MOTU micro express (USB) [84/94] What is drop frame

APPENDIX B: TIME CODE BASICS

60. 23.976 and 24 are the standard frame rates 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. 60 is used for HD 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.

!USB Interfaces Manual Book Page 84 Tuesday, August 16, 2011 11:21 AM