The pixel aspect ratio of a video stream can be computed from the display aspect ratio (usually 4:3 or 16:9) and the frame width and height. For example, a DVD 4:3 video stream encoded at 720x480 would have a PAR of (4*480):(3*720), or 8:9; the same stream encoded at 704x480 would have a PAR of 10:11.
Note that video captured by standard-definition digital camcorders is 720 pixels wide but has a PAR of 10:11, giving a display aspect ratio of a little bit wider than 4:3. This has to be taken into account when converting DV to DVD or other formats.
See also aspect ratio.