Interlacing is a technique developed for transmitting television signals using limited bandwidth. In an interlaced system, only half the number of horizontal lines for each frame of video are transmitted at a time. Because of the speed of transmission, the afterglow of displays, and the persistence of vision, the viewer perceives each frame in full resolution. All of the analog television standards use interlacing. Digital television standards include both interlaced and noninterlaced varieties. Typically, interlaced signals are generated from interlaced scanning, whereas noninterlaced signals are generated from progressive scanning.
Each interlaced video frame consists of two fields. Each field contains half the number of horizontal lines in the frame; the upper field (or Field 1) contains the odd-numbered lines, and the lower field (or Field 2) contains the even-numbered lines. An interlaced video monitor displays each frame by first drawing all of the lines in one field and then drawing all of the lines in the other field. Field order specifies which field is drawn first. In NTSC video, new fields are drawn to the screen approximately 60 times per second, corresponding to a frame rate of approximately 30 frames per second.
Noninterlaced video frames aren’t separated into fields. A progressive‑scan monitor displays a noninterlaced video frame by drawing all the horizontal lines, from top to bottom, in one pass. Computer monitors are almost all progressive-scan monitors, and most video displayed on computer monitors is noninterlaced.
The terms progressive and noninterlaced are thus closely related and are often used interchangeably, but progressive scanning refers to the recording or drawing of the scan lines by a camera or monitor, whereasnoninterlaced refers to the fact that the video data itself isn’t separated into fields.
After Effects automatically separates fields for D1 and DV video footage items. You can manually separate fields for all other types of video footage in the Interpret Footage dialog box.
- Select the footage item in the Project panel.
- Choose File > Interpret Footage > Main.
- Choose an option from the Separate Fields menu.
- Click Preserve Edges (Best Quality Only) to increase image quality in nonmoving areas when the image is rendered at Best quality. Then click OK.
Note: If the field settings in the Interpret Footage dialog box are correct for the input footage and the field settings in the Render Settings dialog box are correct for the output device, you can mix footage items of different field orders in a composition. If either of these settings is incorrect, however, the frames will be in the correct order, but the field order may be reversed, resulting in jerky, unacceptable images.
Determine the original field order
The field order for an interlaced video footage item determines the order in which the two video fields (upper and lower) are displayed. A system that draws the upper lines before the lower lines is called upper-field first; one that draws the lower lines before the upper lines is called lower-field first. Many standard-definition formats (such as DV NTSC) are lower-field first, whereas many high-definition formats (such as 1080i DVCPRO HD) are upper-field first.
The order in which the fields are displayed is important, especially when the fields contain motion. If you separate video fields using the wrong field order, motion does not appear smooth.
Some programs, including After Effects, label the field order when rendering interlaced video files. When you import a labeled video file, After Effects honors the field order label automatically. You can override this field order by applying different footage interpretation settings.
If a file does not contain a field order label, you can match the original field order of your footage. If you are not sure which field order was used to interlace a footage item, use this procedure to find out.
- Select the item in the Project panel.
- Choose File > Interpret Footage > Main.
- In the Interpret Footage dialog box, select Upper Field First from the Separate Fields menu, and then click OK.
- In the Project panel, press Alt (Windows) or Option (Mac OS) as you double-click the footage to open it in the Footage panel.
- If the Preview panel is not visible, choose Window > Preview.
- In the Footage panel, find a segment that contains one or more moving areas.
- Using the Next Frame button in the Preview panel, step forward at least five frames in the Footage panel. Moving areas should move consistently in one direction. If the moving areas move backward every other frame, the wrong field-separation option has been applied to the footage.
This information was reproduced here from the After Effects Help website.