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About D1, DV, and various pixel aspect ratio footage
Pixel aspect ratio specifies the ratio of width to height of one pixel in an image. Frame aspect ratio describes the ratio of width to height in the frame dimensions of an image. For example, D1 NTSC has a pixel aspect ratio of 0.9 (or 0.9 width by 1.0 height). It also has a frame aspect ratio of 4:3 (or 4.0 width by 3.0 height).
Some video formats output the same frame aspect ratio but use a different pixel aspect ratio. For example, some NTSC digitizers produce a 4:3 frame aspect ratio, with square pixels (1.0 pixel aspect ratio), and a resolution of 640 x 480. D1 NTSC produces the same 4:3 frame aspect ratio but uses rectangular pixels (0.9 pixel aspect ratio) and a resolution of 720 x 486. D1 pixels, which are always rectangular, are vertically oriented in systems producing NTSC video and horizontally oriented in systems producing PAL video.
If you display rectangular pixels on a square-pixel monitor without alteration, images and motion appear distorted; for example, circles distort into ovals. However, when displayed on a broadcast monitor, the images will be correct.
When you import D1 NTSC or DV source footage into After Effects, the image looks slightly wider than it does on a D1 or DV system. (D1 PAL footage looks slightly narrower.) The opposite effect occurs when you import anamorphic footage using D1/DV NTSC Widescreen or D1/DV PAL Widescreen. Widescreen video formats have a frame aspect ratio of 16:9.
If a file uses rectangular pixels, After Effects displays the pixel aspect ratio next to the file's image thumbnail in the Project window. You can change the pixel aspect ratio interpretation for individual files in the Interpret Footage dialog box. By ensuring that all footage files are interpreted correctly, you can combine footage with different ratios in the same project or composition and generate output that plays correctly.
When you import footage with either the D1 resolution of 720 x 486, or the DV resolution of 720 x 480, After Effects automatically sets the pixel aspect ratio for that file to D1/DV NTSC. When you import footage with the D1 or DV resolution of 720 x 576, After Effects automatically sets the pixel aspect ratio for that file to D1/DV PAL. However, it is always a good idea to make sure that all files are interpreted correctly by looking in the Project window or the Interpret Footage dialog box. For information about using interpretation rules to automate the import process, see Specifying interpretation rules.
Note: Make sure to reset the pixel aspect ratio to Square Pixels when you import a square-pixel file that happens to have a D1 or DV resolution--for example, an Adobe Photoshop image with a resolution of 720 x 480.