MOTU Digital Timepiece Instruction Manual download pdf (Page 149)

APPENDIX B: SMPTE SYNCHRONIZATION BASICS

147

drop-frame, and 30. 24 is the standard frame rate

for ﬁlm in the US; 25 is the European format for

ﬁlm; 30 is the US standard for audio; and 29.97

drop and non-drop are used for video. Drop frame,

explained in detail in the next section, allows

SMPTE time code numbers to precisely match the

actual elapsed time.

WHAT IS DROP FRAME?

Drop Frame SMPTE time code counts frames at a

rate of 29.97 frames per second but skips two frame

numbers at the beginning of each minute, except

every 10th minute. When the time code display

reaches HH:MM:59:29 (59 seconds and 29 frames

at the end of each minute), the frame count skips

00 and 01 and jumps ahead to HH:MM:00:02. This

jump does not happen at minutes 00, 10, 20, 30, 40,

and 50.

Thus, frame numbers such as 11:14:00:00 and

11:14:00:01 do not exist in Drop Frame: the display

will show a frame at 11:13:59:29 and the next frame

at 11:14:00:02. However, frame numbers at each

tenth minute will not be skipped, such as from

11:19:59:29 to 11:20:00:00, followed by 11:20:00:01

and 11:20:00:02, etc.

Keep in mind that only numbers are skipped, not

actual frames of the picture. In other words, every

picture frame gets a frame number and the

numbers skip every once in a while.

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.

SHOULD I USE DROP FRAME?

Use Drop Frame time code only when it is

absolutely necessary. Drop Frame is required only

with color video projects in which the SMPTE time

code numbers must precisely match the actual

!USB Interfaces Manual Book Page 147 Tuesday, October 10, 2000 12:43 PM

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