Colour video was slowly introduced into broadcast. It was therefore necessary to make it compatible with black and white receivers and to design colour receivers or televisions to be able to receive black and white programming as well. In order to accommodate the extra information needed for colour the b&ws 30 frame/second rate was slowed to 29.97 f/s for colour. Although usually not an issue for non broadcast applications, in broadcast, the small difference between real time (or the wall clock) and the time registered on the video can be problematic. Over a period of 1 hour (SMPTE) the video will be 3.6 seconds or 108 extra frames longer in relation to the wall clock. To overcome this discrepancy drop frame is used.

Drop frame: Every frame :00 & :01 are dropped for each minute change (60 X 2 = 120) except for minutes with 0s (00:, 10:, 20:, 30:, 40: & 50 (6 X 2 = 12, 120 - 12 = 108)