There are lots of head-scratchers in the world of video and I don’t think many are more perplexing than fractional frame rates and drop frame vs non-drop timecode. I won’t get into the technical history of why frame rates like 29.97, 23.98, 23.976 and 59.94 exist but I will try and alleviate some confusion about how they relate (or don’t relate) to timecode.
First off, I want to differentiate between frame rate and timecode. Frame rate is how quickly frames are captured or played back (24.00 frames per second, 29.97 frames per second, etc.,) where as timecode is just a labeling system to give each frame a unique number. There is no inherent link between the two and right off the bat I should make it clear that a frame rate of 29.97 is not the same thing as 30.00, 23.976 is not the same thing as 24.00 and 59.94 is not the same as 60.00.
For example:
A 60 min clip of 30.00 fps footage will contain 108,000 frames.
A 60 min clip of 29.97 fps footage will contain about 108,107 frames.
If we label frame 1 as 00:00:00:00 and count up one number per frame we will obviously get two different final numbers that represent the same duration of time. This is where drop frame timecode comes into the picture. Drop frame timecode will systemically skip (or ‘drop’) timecode numbers so that the timecode readout of the 29.97 footage will stay the same as the timecode readout of the 30.00 footage (which is the same as real time). It will not drop frames of video (even though the name implies that).
Let’s say we have two pieces of rope that are both 60 inches long. Rope A is divided into 60 sections each 1 inch long while Rope B is divided into 120 sections each 0.5 inches long. If I sequentially number the sections of rope, Rope A will be numbered 1 through 60 and Rope B will be numbered 1 through 120. This is analogous to non-drop frame timecode. If I need Rope B to end on the same number as Rope A (60) then I will have to skip numbers along the way to get there. This is analogous to drop frame timecode. I didn’t skip any actual pieces of Rope B I just adjusted the numbering system so it would line up with Rope A’s.
The FCP 7 manual actually has a pretty good write up about frame rates and timecode (it’s where I pulled some of my numbers from) so if you want to dig a little deeper into this I would check it out.