This article discusses field dominance and explains why it is important to have the correct field dominance applied in your Sequence Settings.

NTSC and PAL are both interlaced video formats, as opposed to computer video and some High-Definition video formats, which are capable of using progressive scanning. All video displays, whether analog or digital, work by breaking a single frame of video down into lines of horizontal resolution running across the screen. When we say that a computer uses progressive scanning, we are saying that these lines are drawn one at a time from the top of the screen down to the bottom, in order.

Figure 1 Horizontal Lines of Resolution on a Computer or High-Definition Monitor

NTSC and PAL video is different from computer video and some High-Definition video formats. NTSC runs at 29.97 frames per second (fps), though it is referred to as 30 fps for the sake of discussion. PAL video runs at 25 fps. However, when you record footage with your camcorder, it does not not capture each whole video frame at once. Instead, each video frame is broken down into two fields, with each field containing half the total lines of resolution in the frame.

Figure 2 Horizontal Lines of Resolution on a Standard Definition Television Monitor

The first field is recorded, then the second, and these are laid down to tape in a linear fashion. Both fields are considered to be one frame, but it is important to remember that motion will occur from one field to the next, making it crucial that each field play back in the correct order. A television monitor then displays each recorded frame in two passes: It draws every other scan line on its way down (field 1), and when it's done, it goes back to the top 1/60th or 1/50th of a second later and fills in the missing lines to draw field 2 and complete one whole video frame.

Video capture cards deal with interlacing in one of two different ways. You will hear the terms Upper, or Field 2 Dominant, and Lower, or Field 1 Dominant. Field Dominance refers to how the video fields are recorded into a file when going from the analog world of tape to your hard drive. If you remember that video fields come one after another temporally, as if we were playing 60 frames per second, it becomes a little easier to understand.

You may ask, how do we determine which is Field 1 and which is Field 2? While this point is somewhat academic, if you put your monitor into underscan mode (where you see the entire video picture surrounded by a black frame around the edge) a Field 1 can be seen as the field that begins with the first full line of video on the screen and includes every other line going on down to the bottom. Field 2 is the field beginning with the little half line of video way at the top of the screen, (which is in fact located above the first line of Field 1) and includes all the other alternating lines going on down to the bottom. Even though the first line at the top of the monitor is the half-line, it's really a part of field 2.

Figure 3 Distinguishing Between Field 1 and Field 2 on a Standard Definition Monitor

Because there is motion between one field and another, and because different capture cards handle interlacing differently, it's important to render your digital video with the field dominance expected by your hardware. In Final Cut Pro, the options are referred to as Upper (equivilant to Field 2) or Lower (equivilant to Field 1) Field Dominant. Generally, Upper Field First is used by 640x480 systems, while Lower Field First is most common in professional 720x486 and DV 720x480 systems.

DV-NTSC, DV-Pal, and Targa 720x486 NTCS are all Lower Field Dominant. If you're unsure what the field dominance is with your particular hardware, refer to the manual that came with your capture card to find which dominance your Sequence Settings should use.

Figure 4 Changing Field Dominance in the Sequence Settings

Changing the Field Dominance setting is required only when either the video hardware setup is changed, or when rendering a Quicktime movie which is intended for Web or CD playback. Only full frame broadcast video uses interlaced fields, so if you are outputing a QuickTime movie for multimedia use, interlacing should be set to None.

Unfortunately, extreme movement in recorded video will result in a variance between fields that appears as a tearing or rippling in the still image. This tearing will appear also when interlaced material is played back on a computer's progressive scan monitor.

Figure 5 An NTSC Still Frame Before (Left) and After De-interlacing.

Notice the presence of thin lines due to excess movement between fields on the image to the left.

To eliminate these artifacts, especially when video is being rendered as a QuickTime movie for multimedia use, Final Cut Pro includes a De-Interlace filter in the Effects tab of the Browser.

Figure 6 The De-Interlace Filter Located in the Video Folder of the Effects Tab

When clips with extreme motion are rendering with excess artifacts due to interlacing, this filter may be applied to eliminate the problem. The De-Interlace filter strips out one field of video, and interpolates the remaining field to make up the difference. This can result in a slight softening of the image, but this is preferable to the artifact that it removes.

Still frames exported to Photoshop may have the same artifacts. A similarly named De-Interlace filter extists for the same purpose.

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