Apple Final Cut Express HD - Manual (Page 1053)

Appendix B Frame Rate and Timecode 1053
XIII
Drop frame timecode compensates for the fact that the NTSC format has a frame rate
of 29.97 fps, which is .03 fps slower than the nearest whole number frame rate of 30
fps. Timecode can only be represented by whole numbers, so timecode numbers are
periodically skipped in drop frame timecode. This way, the timecode number always
matches the seconds and minutes of video that have played. NTSC can use either drop
frame or non-drop frame timecode.
Important: No video frames are dropped when you use drop frame timecode. Only the
associated timecode numbers are skipped.
You can think of dropframe timecode like leap years on the calendar. In the case of leap
year, an extra day is added every 4 years except when the year is divisible by 400. This
compensates for the fact that the way we measure our days and the way we measure
our years does not align exactly. Even though the difference is slight, an unacceptable
error accumulates over time unless regular adjustments are made to the count.
More About Drop Frame Timecode and NTSC Frame Rate
NTSC video has a frame rate of 29.97 fps, but the timecode counts at 30 fps. To better
understand this subtle distinction, remember that the main purpose of timecode is to
uniquely label and address each video frame, not to tell time (another name for
timecode is address code).
Consider what it would be like if frames were labeled a different way, without any
reference to time. For example, if each frame had a unique address coded with five
letters of the alphabet, starting at AAAAA, AAAAB, AAAAC, and so on until ZZZZZ,
editors would refer to shots and scenes by their individual five-letter codes. A director
requesting a particular shot could look in the log notes and tell the editor to find frame
ABAAA on a particular tape.
On tape or disk, each frame lasts1/29.97th of a second. Since there is an address affixed
to each frame, the timecode moves at the same rate as the video (29.97 fps).