Re: [Avid-L2] Rookie TC question

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. That
is not actually true. If you count them, there are identical numbers
of frames in a realtime hour of each type of timecode. But there are
108 fewer NAMES of frames.

The problem is that in NTSC there is no such thing as 30fps.

Video is ALWAYS 29.97 in NTSC, but that makes it impossible to come up
with a numbering (timecode) convention that matches real time.

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.

So even when you're watching NDF timecode, you're still seeing the
same number of frames and the same number of frames per second as when
you see DF timecode, but if you number them simply 0-29 before jumping
to the next second and follow that naming convention for an hour, (as
non-drop frame timecode does) then you end up with a program that
looks (by timecode) to be exactly an hour long if it starts at, say
1:00:00:00 and ends at 1:59:59:29, but it's actually 108 frames LONGER
than a real hour. The only time this matters is when you are trying to
broadcast or hit some other specific time length. Otherwise, NDF
timecode is easier to deal with because the frame count is continuous.

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."

On Oct 21, 2009, at 4:34 PM, Dennis Degan wrote:

>
> On Oct 21, 2009, at 7:55 AM, Steve Hullfish wrote:
>
> > Well, the frames are dropped in the NAMING of the frames, as you
> know,
> > and not in the actual frames themselves. My analogy is that drop
> frame
> > time code is like a skyscraper hotel where there's never a 13th
> floor.
> > Of course floor 14 is ACTUALLY the 13th floor, but they named it 14.
> > That obviously doesn't mean that there's a one floor hole in the
> > skyscraper, or that just because you have a penthouse on the 21st
> > floor, you're not REALLY only on the 20th floor.
>
> On Oct 21, 2009, at 11:57 AM, Philip Hodgetts wrote:
>
> > And to go with Steve's excellent explanation, an illustration:
> > http://www.philiphodgetts.com/2009/10/20/what-is-drop-frame-
> timecode/
>
> I now say:
>
> 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."
>
> Dennis Degan, Video Editor-Consultant-Knowledge Bank
> NBC Today Show, New York
>
>
>

[Non-text portions of this message have been removed]

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