Scanning Method

A video frame is made of horizontal lines that are scanned from one side of a display to the other. Progressive video scanning happens when each line of a video frame is scanned, one after another. Interlaced scanning fills the entire frame with only half the lines, which requires half the time, thus doubling the perceived frame rate and reducing flicker.

About Interlaced Scanning

Frame rates lower than 40 fps can cause noticeable flicker. When NTSC and PAL were invented, faster frame rates were not practical to implement. Interlaced scanning was devised to create a perceived frame rate of 60 fps (NTSC) or 50 fps (PAL). Interlaced video scans the display twice, using two fields, to complete a single frame. A single field contains only the odd lines (1, 3, 5, 7, and so on) or the even lines (2, 4, 6, 8, and so on) of the frame. If you could stop the video scanning process to observe a single video field, you would see that every other line is missing, like venetian blinds or a comb.

Because the fields are changing at twice the frame rate, there is less perceived flicker than if each frame was scanned progressively. For example, with NTSC, a field of odd lines is scanned in 1/60 of a second and a field of even lines follows in the next 1/60 of a second, resulting in a complete frame every 1/30 of a second.

Figure. Diagram showing a field of odd lines and a field of even lines combined to create the complete image of a frame.

About Progressive Scanning

Progressive scanning is much simpler than interlaced scanning: each line is scanned consecutively until a complete frame is drawn. Computer displays and many recent HD televisions use progressive scanning.

Here are some significant facts about interlaced and progressive scanning methods:

  • Interlacing provides twice the perceived frame rate with only half the recording or transmission requirements.

  • Progressive scanning is preferred when interlacing artifacts (such as thin flickering horizontal lines) would be unacceptable. Progressive images are often considered more film-like because there are no flickering interlacing artifacts.

  • Computer displays are almost always scanned progressively.

  • NTSC and PAL televisions always use interlaced scanning.

  • Many HD video cameras can record progressive frames.

  • Video destined for computer-only use, such as web video, should always be made progressive.

About Field Dominance

Field dominance is an issue when recording and playing back interlaced video material. With progressive video, there is only one way to play back a video frame: start at line 1 and scan until the end of the last line. With interlaced video, the video player must know whether to scan the odd lines first, or the even lines. In other words, each time a frame is displayed, which field should be displayed first, field 1 or 2? The field displayed first is totally dependent on which field was captured by the camera and recorded first.

Each field is a snapshot in time, so if field 1 is recorded earlier in time than field 2, field 1 must be played back before field 2. If the wrong field order is chosen, each frame’s fields play backward in time, even though each frame as a whole still moves forward. The effect is a very noticeable stutter happening 60 (NTSC) or 50 (PAL) times a second.

Each piece of video equipment and each video format has a preferred field dominance. This prevents you from, say, editing two field 2s back to back, and makes sure that each field is played back in the right order.

Setting Field Dominance in Final Cut Pro

In Final Cut Pro, the field dominance of clips must match the sequence field dominance. Otherwise, the fields stutter during playback because each pair of fields plays back in the wrong order. For example, DV NTSC and DV PAL always have a field dominance of Lower (Even). If you’re working in a sequence and you see that imported clips are flickering, check to make sure the field dominance of those additional clips matches the field dominance of your edited sequence.

Important: You need to change the Field Dominance setting of your projects and sequences only if you change your video hardware setup.

In Final Cut Pro, there are two options for field dominance:

  • 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, whereas Lower is most common in professional 720 x 486 and DV 720 x 480 systems.