frames per second and PAL at 25 frames per second. Each video frame is made of two separate subframes called fields. Each of these fields is an individual snapshot in time. By using fields, the Viewer sees twice as many frames and smoother motion. Even though the fields represent different points in time, they occupy the same video frame. This is achieved through a process called interlacing.
Here comes the fun part. Interlacing weaves together the two fields by starting at the top of the image and taking one line from field 1 (the odd field) and another line from field 2 (the even field) until all 525 or 625 lines are interlaced together. Temporally, the fields always occur in the following order: field 1 and then field 2. Spatially, the ordering is different for NTSC and PAL. For NTSC, the spatial field order starts with field 2, or the even field. For PAL, it is the exact opposite with field 1, or the odd field, being the first spatial field.
Therefore, the interlace process produces two fields of half-height for every broadcast frame. When a television displays these images, it quickly shows the first field only, and then the second field only, and then proceeds to the next frame. Each field sacrifices vertical resolution for the benefit of temporal quality.
When deinterlacing is enabled, it strips out the two fields from each other, placing field 1 at frame 1, and field 2 at frame 1.5. Each field is then copied and moved into the empty spatial place of the removed field. This ensures that all spatial effects are properly handled when rendering fields. This strategy is clever because it doubles the number of frames you have, but keeps the frames within the same duration.
3:2 Pullup / Pulldown
What in the world does 3:2 Pullup / Pulldown mean? This is a technique to temporally convert (resolution not being considered here) film footage to video footage and back again. Given that film uses solid frames and video uses interlaced fields, and that film runs at 24 fps and NTSC runs at 30 fps, you split the film footage into fields and double up 2 out of 5 frames to increase your frames to fill the 30 fps. Pullup is the conversion of film footage from 30fps to 24fps while Pulldown is the conversion of 24fps film footage to 30fps.
Let's use the classic diagram:
We see that the third and fourth video frames have field blending in them to stretch time out. It's therefore called 3:2 because you have three solid frames and two mixed frames.
3:2 Pullup
We can fully reconstruct our original four film frames by extracting the field data from the five video frames. Here comes the odd bit. When you receive your footage, it has probably been edited, so it is not necessarily the case that frames three and four are the mixed frames because all of the clips have been shifted around in the edit. We therefore need to figure out what the first frame is before we attempt to remove the extra fields. To do this, you would go to the first five frames in the clip. If the first frame to have field blending is frame three, you know your first frame should be set to AA. If the first frame to have field blending is frame two, then you know your first frame is BB. You would then set your 3:2 parameter accordingly. If your first frames are a solid color and you can't figure it out, you have to jump to a time range of frames that display the blending and start guessing what the first frame is until the fields go away. Very scientific, isn’t it?
Use the 3:2 setting in the chart below that corresponds to your first frame with field blending.
3:2 Pullup Frame Guide
First frame with field blending
3:2 Setting
BC if followed by a blended frame / CD if followed by a solid frame