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23.98 vs 24 fps in HD


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#1 Andre LeBlanc

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Posted 31 August 2006 - 02:42 AM

Hello,

This is definitly a newbie question, but I was wondering what 24p actually means. Does it mean a frame rate of 24 fps, or does it mean a frame rate of 23.98 fps? Do pro HD cameras (CineAlta, Viper) shoot at both 24 fps, and 23.98, or in the video world does 24 fps always equate to 23.98 fps? If pro HD does both frame rates (24, 23.98) then what would be the benefits of one over the other? Am I wrong in thinking that this sounds suspiciously like NTSC's 29.97 frame rate?

Any help is greatly appreciated! Thanks!
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#2 Brian Dzyak

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Posted 31 August 2006 - 08:17 AM

Hello,

This is definitly a newbie question, but I was wondering what 24p actually means. Does it mean a frame rate of 24 fps, or does it mean a frame rate of 23.98 fps? Do pro HD cameras (CineAlta, Viper) shoot at both 24 fps, and 23.98, or in the video world does 24 fps always equate to 23.98 fps? If pro HD does both frame rates (24, 23.98) then what would be the benefits of one over the other? Am I wrong in thinking that this sounds suspiciously like NTSC's 29.97 frame rate?

Any help is greatly appreciated! Thanks!


24P is 24 Progressive frames. 23.98 is 23.98 Progressive frames. They are distinct and choosing the wrong one could cause significant problems in post.

The summarized version is that choosing 24P gives you a frame to frame match if you want to do a filmout. You would choose 23.98P if your material is NOT meant for a filmout (primarily anyway), but will remain on "video" for broadcast of DVD work. The Cinealta allows the user to pick from a variety of framerates. 59.94Interlaced is the option most closely associated with "normal" television's 29.97 that you refer to above.

The confusion comes in with these lower level cameras that erroneously simplify the framerate choices. Instead of being specific, the choice is 24P when the camera is really shooting at 23.98P. I've run into professional cameramen and Producers who initially do the same. They'll ask for 60I but what they really want is 59.94I. Or they'll say they're shooting 24P when they really need 23.98P.

I'm sure that someone else can get a lot more technical about the explanation :) . I suppose I could too if I dug through my notes, but the gist is that they are different and everyone involved should be on the same page before anything gets shot.
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#3 Michael Most

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Posted 31 August 2006 - 09:18 AM

The summarized version is that choosing 24P gives you a frame to frame match if you want to do a filmout. You would choose 23.98P if your material is NOT meant for a filmout (primarily anyway), but will remain on "video" for broadcast of DVD work.


There is no logical reason to use anything but 23.98 if you're working in the US. True, the slight slowdown is to allow for simpler compatibility with NTSC, however, essentially all sound post facilities in the US use a video based workflow, regardless of whether the final product is film or video. Not to mention the necessity for viewing copies of dailies, assemblies, and other cuts - all usually delivered on video, primarily DVD these days. In short, the only reason to use 24p for a project shot and posted in the US is to be a stubborn purist - but in the process, you're going to create many more problems than you solve.
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#4 David Sweetman

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Posted 31 August 2006 - 12:28 PM

In short, the only reason to use 24p for a project shot and posted in the US is to be a stubborn purist - but in the process, you're going to create many more problems than you solve.


But wouldn't it still be better for a filmout? Or is 23.98 easy enough to alter to true 24?
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#5 Michael Most

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Posted 31 August 2006 - 12:31 PM

But wouldn't it still be better for a filmout? Or is 23.98 easy enough to alter to true 24?


You don't "alter" anything. A frame is a frame, regardless of whether 24 of them pass before your eyes in a second or 23.98 of them do. When you record to film, you turn each video image into a film image. A one to one frame correspondence. The only thing that gets "altered" is the soundtrack, which gets "pulled up" by .1%. This is commonly done by the few remaining companies that produce optical soundtracks. All you do is tell them whether or not to do the pullup.
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#6 Andre LeBlanc

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Posted 31 August 2006 - 03:00 PM

Thanks for the input folks!
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#7 Jon Kukla

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Posted 31 August 2006 - 06:28 PM

The technical reason for the rate difference is the transmission of sound data in the TV signal, which as stated, is always at a constant .1%, hence 23.976p (often rounded up to 23.98, presumably because NTSC's 29.97 only uses two decimal points).
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#8 Michael Most

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Posted 31 August 2006 - 06:44 PM

The technical reason for the rate difference is the transmission of sound data in the TV signal, which as stated, is always at a constant .1%, hence 23.976p (often rounded up to 23.98, presumably because NTSC's 29.97 only uses two decimal points).


No, the reason for the slight slowdown goes back to the invention of NTSC color video. It was decided to make the color signal backwards compatible with existing black and white equipment. The scheme chosen to accomplish this involved piggybacking the color information on the luminance signal, which represented the black and white portion of the image. Black and white receivers would ignore the color subcarrier, and color receivers would use it by combining it with the luminance signal to create a color image. Various technical considerations made it necessary to change the frequency slightly from the 60Hz line frequency that was used for black and white. Hence the creation of 59.94xxxxx NTSC video, and the beginning of a lot of effort over the years to accommodate it.
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#9 Jon Kukla

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Posted 31 August 2006 - 07:26 PM

Yes, you're right. (It's getting late over here...)
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#10 David Mullen ASC

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Posted 31 August 2006 - 11:38 PM

I've transferred many 23.98P movies to 35mm -- it's still a 1:1 frame transfer to film, it's just that it gets projected at 24 fps instead of 23.98 fps, so the audio has to take that into account, which it usually does if the audio was edited to an NTSC downconversion.

The only HD feature I shot at 24P was my first one, "Jackpot", and after the sync problems they had in the mix session, I learned not to shoot at 24P anymore...

Pretty much everyone who is shooting 24P HD is using 23.98P because almost all sound post is done using offline edits using NTSC downconversions.

For cameras like the DVX100, 24P is actually 23.98P anyway. It makes no difference to the image; it's purely an audio / NTSC issue.
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#11 Fabian Remy

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Posted 08 July 2008 - 09:01 PM

I've transferred many 23.98P movies to 35mm -- it's still a 1:1 frame transfer to film, it's just that it gets projected at 24 fps instead of 23.98 fps, so the audio has to take that into account, which it usually does if the audio was edited to an NTSC downconversion.


Hi David. I'm working on a movie that was shot on video (23.98 fps) and that will be printed on 35mm.

I've exported a TIFF image sequence from a Final Cut Pro timeline that was 23.98p (the film was shot and edited on HDV 23.98 fps but
the only frame rate alowed for this image sequence export was 24 fps.

After I did that, I imported this TIFF image sequence into my project to compare it frame by frame with the original sequence, the "cuts" didn't match (as if some frames were added to the image sequence).

why did it happen?
when I export a 23,98 fps sequence into a 24 fps image sequence (TIFF), does Final Cut create frames (like a telecine would "create" frames in a film to NTSC video process)?
how can I check if all the frames were correctly exported, without duplicated or dropped images?

thanks in advance.
(note; I'm a newby and my english isn't that good...)
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#12 Andrew McCarrick

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Posted 08 July 2008 - 09:13 PM

Is the definitive reason why audio post is done in 23.98 ?
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#13 Andrew McCarrick

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Posted 08 July 2008 - 09:20 PM

Is the definitive reason why audio post is done in 23.98 ?

Should have said "Is there a definitive reason why audio post is done in 23.98 ?".
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#14 Jamie Metzger

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Posted 08 July 2008 - 10:56 PM

24P is 24 Progressive frames. 23.98 is 23.98 Progressive frames. They are distinct and choosing the wrong one could cause significant problems in post.

The summarized version is that choosing 24P gives you a frame to frame match if you want to do a filmout. You would choose 23.98P if your material is NOT meant for a filmout (primarily anyway), but will remain on "video" for broadcast of DVD work. The Cinealta allows the user to pick from a variety of framerates. 59.94Interlaced is the option most closely associated with "normal" television's 29.97 that you refer to above.

The confusion comes in with these lower level cameras that erroneously simplify the framerate choices. Instead of being specific, the choice is 24P when the camera is really shooting at 23.98P. I've run into professional cameramen and Producers who initially do the same. They'll ask for 60I but what they really want is 59.94I. Or they'll say they're shooting 24P when they really need 23.98P.

I'm sure that someone else can get a lot more technical about the explanation :) . I suppose I could too if I dug through my notes, but the gist is that they are different and everyone involved should be on the same page before anything gets shot.


Good reply.
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