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.
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Part V
Appendixes
If you edit an hour-long program on NTSC video, the 30 fps timecode will indicate the
last frame of the program is frame 108,000, labeled as timecode 01:00:00:00 (non-drop
frame). However, the table above shows that because the video actually runs at
29.97 fps (each frame is slightly longer than if it were running at 30 fps), one hour has
actually passed at frame 107,892 (3.6 seconds earlier than the 30 fps timecode shows).
What editors wanted, particularly in television environments, was a method of frame
addressing that accurately reflected how much time had passed.
Drop frame timecode was invented to compensate for the discrepancy between
29.97 and 30 fps. Every minute except each tenth minute, two timecode numbers are
dropped from the timecode count. This drop frame mode of 30 fps timecode remains
accurate compared to the actual time passed, with a strange side effect that two
numbers each minute vanish from the count.
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
1800
1800/30ths of a second
= 60 seconds
= 1 minute
=1800/29.97ths
of a second
= 60.06 seconds
= 1.001 minutes
0.001 minutes
0.06 seconds
1.8 frames
18,000
= 18000/30ths
of a second
= 600 seconds
= 10 minutes
= 18000/29.97ths
of second
= 600.6 seconds
= 10.01 minutes
0.01 minutes
0.6 seconds
17.9 frames
108,000
= 108000/30ths
of a second
= 3600 seconds
= 1 hour
=108000/29.97ths
of a second
= 3603.6 seconds
= 1.001 hours
0.001 hours
3.6 seconds
107.89 frames
Appendix B
Frame Rate and Timecode
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V
Timecode on Tape
There are several kinds of timecode recorded on videotape, each stored in a different
part of the video signal. LTC timecode is stored as an audio signal, while VITC is stored
in a line of each video frame.
 LTC (longitudinal timecode) is typically recorded as an audio signal on a dedicated
timecode track. You can add or change LTC timecode on your original tapes even
after they’ve been recorded because it’s recorded on its own independent track.
LTC is also used with professional audio formats, such as DAT and other multitrack
audio recorders. Because LTC is audio, it can be interpreted by a timecode reader
even when the tape is fast-forwarding, but it can’t be read when the tape is paused
or moving extremely slowly.
 VITC (vertical interval timecode) is recorded as part of the video signal, using several
video lines that are normally masked by consumer televisions. You can see VITC as a
series of white dots at the top of the video frame if you view the video on a
professional monitor in underscan mode. Because VITC timecode is part of the video
signal, it can only be changed on your original tapes if you also replace the video
itself, which is rarely worthwhile. The VITC part of the video signal is not captured as
part of a Final Cut Pro clip’s video frame, but the VITC timecode can be captured
directly from the device control connection. Because VITC is part of the video signal,
which most decks can show in slow motion or even pause, VITC can be read at very
slow speeds, but it breaks up when fast-forwarding or rewinding.
Most timecode readers can automatically switch between LTC and VITC if they are both
available. In Final Cut Pro, you can choose this setting in your device control preset:
 LTC+VITC: If you choose this setting, Final Cut Pro looks at both timecodes so that
accurate timecode can be read no matter what speed the tape is playing (LTC is used
for normal and high-speed playback; VITC is used for slow motion and pause).