About Interlaced Scanning
Frame rates lower than 40 fps can cause noticeable flicker. When NTSC and PAL were
invented, faster frame rates were not practical to implement. Interlaced scanning was
devised to create a perceived frame rate of 60 fps (NTSC) or 50 fps (PAL). Interlaced
video scans the display twice, using two fields, to complete a single frame. A single field
contains only the odd lines (1, 3, 5, 7, and so on) or the even lines (2, 4, 6, 8, and so on)
of the frame. If you could stop the video scanning process to observe a single video
field, you would see that every other line is missing, like venetian blinds or a comb.
Because the fields are changing at twice the frame rate, there is less perceived flicker
than if each frame was scanned progressively. For example, with NTSC, a field of odd
lines is scanned in 1/60 of a second and a field of even lines follows in the next
1/60 of a second, resulting in a complete frame every 1/30 of a second.