Apple Final Cut Pro 7 - Manual (Page 1903)

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.
Field 1 Field 2
Frame
About Progressive Scanning
Progressive scanning is much simpler than interlaced scanning: each line is scanned
consecutively until a complete frame is drawn. Computer displays and many recent HD
televisions use progressive scanning.
Here are some significant facts about interlaced and progressive scanning methods:
• Interlacing provides twice the perceived frame rate with only half the recording or
transmission requirements.
• Progressive scanning is preferred when interlacing artifacts (such as thin flickering
horizontal lines) would be unacceptable. Progressive images are often considered more
film-like because there are no flickering interlacing artifacts.
• Computer displays are almost always scanned progressively.
• NTSC and PAL televisions always use interlaced scanning.
1903 Appendix B Video Formats