[MacDV] Re: Help Blacking a Tape/Time Code

[Gregg Gorrie]

on 8/11/03 3:40 PM, Derek Roff at derek at unm.edu wrote:
 
> Drop-frame exists because of the physics of wireless TV broadcasting.
> If a program isn't going to be sent out over the airwaves of
> broadcast television, then the difference in speed between drop-frame
> and non-drop frame has little importance.  For short projects, which
> the original posting mentioned, the difference in run time between
> drop frame and non-drop frame is easy to ignore.  So I am curious as
> to why this project seems to be specified in non-drop frame.
> 
> But it gets weirder.  The original posting quoted the specification
> as saying:  "all material should be striped at 29.97 NDF."  29.97
> frames per second is the drop-frame video speed.  30 frames per
> second is the speed for non-drop frame.  "29.97 NDF" seems like an
> untenable hybrid.
> 
> Can anyone explain "29.97 NDF"?  Am I suffering from brain lock, or
> is the film festival's specification confused?

Not sure where you got your misinformation concerning drop frame and
non-drop frame time code, but here are the facts:

1) Due to engineering limitations of the day, when NTSC color TV (North
America and Japan) was brought in a compromise was made. To make color
broadcasts backwards compatible with black and white television sets, they
changed (slowed down) the frame rate from 30 fps to 29.97 fps.

2) Because 29.97 fps still uses a 30 frame count (:00 --> :29), there is a
discrepancy between the length of a program according to the time code and
the real length of the program according to the "wall" clock. For example,
if a program using 29.97 non-drop format starts at 01:00:00:00 and ends at
02:00:00:00, it's true length according to "real" time is one hour plus 108
frames (remember, 29.97 fps is just 30 fps slowed down about .1%)

3) This time discrepancy was unacceptable to broadcasters who run on
precision time, so drop frame time code was created. Drop frame numbering
"skips" 2 frames (:00, :01) every minute EXCEPT for minutes ending in 0 (ie
00, 10, 20, etc.). The net result is that 29.97 DF code "catches up" with
real time and the time code length matches the wall clock time. There is no
change to the actual video itself, drop frame is just a numbering scheme
(sometimes called compensated time code).

4) ALL NTSC video (both DF and NDF), is 29.97 fps - NDF is NOT 30 fps.
Elsewhere in the world (PAL, SECAM at 25fps) there was no need to change the
frame rate when they converted to color TV, hence they had no need to create
a drop-frame system.


The fact that this thread exists demonstrates just how much confusion those
decisions (made half a century ago) still create.
-- 
Gregg