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GH1-1080/24p It Doesn't Drop Frames; It Reapeat Some Frame...

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    #16
    "Pulldown" refers specifically to the technique used to convert 24p footage to 60i. It is used by the telecine process in order to turn film (which is always shot at 24 fps for "normal" motion) into an NTSC video signal, which is 60i. Please read the relevant Wikipedia article for an explanation on why it's called "pulldown" in the first place.

    You're confusing pulldown with interlacing. Interlacing merely refers to an encoding scheme used for the efficient transmission of a video signal. Deinterlacing refers to any method of constructing a progressive signal from an interlaced signal. That's probably why you assumed that "removing pulldown" (AKA deinterlacing, in your mind) meant that you would be getting a 30p signal from 60i. While some deinterlacing methods weave the fields together (getting you 30p), the reverse telecine process actually reconstructs the original 24p signal through removing the pulldown scheme.

    The vast majority of consumer and prosumer 24p camcorders use pulldown because the manufacturers want the video to have backwards compatibility with NTSC video systems, such as older TVs. There's nothing weird or unusual about it. There should be no more confusion now.
    Last edited by ydgmdlu; 05-13-2009, 03:41 AM.
    Current cameras: Panasonic LUMIX DMC-GH2 | Sony HDR-SR11 (infrared modded)

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      #17
      Ydgmdlu, thanks for letting this so clear. I was confused in the first place because that was exactly how I knew about telecine, interlacing, and pulldown.
      Then in fact a camera shooting at 24p IS 24P and DOES NOT need anything else except removing artifacts of bad desinterlacing while converting 60i to 24p.
      Am I right?

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        #18
        No. You have to find out how any given camcorder encodes the 24p signal. There are four methods in which it's done.

        The first method is to encode 24p as 24p. Canon prosumer camcorders, such as the XH-A1 and the XL-H1, do this in their fake 24p mode. Since this is already 24p, nothing more needs to be done about it.

        The second method is to encode 24p as 24p with a pulldown flag. The pulldown flag tells the playback device to perform a on-the-fly, real-time telecine on the video stream. Pulldown flags are a feature built into MPEG-2, MPEG-4, and AVC. They are most commonly used on DVDs. You can get the original 24p signal simply by ignoring the pulldown flag. However, I am not sure if any camcorders use this method. Some HDV camcorders might, but I'm pretty sure that the DVX100 doesn't, since the DV codec doesn't have this feature, as far as I know.

        The third method is to encode 24p as 60i with the pulldown information in metadata. A decoder or NLE can use that information to perform an inverse telecine to reconstruct the 24p signal. This is obviously an inferior method, as per-frame bandwidth is lower than that of native 24p.

        The fourth method, which is what the GH1 uses, is to encode 24p as 60i. There's no easy way of doing inverse telecine because the pulldown pattern (i.e. "cadence") is unknown and could change at every cut.

        There is no such thing as "bad deinterlacing," other than simply weaving the fields together. There are many different deinterlacing methods, each with strengths and weaknesses, but there will never be artifacts to remove if a deinterlacing algorithm has done its job. The only artifacts there could be are ghosting and duplicated frames. Ghosting can't be removed, at least not easily, and duplicate frames are not an intra-frame problem.

        There could be artifacts if inverse telecine (IVTC) is not done properly. This happens when the wrong cadence/pattern is used in the IVTC process. The only way to remove the artifacts would be to break up the image into fields and perform deinterlacing all over again using a different method. There are never any artifacts if IVTC is done properly. What's frustrating about GH1 24p footage is trying to find a reliable IVTC method.
        Last edited by ydgmdlu; 05-13-2009, 12:46 PM.
        Current cameras: Panasonic LUMIX DMC-GH2 | Sony HDR-SR11 (infrared modded)

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          #19
          According to all this arcanum, in your experience, which could be the best method to have real 24p in the end? Shooting 720/30p and converting them to 24p using something like Magic Bullet Frame Plus? The first time I heard about the GH1 with a 1080/24p, I was really excited... Converting footage 29p, 60i etc... to 24p using any of the famous plugins is time consuming.

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            #20
            Please read the GH1 workflow forum. There's plenty of discussion there about how to get what you want. This thread is extraneous.
            Current cameras: Panasonic LUMIX DMC-GH2 | Sony HDR-SR11 (infrared modded)

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              #21
              Adam Wilt does a great job of explaining pulldown in relation to the DVX. I haven't read this in years but it's worth reading just for his clarity.

              http://www.adamwilt.com/24p/

              And specific to 24P and why we have to deal with pulldown.

              http://www.adamwilt.com/24p/#24pRecording

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