Drop Frame Timecode

Drop-frame timecode (DF) is a counting convention that solves a specific arithmetic problem in NTSC video: the actual frame rate is 29.97 fps (or 23.976 in film-pulldown contexts), not exactly 30. If a timecode counter ticked at exactly 30 frames per second of timecode but only 29.97 frames of video passed in a real second, the timecode would drift ahead of wall-clock time by about 3.6 seconds per hour — disastrous for broadcast where program lengths must be exact. Drop-frame fixes this by skipping two frame numbers at the beginning of every minute (e.g., 00:01:00:00 jumps to 00:01:00:02) except every tenth minute (00:10:00:00, 00:20:00:00, etc.). Over the course of an hour this drops 108 frame numbers total, exactly compensating for the 0.03 fps deficit. Importantly, no actual frames are dropped — only the labels are skipped; the video plays continuously. Drop-frame timecode is signaled by semicolons in the display (HH:MM:SS;FF) instead of colons. It is the standard for NTSC broadcast deliverables in North America, Japan and certain other regions; non-drop is standard for PAL (25 fps, no fractional rate), film (24 fps), and modern non-broadcast contexts. SMPTE ST 12 codifies the algorithm.