White Paper

June 25, 2001

Digital Video - Drop frame / Non drop frame
By Glenn Koenig

Ok. I won't go into the history on WHY NTSC runs at 29.97 frames per second, there are other people, such as Mark Schubin at Videography magazine that can tell you much better than I can. (You wanted simple, ... this takes a lot of words but is as simple as I can think to put it.) What it means to you: If you pick up a camera and shoot regular NTSC video, each frame is just slightly longer than 1/30th of a second (only 29.97 go by in a second, so a full 30 of them is 1/1000 longer than a second). If you're using time code, the time code counter faithfully counts frames and clicks over to count 1 second after 30 frames, one minute after 60 seconds, etc. But since the frames are longer than 1/30th of a second, your time code counter will eventually show 1 hour when in fact you've been shooting for more than 1 hour. Actually, you will have shot exactly 3.6 seconds longer than the 1 hour and for most of us, this is not a big problem (this is known as non-drop frame time code). But in the world of broadcast television, 3.6 seconds every hour is enough to get their entire schedule out of whack so they wanted an ALTERNATIVE WAY of counting time code so that when the time code counter showed 1 hour, you had really been shooting for exactly 1 hour, not 3.6 seconds longer. So the time code counter skips a count every so often, kind of like having leap years in reverse (you miss a day instead of adding one) to make it match 'real' time (this is 'drop frame' time code). You can do the arithmetic if you want. Keep in mind that there are no frames ACTUALLY dropped, just frame NUMBERS that are skipped. The video you record is still the same video in either system. Just as in real life, the earth is revolving around the sun at the same speed as always, it's just that our calendar gets a little off as we go along, so we make a correction in how we count the days. We're still getting older just as fast (or slowly, for you optimists). The important thing is that most people don't need to use drop frame. Set your system to non-drop frame unless exact time is very important to you.

For film that runs at 24 frames per second (fps), in the old days, they used to show 4 frames of film a little too fast (1/30th sec. each) and record each of them as one video frame. Then they'd show the 4th frame of film a second time and record that as the 5th frame of video. A special cam in the projector just left the 4th frame in the gate for a second splash of light. This worked because 4/24ths of a second is the same as 1/6th of a second of film and 5/30ths of a second of video, so it made the total run time of the movie on video come out the same but the movie had a 'time jerkiness' to it in a strange way when you watched the video made from it. Now, they take advantage of the fact that NTSC video is really 60 fields (half frames, interlaced with each other to form 30 frames) per second so they use what is called "3/2 pulldown" (say "three two pulldown"). This means that some frames of film are used to record 3 fields of video (a frame and a half) and some to record 2 fields of video (one frame). They alternate this (I forget the exact sequence) so that they end up going through 4 frames of film for every 5 frames of video that is recorded. This makes the result on video look smoother. A drawback crops up when someone has converted a film to video, will then edit the video, but then expects to make an edit decision list (of frame numbers of each clip in the movie) and go back and cut the film negative to match. Well it doesn't take long to discover that if you cut the video between certain frames, your edit decision list will have you trying to cut film frames in half if you wanted the exact same edit points for the clip. So some computerized editing systems were designed to remember which frames of video you "shouldn't" cut between and automatically prevented the editor from cutting there. Later, the people with the white gloves (literally!) who have to cut the film negative to "conform" to the edit decision list aren't expected to do the impossible.

Sorry about writing a book, here. Perhaps I should (write a book)? There are books out there that probably explain all this in a hundred different ways. I hope this one works for you.

Glenn Koenig
Open Eyes Video Arlington, Mass.

copyright © Glenn Koenig 2001

Most DV cameras default to drop frame. You don't need to set them. Unless you have a Sony DSR 200 or better, it will always be drop frame. When you see that semi-colon between the seconds and frames, you know it is drop frame.

TC drop or Non-drop HAS NOTHING to do with audio-video sync. TC is merely a counting system, and has no effect on playback speed.

Ralph Fairweather.


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