Apple Final Cut Express HD - Manual (Page 1035)

Appendix A Video Formats 1035
XIII
Scanning Method
A video frame is made of horizontal lines that are scanned from one side of a display to
the other. Progressive video scanning happens when each line of a video frame is scanned
one after another. Interlaced scanning fills the entire frame with only half the lines, which
requires half the time, thus doubling the perceived frame rate and reducing flicker.
About Interlaced Scanning
Frame rates lower than 40 fps 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 1/60th of a second and a field of even lines follows in the next
1/60th of a second, resulting in a complete frame every 1/30th of a second.
Field 1 Field 2
Frame