Apple Final Cut Pro 7 - Manual (Page 1925)

You can think of drop frame timecode as being like leap years on the calendar. In the
case of leap years, an extra day is added every 4 years except when the year is divisible
by 400. This compensates for the fact that the way days are measured and the way years
are measured do 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 the 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 lasts 1/29.97 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 is labeled 00:00:00:00.
The 29th frame is labeled 00:00:00:29, and the 30th frame is labeled 00:00:01:00. Again,
just because a frame is labeled 00:00:01:00 does not mean that 1 second has passed. The
frame could just as easily have 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 passed.
Error between
timecode number and
real time
Time passed
(29.97 fps) Timecode labels
(30 fps)
Frame count
0 0 0 0
Negligible 1/29.97 of a second 1/30 of a second 1
0.001 seconds = 30/29.97 of a second
= 1.001 seconds
= 30/30 of a second
= 1 second
30
0.002 seconds = 60/29.97 of a second
= 2.002 seconds
= 60/30 of a second
= 2 seconds
60
1925 Appendix C Frame Rate and Timecode