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Synchronization
Drop Frame (df): One technical problem was the frame
rate of 29.97 fps. Only whole frames can be counted, but
counting up to the 30th frame in every second would
cause a deviation between the time code time, and the ac-
tual time (a difference of about 5.4 seconds for a typical
feature video). So, the following trick was employed: in ev-
ery minute 2 frames are left out (“dropped”), except in
minutes that can be divided by 10.
This may sound complicated, but is actually simpler than
“one leap-year every 4 years except in years divisible by
100, apart from those also divisible by 400”!
To describe this method, “drop” or “df” is added after the
frame rate. Because it is used so often with 30 fps, it is
more common to indicate non-usage by adding “non
drop”, “nd” or “ndf” to avoid confusion.
The 29.97 nd format is seldom used (just like the point-
less 30 df), because the timecode time “drifts” from “real”
time.
Frame Rates: The SMPTE frame rate must be set the same
for all connected devices; you can’t have different frame
rates in one system. The diagram below shows the various
frame rates with the respective duration of a frame, or sub-
frame:
In Europe 25 fps is used both in the audio field and for syn-
chronizing television or video productions.
American audio productions use mainly 30 fps, but with
video the frame rate is nearly always 29.97 df.
International audio productions are recorded with 25 fps,
and 30 fps on different tracks.
LTC / VITC: The 80 bits per frame of the SMPTE time
code can be stored in two different forms:
– As a sound signal on a sound track. This is called longi-
tudinal time code (LTC).
– As a signal in the scanning gap of the video picture.
(The complete picture information is transferred
slightly quicker than the duration of a frame. There is a
short gap, while the electronic beam of the cathode ray
Frame Rate [fps] Frame [ms] Bit [µs] Source
24 41.67 520.8 Video
25 40 500 PAL
29.97 drop 33.37 417.1 —
29.97 33.37 417.1 NTSC color
30 drop 33.33 416.7 —
30 33.33 416.7 NTSC S/W