Apple Final Cut Pro 5 - Manual (Page 1735)

384 Part V Appendixes
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).
Now, instead of using a five-letter code to uniquely tag each frame, consider using an
address code in the format 00:00:00:00. Remember that these numbers don’t reflect
time, they are simply unique identifiers. The first frame of NTSC video will be labeled
00:00:00:00. The 29th frame will be labeled 00:00:00:29 and the 30th frame will be
labeled 00:00:01:00. Again, just because a frame is labeled 00:00:01:00 does not mean
that one second has passed. The frame could just as easily been named AAABD, in
which case there would be no temptation to read the label as a time value. Only the
frame rate of the video can determine how much time has passed by the 30th frame. In
the case of NTSC video, 0.999 seconds have passed by frame 30. By frame 1800, 60.06
seconds have past.
Frame Count
Timecode labels
(30 fps) Time passed
(29.97 fps)
Error between
timecode number
and real time
0 0 0 0
1 1/ 30th second 1/29.97th second Negligible
30 = 30/30ths of a second
= 1 second = 30/29.97ths
of a second
= 1.001 seconds 0.001 seconds
60 = 60/30ths of a second
= 2 seconds = 60/29.97ths
of a second
= 2.002 seconds 0.002 seconds