Scan Lines

A PAL television picture is updated 50 times a second. On a standard display, the first 50th of a second contains lines 1,3,5,7 and so on, the second 50th (the picture is made up of lines 2,4,6,8 and so on. This makes up the 25 frames per second. This is known as interlacing and can be seen quite clearly on DVD menus where the screen flickers slightly. On a larger screen these scan lines are more apparent.