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PAL to NTSC conversion with 25p

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Julian Ford
PAL to NTSC conversion with 25p
on Jul 22, 2005 at 2:02:34 am

Hi guys,
I am just doing a bit of research into converting our PAL showreel into NTSC. I am a little hazy on interlacing so bear with me and ask the right questions and hopefully we can solve with this quickly! We master out all of our jobs from Flame which does not operate in fields I believe. I have subsequently captured these jobs into my G5 using a blackmagic SD card into final cut pro. The capture settings for fields was set to none. I now have these quicktimes that I want to convert to NTSC. I believe the blackmagic card usually operates in upper field first. Does this make a difference given that I think the footage was progressive to start with?
Next, I have been looking at these twixtor / fields kit tutorials around the place. Now, is there any benefit to converting my 25P footage to 50P befroe performing a frame rate conversion (and do i need fields kit for such a conversion?)

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shin kurokawa
Re: PAL to NTSC conversion with 25p
on Jul 22, 2005 at 1:58:20 pm

RE: fields in flame
Usually one would deinterlace the source clip
(making it 2x the length by separating fields),
process/roto it, then reinterlace it again.
You can set the field dominance in flame in
the Format menu. You can also export image seqs
that you can import into your other machines
without going the tape route.

RE: 25p->50p
Depending on what you need to do, there maybe
some benefit in twixtoring a 25p to 50p,
in terms of visual quality, but you may need
to weigh it against the longer work hours spent. :)

I assume what you really want to do is to
convert a progressive PAL to progressive NTSC.
There are 2 basic methods:
1) 25p -> 30p
2) 25p -> 24p + 3:2pulldown
The differences are...:
#1 requires increasing the framerate.
#2 reduces the framerate, but uses the standard
pulldown to simulate the normal movie-on-TV(NTSC)
look. In this case, the result is interlaced,
but if you have a TV that removes pulldown on
the fly (there are many on the market now) then
you can actually look at the progressive content.
Similarly, if you have a DVD encoder that detects
pulldown, you can save a lot of space by
encoding it progressive. The pulldown can
be added to 24p clips automatically using
for ex., a video i/o card such as the BMD or
Igniter. If not, there's a need to render

Now, we could do the above in 2 ways:
with temporal reinterpretation using Twixtor,
or without Twixtor. For instance, if you
simply stick a 25fps clip in a 30fps
timeline w/o Twixtor, you would get a speedup
and decrease in TRT (total running time).
Most people find this annoying since the
resulting action looks funny and the audio pitch
shift is very noticeable (although audio can be
pitch corrected using an external application),
but many find it acceptable in the case
of 25p->24p (or 24p->25p) where the
speedup/down factor is fairly small --
in fact, this is done all the time around
the world! :) With Twixtor, you can preserve
the TRT if you need to; in both #1 and #2
above, Twixtor will actually create/interpolate
new inbetween frames.

(I should also add that 25p->60i w/Twixtor
interpolation is possible. In this case
you will no longer see a progressive image
sequence - unless there's no motion whatsoever.
Where there's motion, things will look much/
overly 'smoother' because of the framerate

Note: we're only talking about framerates
here. There's also the issue of pixel aspect
ratio and resolution differences and so on.
For more information, check out the basics
tutorial on the Twixtor page:


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