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Question about a double frame after converting 23.976 to 24 fps

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Chris JonesQuestion about a double frame after converting 23.976 to 24 fps
by on May 16, 2012 at 4:21:57 pm

Hi, I'm trying to understand what is going on with the frame rate conversations more than having a problem:

I have an old short film that was shot on 16mm, telecined (years ago) in SD to tape at 29.97.

Recently I had the original film rescanned at true 24fps in HD.

I took the the edited short film on NTSC tape and did a reverse telecine in compressor then made a true 24fps version (from 23.976 and laid it out in in FCP7 (24fps timeline)

I then took the newly scanned files (true 24fps) and have been matching the new footage over the old footage and doing a frame by frame match up.

I have found that twice (I'm 3 min into the process) double frames in the converted original material (when I say double frame I mean 2 incidental frames - one after the other.

I'm always trying to have a greater understanding of frame rate conversions.... I curious why I'm getting the occasional double frame?

Is it coming about during the reverse telecine or when I converted from 23.976 to 24 fps?

Thanks :)


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Shane RossRe: Question about a double frame after converting 23.976 to 24 fps
by on May 16, 2012 at 7:23:49 pm

Is the original footage STILL 29.97? If so, that's why you see double frames. 24fps needs to fill the 30fps frame rate...so 6 frames will repeat. Typically spread between fields...a typical 3:2 pulldown.

is that what you mean? Or what?

Shane
Little Frog Post
Read my blog, Little Frog in High Def


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Chris JonesRe: Question about a double frame after converting 23.976 to 24 fps
by on May 16, 2012 at 9:46:39 pm

The original edit of the short film was 29.97 (shot on 16mm 24fps) That as done 10 years ago.

I put a copy of that in compressor and did a reverse telecine on it. Which should have given me 23.976.

I then took THAT footage and converted it to a true 24fps.

In that footage I'm finding the duplicate frames.

I thought since I was reversing telecining the 29.27, then changing that new frame rate from 23.976 to 24. In a true 24 fps timeline I would have a clean 24fps with no duplicate frames. Which is not the case.

I'm curious to know why in hopes that one of these days I will no longer be perplexed by these frame rate conversion issues :)


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Chris JonesRe: Question about a double frame after converting 23.976 to 24 fps
by on May 17, 2012 at 12:34:08 pm

I think since the video was edited, the reverse pulldown does not work perfectly. Although it does say Compressor should adjust for this. But I'm not sure if that means it will or will not leave double frames.

"If you take these telecined clips and edit them as NTSC video, the result will be a final video file that has a broken cadence with an inconsistent 3:2 pattern. It is much more difficult to remove the telecine from this clip since you have to constantly verify the cadence to make sure you don’t inadvertently choose incorrect fields when creating the 23.98 fps video.

The Reverse Telecine feature included with Compressor automatically detects broken cadences and adjusts its processing as needed."


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Dave LaRondeRe: Question about a double frame after converting 23.976 to 24 fps
by on May 17, 2012 at 10:01:38 pm

[Chris Jones] "I have an old short film that was shot on 16mm, telecined (years ago) in SD to tape at 29.97...
I took the the edited short film on NTSC tape and did a reverse telecine in compressor then made a true 24fps version (from 23.976 and laid it out in in FCP7 (24fps timeline) "


Two things here:

First, when you originally edited the film transfer in 29.97, the 3:2 pulldown was still present in the footage. It's so unlikely -- verging on the impossible -- to cut such footage at the same point in the pulldown cadence that you can be assured there are cadence mismatches throughout the completed edit. Even if you did a reverse telecine on it, the process needs a consistent cadence, and you don't have it. So weirdness ensued.

Second, I'm scratching my head why you insisted on real-live 24fps and didn't simply leave it at 23.976.




[Chris Jones] "The Reverse Telecine feature included with Compressor automatically detects broken cadences and adjusts its processing as needed.""

FCP also says you can export straight from the timeline to Compressor. It works... sometimes. Most will tell you not to do it. That "fixing broken cadences" deal isn't the most reliable feature in FCP.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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Chris JonesRe: Question about a double frame after converting 23.976 to 24 fps
by on May 18, 2012 at 1:14:36 am

Thanks for the explanation. That makes a lot of sense. I'm one more step towards understanding this stuff :)

"Second, I'm scratching my head why you insisted on real-live 24fps and didn't simply leave it at 23.976."

I'm glad you asked me this since after redoing the short film I'm re-editing a feature film I shot on film to HD (I have rescanned all the film to DPX).

If you can scare me away from editing it at true 24fps please do so. I've been debating this for a while. I've read about it, heard the arguments and know everyone says to edit at 23.976.... but most of the arguments I've heard were against people shooting video at true 24.

This is my thinking behind editing at true 24:

First, the film was shot at true 24 as was the sound.

To edit at 23.976 I would have to convert all the film and audio to 23.976 to edit it at that frame rate. Which would be a pretty big job considering the dozens of audio tapes.

I would then have to convert it back from 23.976 to true24 if I ever want to go to a film festival.

Also I like the idea of having a master where the audio has not had the speed changed (I've never really understood if converting from 24 to 23.976 compresses the audio, or it just plays in the timeline at a different speed)

It seems I can just edit everything in its native true 24, then when I'm done, convert the final product to 23.976. I would just be doing it later rather than sooner.

But I don't really understand the inner workings of computers and video as I understand film. If there something I'm missing?

I'm very open to any argument against what I'm doing. I don't want to be far down the road and say "Ohhhhh THAT is why everyone says to edit at 23.976.... wow.. I'm really messed up now..."


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Dave LaRondeRe: Question about a double frame after converting 23.976 to 24 fps
by on May 18, 2012 at 6:32:42 pm

Yeah, you're stuck at 24, I guess. Bummer. I hate mixed frame rates.

Dave LaRonde
Sr. Promotion Producer
KCRG-TV (ABC) Cedar Rapids, IA


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