Alteration of timecode to match the 1000/1001 frame-rate offset of NTSC transmissions and many newer HD and UHD video formats used in territories that broadcast or used to broadcast ‘NTSC’ analog color signals – including the USA, Canada and Japan. The timecode is modified by dropping two frame counts from each minute except for each 10th minute.
For historical reasons 525- line NTSC color transmissions with a nominal frame-rate of 30 fps actually runs at 29.97 fps, 720 and 1080-line HD as well as 2K and 4K UHD all include the 1000/1001 offset frequencies of nominal 24, 30 and 60Hz frame rate. So even 24 fps movies run at 23.97 Hz.
With drop-frame timecode, the timecode is locked to the video and it needs to make up 1 in 1001 frames. It does this by counting two extra frames every minute while the video remains continuous. So 10:35:59:29 advances to 10:36:00:02. In addition, at every ten-minute point the jump is not done. This maintains the timecode time almost into step with the real-time reference.
Timecode that does not use drop-frame is then called non drop-frame time-code. Confusion arises when the wrong one is used!
See also: 1000/1001
Website: www.dropframetimecode.org