From here:
https://blog.frame.io/2017/07/17/timecode-and-frame-rates/
What is Drop Frame Timecode?
Remember that the NTSC frame rate is 29.97fps instead of 30fps, which means that .03 frames are unaccounted for every second. Since timecode can only count in whole frames, after an hour there should be 30fps x 60sec/min x 60min/hr = 108,000 frames. Because NTSC is 29.97fps, after an hour there will be 29.97fps x 60sec/min x 60min/hr = 107,892 frames.
So there’s a discrepancy of 108 frames in NTSC, which means that after one hour of real time, the timecode on your recording would be behind by 3.6 seconds (108 frames/30fps = 3.6sec). The timecode count would be 01:00:03:18.
How does drop-frame work?
Drop Frame Timecode works by dropping two frame numbers from each minute except every tenth minute. Your recording is unaffected because it drops frame numbers, not actual frames! Because it drops those numbers, at one hour in real time, your timecode will increase by exactly one hour.
It looks like this:
Note that drop-frame timecode is written with a semicolon at before the last number, instead of a colon.
Logically, you use Drop Frame (DF) timecode when you shoot material at 29.97fps or 59.94i (59.94 interlaced) because it’s meant for TV broadcast. The general confusion around all these identical-looking frame rates means that sometimes people still refer to this as 30fps or 60i even though that’s technically incorrect. If you look back at the various frame rates and the standards they apply to, the only one left that is 30fps is ATSC, which is compatible with 29.97fps / 59.94i. 30fps and 60i are uncommon but unfortunately, some recording devices do record in those formats, so it’s important to make sure that your frame rate is exactly what you think it is.
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