Alteration of timecode to match the 1000/1001 speed offset of NTSC transmissions and many newer HD and UHD video formats used in ‘NTSC’ countries – including the USA, Canada and Japan. 525-line NTSC at a nominal 30 f/s actually runs at 29.97 f/s, 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 f/s 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 brings the timecode time almost exactly into step with the video.
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