> Again, this explanation is not quite right - though it is right
mathematically, it isn't right semantically. I point this out because
your post says that there are 108 fewer frames in DF than in NDF.
I state:
I quite clearly did NOT say that. I stated the difference between the
number of actual frames between 29.97fps NTSC video and 30fps video.
Steve also said:
> The problem is that in NTSC there is no such thing as 30fps.
I repeat:
I know. I said as much and carefully indicated that NTSC is 29.97.
The point of my discussion was not about what's available NOW. My
discussion concerned the origins of the need for Drop-Frame TC. It
came from the fact that originally, B&W video WAS 30fps (early on, it
was even locked to the incoming AC power frequency of 60Hz, which at
the time was pretty reliable). When NTSC color 29.97 video was
introduced (before there was time code), the change in frame rate was
not considered a problem (oh look how wrong THAT turned out to be!).
The need for time code complicated things in ways we may never be rid
of.
Steve offered:
> The thing that trips so many people up (not saying either one of you,
Philip or Dennis, but those who read explanations like this) is that
the FRAMES themselves aren't being skipped, it's the NAMING of the
frames that's being skipped.
I say:
That does trip up a lot of people. But if those who read explanations
like mine do not understand the intended meaning of the discussion, I
can't help them. I try to write my explanations as clearly as I can.
The problem is not in the explanation, the problem is in what's being
explained. I was not referring to the confusion of skipped frames vs
skipped numbers. That was never mentioned because it's not what was
being discussed. If those reading my work are confused by what I
write, that's one thing. Perhaps I need to work a little harder at
making things more clear. But I cannot help those who don't understand
what's even being discussed.
Steve:
> Just to remember that DF vs. NDF is really simply a naming
convention, NOT a change in speed, NOT a change in the actual number of
frames per second and NOT actual "dropped frames."
I clarify:
My comments were about the origin of the need for DF, NOT the
differences between DF and NDF. Therefore, we are BOTH correct. I
described the difference between 29.97fps NTSC video and exact 30fps
video. There are 108 fewer ACTUAL frames in NTSC color video (running
at 29.97fps) than there are in exact 30fps video (which used to be B&W
video in the US and now is hard to find here). NTSC 29.97fps is
actually running at a slower frame rate than 30fps video (hence the
frame rate designation of 29.97). As I said, there are 108 fewer
frames in 29.97fps than there are in 30fps video. In 30fps video, an
hour-long program would be an hour of TC without any compensation
needed. But if you run the video at 29.97fps, you'll have fewer frames
and of course fewer frame numbers, making it to be less than an hour of
TC numbers once you've reached an hour of actual clock time. Since the
point is to make the time code numbers equal accurate clock time,
numbers (not frames) must be skipped when using 29.97fps NTSC video in
order to get DF time code to accurately reflect real clock time.
Dennis Degan, Video Editor-Consultant-Knowledge Bank
NBC Today Show, New York
- - - - - - - - - - -
On Oct 21, 2009, at 4:34 PM, I previously wrote:
> This is what I commented on the above-referenced blog -
> "Phillip, your diagram doesn't explain WHY there is a need for Drop
> Frame Time Code. Here's my explanation:
> The difference in the total number of frames between 29.97fps NTSC
> video and exact 30fps video is approximately 108 frames per hour. IOW,
> there are 108 FEWER frames of video at 29.97fps than there are in
> 30fps video. In order to make 29.97fps NTSC video time code run at
> accurate clock time (where an hour of video displays exactly one hour
> of DF
> TC), those 108 frame numbers must be skipped so that the clock time
> (as represented by time code) will 'catch up' to actual clock time and
> be accurate at the end of the hour.
> It was decided (I think SMPTE made this the standard) to skip 2 frames
> every minute except on the tens-of-minutes. There are of course 60
> minutes per hour. If you skipped 2 frames for each minute, that would
> be 120 skipped numbers, which is too many skipped frame numbers. By
> NOT skipping frame numbers on the tens-of-minutes, there would be 6
> occasions when numbers would NOT be skipped each hour. Since each
> skipping occurrence skips 2 frames, that means 12 frames would NOT be
> skipped on those special instances. The result is 120 minus 12, which
> is of course 108; the exact number of frames that must be skipped each
> hour."
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