DigitalVideoAndHDTVAlgorithmsAndInterfaces.pdf

The final field in an hour of
480i29.97 video has DF code
hh:59:59;29 and NDF code
hh:59:56:23.
hh:mm: ss: ff
xx:x0:00:00
xx:x1:06:20
xx:x2:13:10
xx:x3:20:00
xx:x4:26:20
xx:x5:33:10
xx:x6:40:00
xx:x7:46:20
xx:x8:53:10
Figure 32.1
Periodic dropped
timecode numbers
Dot present for field 2
Comma for dropframe
Figure 32.2 Timecode
displayed, or represented in
ASCII, has the final delimiter
(separating seconds from frames)
selected from colon, semicolon,
period, and comma, to indicate
dropframe code and field 2.
Counting frames at the NTSC frame rate of 29.97 Hz is
slower than realtime by the factor 1000 ⁄1001, which, in
NDF code, would result in an apparent cumulative error
of about +3.6 seconds in an hour. To make timecode
correspond to clock time, approximately once every
1000 frames a frame number is dropped – that is,
omitted from the counting sequence. Of course, it is
only the number that is dropped, not the video frame!
Frame numbers are dropped in pairs in order to maintain
the relationship of timecode (even or odd frame
number) to NTSC colorframe (A or B).
Dropping a pair of frames every 66 2 ⁄3 seconds – that is,
at an interval of 1 minute, 6 seconds, and 20 frames –
would result in dropping the codes indicated in
Figure 32.1 in the margin. Although this sequence is not
easily recognizable, it repeats after exactly ten minutes!
This is a consequence of the ratios of the numbers: Two
frames in 2000 accumulates 18 frames in 18000, and
there are 18000 intervals of 1 ⁄30 second in 10 minutes
(30 frames, times 60 seconds, times 10 minutes). To
produce a sequence that is easy to compute and easy to
remember, instead of dropping numbers strictly
periodically, this rule was adopted: Drop frame numbers
00:00 and 00:01 at the start of every minute, except the
tenth minute. In effect, a dropped pair that is due is
delayed until the beginning of the next minute.
Figure 31.2 depicts the convention that has emerged to
represent field identification and of the use of dropframe
code in timecode displays.
Dropframe does not achieve a perfect match to clock
time, just a very good match: Counting dropframe code
at 30 ⁄1.001 frames per second results in timecode that is
about 86.4 ms late (slow) over 24 hours. If the residual
error were to accumulate, after 11 or 12 days timecode
would fall about one second later than clock time. If
a timecode sequence is to be maintained longer than
24 hours, timecode should be jammed daily to reference
clock time at an innocuous moment. No standard
recommends when this should take place; however, the
usual technique is to insert duplicate timecode numbers
CHAPTER 32 TIMECODE 383