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davide sorasio

why 23.976 and not 24 fps??

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23.976 is still 24 progressive frame. 23.976 is really a hardback to the old NTSC standards adopted in the early days of television. Has something to do with timecode and compatibility with older broadcast systems. In a modern sense, you can shoot either frame rate. Just keep in mind that you want to shoot the SAME frame-rate across the project. If one camera shoots 24 and the other 23.976 you'll have a harder time matching them.

 

I'd say as long as all your camera's and project files will be 24.00, the shoot at 24.00. That is DCI spec, and if anything needs to be delivered 23.976 (would that even happen?), it can be done in post. 24 even frames per second just seems more 'fluid' to me.

Edited by Landon D. Parks

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I'd say as long as all your camera's and project files will be 24.00, the shoot at 24.00. That is DCI spec, and if anything needs to be delivered 23.976 (would that even happen?), it can be done in post. 24 even frames per second just seems more 'fluid' to me.

 

The right way to approach this is to work backwards from the deliverables format. If it's DCP, then 24. If it's broadcast, or if you want to make a DVD, then 23.976. 23.976 exists because it's the only way to encapsulate a 24fps progressive image inside a 29.97 interlaced package (NTSC), which is done using 3:2 pulldown. This pulldown can be undone if the cadence isn't broken, and a progressive image displayed even if it exists inside an interlaced stream. This is how it works on DVD, and on 1080i (in most cases).

 

We usually recommend 23.976 because it's easier to get to more formats from there (NTSC, Progressive DVD, Progressive Blu-ray, 25fps). Changing to 24fps isn't particularly hard, but most of the common delivery formats work natively with 23.976, and only a couple (DCP, Blu-ray) support hard 24fps. And Blu-ray also supports 23.976

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Truth is that most digital projects shot for either broadcast or cinema use 23.976 instead of 24 in the U.S.

 

It's nothing to do with timecode.

 

From Wikipedia on NTSC:

In January 1950, the Committee was reconstituted to standardize color television. In December 1953, it unanimously approved what is now called the NTSC color television standard (later defined as RS-170a). The "compatible color" standard retained full backward compatibility with existing black-and-white television sets. Color information was added to the black-and-white image by introducing a color subcarrier of precisely 3.579545 MHz (nominally 3.58 MHz). The precise frequency was chosen so that horizontal line-rate modulation components of the chrominance signal would fall exactly in between the horizontal line-rate modulation components of the luminance signal, thereby enabling the chrominance signal to be filtered out of the luminance signal with minor degradation of the luminance signal. Due to limitations of frequency divider circuits at the time the color standard was promulgated, the color subcarrier frequency was constructed as composite frequency assembled from small integers, in this case 5×7×9/(8×11) MHz.[7] The horizontal line rate was reduced to approximately 15,734 lines per second (3.579545×2/455 MHz) from 15,750 lines per second, and the frame rate was reduced to approximately 29.970 frames per second (the horizontal line rate divided by 525 lines/frame) from 30 frames per second. These changes amounted to 0.1 percent and were readily tolerated by existing television receivers.

 

So analog color NTSC runs at 29.97 fps / 59.94i. This frame rate has been carried over into digital HD broadcasting in the U.S.

 

So throughout the 2000's, the main reason you shot at 23.976 fps instead of 24P was audio post in the U.S. Even movies shot at true 24 fps had to deal with this because a telecine transfer to NTSC for dailies and NLE post changed the frame rate to 23.976 fps, so the frame rate didn't get restored to 24 fps until the movie was finished to film and projected at 24 fps. So after a movie was edited offline, tape copies were sent to sound post for cutting and mixing sound, so they were working with material running at 23.976 fps whether or not it was shot at 24 fps or 23.976 fps. If the final product was for broadcast, it didn't matter because it was going to get shown at 59.94i with a 3:2 pulldown anyway. But for material destined for theatrical projection, at some point before or after the final mix (depending probably on whether they were mixing to a video copy or a print), the speed had to be corrected back to 24 fps.

 

So it because easier for digital projects to just shoot at 23.976 fps instead of 24 fps so that the sound post could stay at 23.976 fps all the way to the mix, after which one could decide if one needed a true 24 fps version.

 

Now that most sound people get digital files instead of tape copies of the offline cut to work from, it would be possible to eliminate dealing with 23.976 and stay at true 24 all the way through post, but it's been hard to change the industry, especially since so much post work is still done for television broadcast compared to for theatrical release.

 

My own experience shooting one of the first 24P movies in 2000 was that since it was for theatrical, I chose 24 instead of 23.976 in the camera menu. Later when I asked the editor how the sound mix was going, she said "fine, except that the entire reels are drifting slightly out of sync and I'm having to manually adjust them." Live and learn... this was probably the first 24P HD feature ever posted in Los Angeles at the time. Ever since then, I've stuck with 23.976 for digital features and haven't had a problem. But I think one could choose true 24 fps today and make it work.

 

As Landon says, DCI requires true 24 so digital movies shot and finished at 23.976 get converted to 24 for the theatrical DCP.

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Ah, I was not aware that 23.976 was still needed for digital broadcast. I guess I just like the idea of shooting 24p rather than an off frame rate. Film has been shot that way for over a hundred years. That is not to say you should take my advice and shot 24 progressive (some camera's don't even offer it as an option), but that is the way I shoot on my GH4 and have had no problem with DVD output. I also record sound in-camera rather than separate, so that whole sync-issue really doesn't exist in my workflow.

 

As others have said, most people shoot 23.976, and doing so will not be detrimental to your workflow. Just keep in mind that if you ever plan to do a DCP (lots of major festivals are moving toward this now), you'll need to have a 24.00 progressive JPEG2000 sequence.

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The camera may even be running at 23.976, even though it says 24 fps on the camera (takes less space).

 

No it doesn't. It just plays at a different speed. Every frame is the same size.

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No it doesn't. It just plays at a different speed. Every frame is the same size.

 

I think Brian is referring to the camera manufacturers naming 23.976p as '24p' or '24f' on their cameras. I don't know how common this is now, but back when there was no real cameras shooting TRUE 24P, 24P was still often used in the menu's and on literature to refer to 23.976.

Edited by Landon D. Parks

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I think Brian is referring to the camera manufacturers naming 23.976p as '24p' or '24f' on their cameras. I don't know how common this is now, but back when there was no real cameras shooting TRUE 24P, 24P was still often used in the menu's and on literature to refer to 23.976.

 

Ahh, ok. That makes sense!

 

Usually 23.976 is abbreviated as 23.98 (but they're not the same thing, and on long form material, that extra .004 makes a difference for audio sync!). Since we primarily deal with film and with tapes that have already been made, I'm not too familiar with how the camera manufacturers do that. If they're really abbreviating 23.976 as 24, that seems like a really bad idea!

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The blackmagic cameras have both 23.976 and 24 and I choose to film in 24 for everything but I get asked all the time why I do that.  The biggest reason for me is because everything is digital now.  I also really like whole numbers instead of decimals. It just seems to make sense to film 60fps and easily convert to 24fps, it's an even 2.5. (Pocket 4k just released 75fps but I use 72fps instead also, I like even numbers, 3 times factor.). I also record audio straight to camera so I don't have to worry about syncing later.  Am I wrong to think like that?

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I think the answer is, if you are shooting in the U.S., check with the audio post people (if they've been selected before the shooting begins!) about which frame rate they prefer.  There is really no technical reason not to shoot 24fps these days, but the audio post guys often prefer to stick with the old 23.976fps rate, perhaps because their sound fx libraries are in 23.976fps.

It's also important for everyone in the post workflow to understand that the frame rate needs to be exactly the same.  I once shot a film at 23.98 and the transcodes were done to 24fps by adding a duplicate frame every 501 frames (I think).  During color correction we started noticing the duplicate frames, and of course, that often put sound out of sync by a frame or two on a long take.  In the end, the error needed to be corrected in editorial, and the entire film needed to be re-conformed.

And yes, there are some, usually "prosumer" cameras that shoot 23.98 when the camera is set to 24fps.  It's best to shoot a test, download the clip, and open in editing software to confirm the true shooting frame rate before filming begins.

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3 hours ago, Bruce Greene said:

perhaps because their sound fx libraries are in 23.976fps.

I'm a little confused, how can sound effect files have a correlation to framerate?

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4 hours ago, Bruce Greene said:

Timecode?

I understand timecode for long stretches of audio to be synced but why would a sound designer need timecode for a 5 second clip of foley?

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It's all about sound editing using multiple tracks of synced effects and music, so it just depends on the people doing the sound editing and mixing. You can resolve back to 24 or 23.976 at the end but everyone has to be working at the same rate.

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13 hours ago, Max Field said:

I understand timecode for long stretches of audio to be synced but why would a sound designer need timecode for a 5 second clip of foley?

I'm way out of date on the detail, but you can only say it's 5 seconds long because of timecode- the clip needs to be told how long it is. Years ago it would have been a certain length of magnetic film- 120 frames at 24fps or 125 at 25- but you still needed to know the frame rate. The "timecode" was the sprocket holes. Now that information comes as metadata.

Edited by Mark Dunn

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This is digging deep in the skull and I could be wrong, but I believe in the move from B&W television to color, the frequency of video in the US was changed to eliminate artifacts from the color encoding process.  For some reason color encoding didn't tie well to the electrical line frequency in tube TVs, which in B&W was 60.00hz.  Up to that point B&W TV and Film were both tied to 60.000hz line frequency motors (or generators in the case of TV).  I don't know enough about the process to know where that .06hz disappears to in color land.

59.94/2.5 = 23.976 i.e "24p" in video speak

60.0/2.5 = 24 or "filmout 24fps" in video speak

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