Drop Frame vs. Non-Drop Frame rate

By Mickey on January 30, 2008 5:41 PM | Permalink | TrackBacks (0)

I have been asked many times to explain the difference between Drop Frame and Non-Drop Frame. I found a fairly detailed explanation the will hopefully help you understand the difference. US TVs display 29.97 frames a second. When NTSC was defined, they found out there were some problems with the video signal carrier fequency running at 30 frames per second (fps). So they decided to shift things by 1/1000 to 29.97 fps which is 0.999 of 30 fps.The two of the more commonly used combinations of time display settings are 30 fps drop-frame timecode and 30 fps non-drop-frame timecode. When the frame rate is a non-integer number as is the case with the NTSC frame rate of 29.97 frames per second a compromise of one sort or another must be made in displaying time. Either the time display can accurately show clock time (after one hour, the time display shows 1:00:00:00) or the time display can be continuously numbered (frame n is always followed by frame n + 1, modulo the number of frames per second). Drop-frame timecode does the former; non-drop-frame timecode does the latter. In the case of NTSC 30 fps drop-frame timecode, two frame numbers are skipped for each minute, except for every tenth minute. Drop-frame timecode is conventionally indicated by separating the time units with semicolons. The most common case for which drop-frame versus non-drop-frame timecode is relevant is 29.97 fps NTSC, but it also applies to 23.976 fps and 59.94 fps.