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Trying to understand 23.976

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David Smith
Trying to understand 23.976
on Nov 17, 2011 at 8:38:43 pm

I've been reading and trying to understand this frame rate for some time now. I know the history of fraction frame rates - making color work on what was an originally B&W system and so on. I understand the basics of converting 24fps to 29.97 (by adding frames). But I still don't fully understand all aspects of working with 23.976.

Any help would be greatly appreciated!

My first question is... when 24fps film is slowed by .1% and transcoded to 23.976.... how exactly is it doing it?

Does the process remove any frames?


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John Heagy
Re: Trying to understand 23.976
on Nov 17, 2011 at 9:22:21 pm

[David Smith] "Does the process remove any frames?"

No, it's just slowed down. Anything shot at 24.00 is 100.1% longer when conformed to 23.976.

The same is true for 30.00 vs 29.97. SD is the reason why we are still using 29.97 even though HD supports 30.00. One can't have an SD and HD broadcast of the same show be different lengths. Once online distribution kills tradition broadcast is when 30.00 can be considered.


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David Smith
Re: Trying to understand 23.976
on Nov 17, 2011 at 9:38:00 pm

[John Heagy] "No, it's just slowed down"

Thanks for clearing that up.

I still can't get my head around how it's slowing it down. I shot film for so long that digital and video are still baffling to me.

If I have film that was shot 24fps. The frame was exposed to light for 1/48th of a second and the rotating shutter blocked the film for another 1/48th of a second. And it was projected the same way.

I understood NTSC 27.27 a little bit with fields and interlaced, etc.

But I'm having a hard time understanding how 24fps is actually "projected" from a computer to a screen (is it on the screen for 1/48th of a second?

Does 60 hertz mean 1/60 of a second? I've read up on this a bit but it just all goes over my head.


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Michael Gissing
Re: Trying to understand 23.976
on Nov 17, 2011 at 10:31:37 pm

Actually when your film was projected it ran approximately at 24 fps. The critical info is frames per second. It is just a measure of speed of frame display over time. Some projectors ran fast, some slow.

In video it means the frame rate per second is fixed at various rates. 23.976 is an exact rate of display for the TV or projector. by running .1% slower than 24, it means running time is slightly longer and pitch is slightly lower but because the difference is so small it doesn't noticeably affect pitch. If you are syncing audio the slow down does matter as sync will gradually drift over time.


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David Smith
Re: Trying to understand 23.976
on Nov 17, 2011 at 11:02:11 pm

[Michael Gissing] "Actually when your film was projected it ran approximately at 24 fps"

That's what I meant. It's just during that 1/24th of a second for each frame, part of that time period the frame is not moving and part of the time it is moving. When it's moving the screen is dark. One fact I always found interesting is when you are in a theater and watching a 2 hour movie, you are actually end up watching a dark screen for 1 hour of that time.

What I don't understand with video is how the hertz ties in with the speed the images are showing. I think it got more complicated after NTSC with digital.

I understand now it has to play at fractions to fit in with all media, but I'd like to understand what is actually going on.

Maybe I'm thinking it's more complicated than it is. I can visualize analog audio being slowed down .01 percent but digital audio being slowed down confuses me.

If analog is slowed. The machine just spins slower. But with digital doesn't it have do somehow fit into 60 hertz in some way? This is what I really don't understand.


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Michael Gissing
Re: Trying to understand 23.976
on Nov 17, 2011 at 11:10:04 pm

You are definitely over-thinking it. Video doesn't have a display shutter. That's all. Think of it as an image refresh rate. A computer screen can have a refresh rate of 60, 70, 100 hz regardless of the mains frequency. Same with video rates. They are no longer tied to mains so forget that aspect.

It is just purely the frame rate per second and the display equipment synchronises the refresh rate to the data.


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David Smith
Re: Trying to understand 23.976
on Nov 17, 2011 at 11:53:12 pm

[Michael Gissing] "
It is just purely the frame rate per second"


Ok thanks! It sounds like I was making it more complicated than it needed to be.

Now I can ask my more meaty question:

I shot 90 percent of a 16mm feature film some years ago that I now want to finish the last 10 percent on HD.

The film was shot 24fps with sound. I may go back to film after editing.

The film has all been scanned to DPX files. I'll be using proxy ProRes for editing in FCP7 but finish in Resolve with the DPX files.

When I shoot new HD footage, I will record sound to an external recorder and sync later.

The old audio was recorded 44.1 16 bit. And the new audio will be recorded 96khz 24 bit - all he old audio has been upsampled to 96hkz 24 bit already.

Since I may go back to film should I shoot the new HD footage at 24 fps? Edit in 24fps. Then when I'm done with the edit just slow the entire movie down to 23.98 for use with everything else except the film version?

Are there pit falls in transcoding picture and audio from 24fps to 23.98 and visa versa?

Thanks in advance again!


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Michael Gissing
Re: Trying to understand 23.976
on Nov 18, 2011 at 5:47:44 am

All audio for FCP and in general for digital video should be 48khz 24 or 16 bit. 96khz will have to be downsampled and all your old 44.1khz should be sample rate converted before syncing.

No reason to shoot video double system unless you really want to. I would edit everything in 23.976. When it goes back to film, everything will run .1% faster which is of no consequence for pitch. Because your film footage will be running at 23.976, convert the audio to 47952khz and set the file flag to 48khz so the audio runs .1% slower to match and hold sync.

This article explains it and says how to do it in ProTools.
http://protools.dudesnetwork.com/2009/05/18/audio-pull-updown/


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David Smith
Re: Trying to understand 23.976
on Nov 18, 2011 at 6:45:57 am

[Michael Gissing] "When it goes back to film, everything will run .1% faster which is of no consequence for pitch"

Thanks for all your help!

Just curious. If audio is slowed down .1% does that affect pitch?

I've always been curious if you slow down or speed up an entire project by .1% if it has any negative impact on the audio and video.


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Michael Gissing
Re: Trying to understand 23.976
on Nov 18, 2011 at 6:55:41 am

.1% is imperceptible for pitch. 6% is roughly a semi tone in music.


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Nick Meyers
Re: Trying to understand 23.976
on Nov 20, 2011 at 1:46:03 am

another approach would be to simply stay a 24fps until you finish your edit / mix / grade, etc.
then change your master to 23.976
this seems simpler to me as you don't need to mess with your sound to get it to sync.


nick


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