How does premiere export 23.976 timeline to 24p output?

[lcagaiden]

I'm just curious how this actually works.

I have a 23.976 project. But I've been asked to export it as a 24p file. It's easy enough to do, i was a bit nervous of sound sync issues or strange image glitches but everything's actually perfectly fine.

Just trying to understand - how does Premiere actually do that? How does it change the framerate without affecting the sync or the smoothness of the video??

[Averdahl · Accepted solution]

I have seen glitches with that workflow.

But, ask them if they really want 24.00 fps. Many people call both 23.976 and 24.00 for simply 24p. So by asking them what 24p means in their world can save you some work.

[lcagaiden]

Yeah, just the idea of doing that made me nervous but i just watched the 24p render all the way through and everything looks perfectly fine - i'm happy about that but at the same time oddly suspicious haha, would like to understand how this actually works.

And i know what you mean about people calling both 24p.  Actually this is a screener for a major film festival (Berlin) and in their submission file specs they specifically ask for a 24p or 25p file.  I went ahead and uploaded my existing 23.976 file there anyways, only to find in their preview that that file ends up having a sync issue when played back from their site.  I asked them to clarify what's going on but they're slow to reply and i got a deadline to meet - so figured i'd try and see if the actual 24p file would work better.

[salvo34]

You have to dig deep into google and look at that issue via SMPTE specs for stuff.. and it's very confusing. Basicly in the U.S. especially ( where TV was 30 fps ( 29.976 ) based on the power grid of 60 cycles per second ... and the need to have code in the interleaved frames ( so the electron gun knows when to go back to the top and show the next field ) got added to the bottom of the last half frame....

Hard to explain and I don't understand the tech aspects like an engineer would... In Europe it's a different power grid ( 50 hz per second ? ).

Anyway, the reason people shot that weird 29.976 was to essentially make stuff easier to 'translate' to other specs ( PAL and Japan have different stuff than the U.S. ).

So, when shooting film cameras ( real 24 fps ) for theatre, if you wanted to convert to 30 ( or other power grids ) that stuff started popping up to make the timing work out better for the conversion from film to TV.

It gets really confusing for me.. but it is what it is.

I worked on film jobs for TV commercials where the film camera would shoot 30 fps just because everyone knew it would never go to the theatre... and only to a TV broadcast... and so it was more or less a 'frame to frame' conversion ( telecine ) ... 

There are things called drop frame and non drop frame, where over time the discrepency re: sound sync etc. gets corrected during the process....

It sounds to me like the people you are dealing with want to show it using a real film projector... ( not TV) and they prefer to have that product from you. The NLE should do a decent job of exporting 24 from 23.976 or whatever... so they are happy.

🙂

[Qengineering]

salvo34

I respect your attempt at the explanation but the truth is that the U.S. interlaced field (not frame) rate was 60.0 until they had to create color television and make it reverse compatible with existing B&W TVs. The math used to create the color sub-carrier subsequently worked out to 29.97 fps (not 29.976). Now, 23.976 was a mathematical derivative of 29.97 for compatibility with 24fps motion picture film.

Anyway... files are converted from 23.976 to 24.0 fps by speeding them up an inperceptable ~1/1000%

The only time I have ever been asked for 24 fps is for projection in theaters.

[salvo34]

thanks...

[lcagaiden]

Anyway... files are converted from 23.976 to 24.0 fps by speeding them up an inperceptable ~1/1000%

Thank you - that's what i suspected, but i compared by two renders (23.976 and 24p) - and their length is precisely the same.  Which makes no sense to me.  Cause if Premiere is indeed stretching the frames longer or shorter, wouldn't the whole overal film end up being a second or few seconds longer?  It's a 1h20m project, so i imagine there would be some difference in runtime, no?

[Qengineering]

Well, now that could depend on how you're measuring the duration.

If you're using a video player or Premiere and simply checking the duration on the counter, it won't show a difference, you still have essentially 24 (23.976 or 24) frames per second and the ticker will turn over to 1 second after every 24 frames. If you could actually use (and trust) a stopwatch to measure the duration, you would see a 1/1000 difference in duration or about 5 seconds over a 1 hour 20 minute time span (if I have my math right?).

Now, you've opened a different can of worms that salvo34 eluded to with the difference between non-drop and drop frame time code. In the 29.97fps world, 1 hour of non-drop frame timecode is not the same duration as 1 hour of drop-frame time code but a video player or Premiere will call them both 1 hour.

[Qengineering]

Hint: the drop-frame 29.97 fps timecode is exactly 1 hour, the non-drop is ~3.6 seconds longer in true duration.

The ONLY people who actually care about this accuracy are broadcasters because they all have to have the network affiliates on the same clock when jumping from network programing to local and back. If broadcasters used non-drop TC (where every single frame has a sequencial assending number) then all programming would be off by 3.6 seconds every hour. I deliver 30 second spots for broadcast several times a week and I use non-drop (don't tell anyone!) because the difference is undetectable in 30 seconds.

[NRS-Media]

Hi lcagaiden,

If you export 24fps from a 23.976fps sequence, Premiere will repeat a frame at certain intervals because 23.976 is slower. It's hard to noticed because it doesn't happen every second, as it would if you exported 25fps from a 24fps sequence.

The only way to do this properly is to export your sequence in its native 23.976fps, then import that file into your project. In the project window, right click on your video go to Modify > Interpret Footage. A window will open that will allow you to set a frame rate manually for this clip. Choose "Assume this frame rate" and type in 24; click okay. The video will now play back at exactly 24fps. Create a new sequence that is exactly 24fps, name it accordingly, and drop your modified clip in it. You can now export this sequence to create a 24fps file that will not have repeated frames. When you export, the audio will be processed because you have changed the speed of the audio and it has to be resampled.

Cheers,

[kinoks]

I would also apply Audio Pitch Shift -1 Cent. Right?

[MyerPj]

This is an old thread. You can always try it and see what you think. I doubt if that will be worthwhile, especially in PP.

[NRS-Media]

Hi Kinoks,
Pitch shifting the audio between 23.976fps and 24fps will be imperceptable and unnecessary. If you were doing a speed change from 23.976 or 24 to 25, then a pitch shift would be useful as the audio would be playing approximately 4% faster.

24fps is only 0.1% faster than 23.976fps.