|Posted on February 28, 2010 at 12:53 AM|
Frame rate is the rate at which videoplays back frames. Black and white video ran at a true 30 frames persecond (fps). When the color portion of the signal was added, videoengineers were forced—for various technical reasons related to thephysical circuits—to slow the rate down to 29.97 fps. This slightslowdown of video playback leads to distortions in the measurement ofvideo vs. real time. Video is measured in indivisible units calledframes. Real time is measured in hours, minutes, and seconds.Unfortunately, a second is not evenly divisible by 29.97 fps. Let'slook at the mathematical relationships involved here:
A frame rate of 29.97 fps is 99.9% as fast as 30 fps. In other words, it is 0.1% (or one-thousandth) slower:
29.97 fps / 30 fps = .999 (or 99.9%)
100 - .999 = 0.1% slower
Conversely, a frame rate of 30 fps is 0.1% (or one-thousandth) faster than 29.97:
30 fps / 29.97 fps = 1.001 (or 100.1%)
(The actual value is 1.001,001,001, ..., 001 repeating infinitely.1.001 is enough precision for our calculation, given that the nextsignificant digit is the one-millionths place. No video program is longenough that the stray millionths of a second per hour will add upenough to throw the frame count off again.)
One hour's worth of "true 30 fps" video contains exactly 108,000 frames:
(30 frames/sec) * (3600 sec/hour) = 108,000 frames
However, if you play back 108,000 frames at 29.97 fps, it will take longer than 1 hour to play:
(108,000 frames) / (29.97 frames/sec) = 3,603.6 seconds = 1 hour and 3.6 seconds
(Actual value is 3,603.603,603, ..., 603 repeating infinitely.
Again, 3,603.6 is sufficient for video timecode, given that
the next loss of precision is three one-thousandths of a second perhour. You would have to make a video over 11 hours long before you wereoff again by a single frame.)
This is notated in timecode as 01:00:03:18. Thus, after an hour it is108 frames too long. Once again, we see the relationship of 108 framesout of 108,000, or one-thousandth.
Now let's apply that discrepancy to 1 minute of video. One minute, or60 seconds, of 30 fps video contains 1800 frames. One-thousandth ofthat is 1.8. Therefore, by the end of 1 minute you are off by 1.8frames.
Remember, however, that frames are indivisible; you cannot adjust by afraction of a frame. You cannot adjust by 1.8 frames per minute, butyou can adjust by 18 full frames per 10 minutes.
Because 10 minutes is not evenly divisible by 18 frames, we usedrop-frame timecode and drop two frame numbers every minute; by theninth minute, you have dropped all 18 frame numbers. No frames need tobe dropped the tenth minute. That is how drop-frame timecode works.When you use drop-frame timecode, Premiere 5.x renumbers the first twoframes of every minute, except for every tenth minute.
NTSC and the drop-frame numbering system
There are three fundamentally important things to remember about NTSC and drop-frame timecode:
• NTSC video always runs at 29.97 frames/second.
• 29.97 video can be notated in either drop-frame or non-drop-frame format.
• Drop-frame timecode only drops numbers that refer to the frames, and not the actual frames.
We will examine the ramifications of these rules below.
NTSC video always runs at 29.97 frames/second
Unlike "true 30 fps" video, an hour's worth of NTSC video does not have108,000 frames in it. It has 99.9% as many frames, or 107,892 frames,as described earlier. Again, at the rate of 1.8 less per minute, anhour of NTSC video has 108 frames less than an hour of "true 30 fps"video:
108,000 * 99.9% = 107,892 frames in an hour of NTSC video
108,000 - 107,892 = 108 frames difference
If we were to sequentially number each of these frames using the SMPTETimecode format, the last frame of the video would be numbered00:59:26:12:
108 frames = 00:00:03:18 in timecode format
01:00:00:00 - 00:00:03:18 = 00:59:26:12
That is 3 seconds and 18 frames shorter than an hour-long video.Drop-frame timecode is a SMPTE standard that maintains time accuracy byeliminating the fractional difference between the 29.97 fps frame rateand the 30 fps
When you use drop-frame timecode, Premiere 5.x adjusts the framenumbering so that an hour-long video has its last frame labeled01:00:00:00.
Timecode measures time in Hours:Minutes:Seconds:Fractions-of-secondscalled frames. However, in NTSC video, a frame is not an even fractionof a second! Thus, NTSC timecode is always subtly off from real time—byexactly 1.8 frames per minute. Drop-frame timecode numbering attemptsto adjust for this discrepancy by dropping two numbers in the numberingsequence, once every minute except for every tenth minute (see thepreceding section, Mathematics of 29.97 video, for details).The numbersthat are dropped are frames 00 and 01 of each minute; thus, drop-framenumbering across the minute boundary looks like this:
..., 00:00:59:27, 00:00:59:28, 00:00:59:29, 00:01:00:02, 00:01:00:03, ...
Note, however, that you are off by only 1.8 frames per minute. If youadjust by two full frames every minute, you are still off by a little.Let's go through a sequence of minutes, to see how far off we are eachminute, and where each adjustment leaves us: Thus, 00:10:00:00 indrop-frame is the same as 00:10:00:00 in real time! Also, 10 minutes ofNTSC video contains an exact number of frames (17,982 frames), so everytenth minute ends on an exact frame boundary. This is how we can getexactly 1 hour of video to read as exactly 1 hour of timecode.
29.97 Video can be notated in either drop-frame or non-drop-frame format
You can notate 29.97 video using drop-frame or non-drop-frame format.The difference between the two is that with drop-frame format the frameaddress is periodically adjusted (once every minute) so that it exactlymatches real time
(at the 10 minute mark), while with non-drop-frame format the frameaddress is never adjusted and gets progressively further away from realtime.
Minute Start Position Frames Lost Drop Frame Adjusted Position
01 1.8 lost this minute drop 2 to correct 0.2 ahead
02 0.2 ahead 1.8 lost this minute drop 2 to correct 0.4 ahead
03 0.4 ahead 1.8 lost this minute drop 2 to correct 0.6 ahead
04 0.6 ahead 1.8 lost this minute drop 2 to correct 0.8 ahead
05 0.8 ahead 1.8 lost this minute drop 2 to correct 1.0 ahead
06 1.0 ahead 1.8 lost this minute drop 2 to correct 1.2 ahead
07 1.2 ahead 1.8 lost this minute drop 2 to correct 1.4 ahead
08 1.4 ahead 1.8 lost this minute drop 2 to correct 1.6 ahead
09 1.6 ahead 1.8 lost this minute drop 2 to correct 1.8 ahead
10 1.8 ahead 1.8 lost this minute drop 0
At the end of an hour-long video, the frame address for drop-frameformat will be 01:00:00:00, while the frame address for non-drop-frameformat will be 108 frames lower (remember, 108 frames out of 108,000,or 0.1%) at 00:59:56:12.
Conversely, at the point where the frame address for non-drop-frameformat reads 01:00:00:00, the frame address for drop-frame format wouldbe 01:00:03:18. Remember, this is longer than 1 hour of real time: 3.6seconds out of 3600, or 0.1%.
Either numbering system could have been used for this theoretical videoprogram. No matter which timecode format you use, the frame rate—29.97fps—would be the same, and the total duration of the program—in realtime—would be the same. The only difference is which address code getsstamped on what frame number.
Drop-frame timecode only drops numbers that refer to the frames, and not the actual
This is nothing complicated; just remember to keep your terminologystraight. Much analog video equipment uses drop-frame SMPTE timecode.Just imagine if analog video were to drop the actual frames! First, itwould visually disturbing to literally drop two frames every minute.Second, and more importantly, analog video equipment is
governed by a certain amount of tape moving past the heads at a certainspeed. Even if the equipment didn't display two frames, there is no wayfor the physical mechanism to make up for the lost time. This is notthe same as with digital video, where a capture or playback device willdrop frames because it simply can't keep up with the amount of databeing streamed through it. Also, when we talk about being 1.8 framesahead or behind, we are referring to the frame numbering scheme being
ahead of real time. It does not refer to the video track being ahead orbehind the audio track; audio that drifts away from its video is adifferent issue,
In summary, "dropped frames" refers to a playback or capture issuerelated to data rates and hardware capabilities; drop-frame timecoderefers to a frame-numbering convention.
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