About Drop Frame and Non-Drop Frame Timecode
NTSC video (black-and-white) originally had a frame rate of 30 fps, so the timecode
counted at 30 fps. However, NTSC color video (the only kind of NTSC video in use
today), has a frame rate of 29.97 fps. This subtle difference between 30 fps and 29.97 fps
seems practically negligible and, in many cases, ignoring this discrepancy is fine. But
not always. What editors needed, especially in expensive broadcast markets, was
timecode that accurately reflected the exact duration of a program on tape.
There are two types of 30 fps timecode for use with NTSC video: non-drop frame and
drop frame. Non-drop frame timecode is simple: for every frame of video, there is a
corresponding timecode number. The timecode increments without any compensation.
In almost all cases, timecode is non-drop frame. In fact, drop frame timecode only
matters in the case of NTSC video.
Hours Minutes Seconds Frames
Frame Rate and Timecode
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.
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.