User guide

APPENDIX B: TIME CODE BASICS
84
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