In interlaced video, a frame is divided into two fields . Each field contains every other horizontal line in the frame. A TV displays the first field of alternating lines over the entire screen, and then displays the second field to fill in the alternating gaps left by the first field. The field that contains the topmost scan line in the frame is called the upper field , and the other field is called the lower field . When playing back or exporting to interlaced video, make sure the field order you specify matches the receiving system, otherwise motion may appear stuttered, and edges of objects in the frame may break up with a comb-like appearance.
Interlaced video describes a frame with two passes of alternating scan lines.
Progressive-scan video describes a frame with one pass of sequential scan lines. Television signals are interlaced because of the nature of early televison sets and the nature of human vision. The United States uses a 60Hz power cycle, but early television sets were only able to display at a 30 fps frame rate. Interlacing two 30 fps fields achieved an effective 60 fps frame rate, which solved the problem of low bandwidth and was high enough to provide adequate persistence of vision. This early solution has made interlacing at 30 fps the US standard.