Why is interlacing used?
To produce the illusion of smooth motion to the human eye the image rate must be at least 24 fps. However at 24 fps the image flickers badly. To remove the flicker 48 fps are needed. The minimal way to do this is to show each image frame twice 24 times a second. That is what you see when you watch a film at the cinema. An interlaced standard such as 1080i25 shows half the lines of the picture on each frame so that the illusion of motion is achieved without flicker for the most efficient use of bandwidth. When you watch a film on TV, each frame is scanned into a pair of interlaced frames which are shown successively. TV cameras capture the half frames at staggered intervals so the motion should be smoother when watching a sequence from a TV camera with an interlaced standard. However the 1080i standard is most appreciated over 720p because of its higher resolution for still images.
