MOTU Micro Lite [143/166] Why does drop frame exist

APPENDIX B: SMPTE SYNCHRONIZATION BASICS

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WHY DOES DROP FRAME EXIST?

Video was ﬁrst introduced in black and white and it ran at

exactly 30 frames per second. Years later, color video was

developed. The Drop Frame format was developed to

compensate for an accumulating timing error in color

video, which runs slightly slower than black and white

video. Color video frames actually run at a rate of 29.97

frames per second, which is slightly slower than exactly 30

frames per second. Over a period of time, this difference

causes the time code that is counting the frames to fall

behind actual elapsed time.

For example, let’s say our video program is 60 minutes

long. When shown in black and white video at exactly 30

frames per second, it will be precisely 60 minutes long. In

addition, the time code that counts the frames will show

01:00:00:00 (exactly one hour’s worth of frames) on the

ﬁnal frame. So far, so good.

Now, if we play a color version of the same program, it

actually runs slower at 29.97 frames per second so that the

actual elapsed time is 60 minutes and 3.6 seconds! Here’s

where the discrepancy arises: the time code that counts the

frames shows that one hour’s worth of frames has gone by,

which is 01:00:00:00 on the ﬁnal frame. But this does not

match the actual elapsed time, which is 01:00:03:18! In

broadcast situations, where edits are calculated down to

fractions of a second, 3.6 seconds is a long, long time––too

large a degree of inaccuracy.

Drop Frame time code ﬁxes this problem by skipping

ahead every once in a while as it counts color video frames

to catch up with actual elapsed time. The result is that over

the period of several minutes, the time code matches the

actual elapsed time.

It is important to note that since frames are dropped only

once every minute, Drop Frame time code does not always

reﬂect the exact actual elapsed time: it may be up to a 10th

of a second faster or slower than actual elapsed time,

depending on how recently the last frame number was

dropped.