So what is drop frame coding?

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.

Time code for video is displayed in drop frame coding as HH;MM;SS;FF format. The semi-colon denotes drop frame coding. Non drop frame coding uses a colon (HH:MM:SS:FF). No fractions of frames are displayed in either type of coding. To account for these fractions, drop frame coding is used. Frames are not dropped! Only the count of frames is dropped. If we counted the frames in a 10 minute section of video, it will be exactly 17982 frames which is 29.97 fps. Desktop video applications that do not use drop frame coding will have severe audio / video sync problems. After about 8 minutes, the video will be almost a half second ahead of the audio! (8 min * 30 fps = 14400 frames, 8 min * 29.97 fps = 14385.6 frames, the difference is 14.4 frames or about half a second). This only applies to rendering video to NTSC. 30 fps works just fine on computers (if your computer has enough power to spit out that kind of video).

So how do you "count" time in drop frame? The first minute has 1800 frames (30fps). The next 9 minutes have 1798 frames each. There is no 01 or 02 frame count in those 9 min. Time code for the first minute goes from 00;59;29 to 01;00;02. The next 9 minutes are similar. The 10 minute time code goes from 09;59;29 to 10;00;00. The cycle repeats as if it was at time code 00;00;00.

The following figure should help a little.

So how far "off" is the time code in drop frame coding? The following table displays the difference in number of frames for the first 12 minutes of video. Notice how the entire cycle repeats every 10 minutes. The time code is never off of "actual time" by more than 2 frames and only off by 1 on the average.