Interlaced video is video
captured at 50 pictures (known as fields) per second, of which every 2
consecutive fields (at half height) are then combined into 1 frame.
Interlacing was developed many years ago for the analog TV world and is
still used widely today. It provides good results when viewing motion
in standard TV pictures, although there is always some degree of
distortion in the image.
To view interlaced video on
e.g. a computer monitor, the video must first be de-interlaced, to
produce progressive video, which consists of complete images, one after
the other, at 25 frames per second. See also Progressive scan.