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23.976 fps vs. 24 fps? (1 reply and 12 comments)

Sairaj Batale
1 month ago
Sairaj Batale 1 month ago

What's the point of a 23.976 frame rate, as opposed to 24? will it ever matter in post-prod. considering I've heard it causes problems with audio sync? Or should I just stick to 24 fps at 172.8 shutter?

dmullenasc
1 month ago
dmullenasc 1 month ago

The shutter is a separate issue, use whatever shutter angle you need to. Audio can be recorded for 24 fps or 23.976 fps, you just have to tell the recordist.

The reason for 23.976 fps is a bit complicated, it stems from the fact that NTSC video is 59.94 Hz (not 60 Hz) and so is 1920 x 1080 HDTV in the U.S.  So even if you shoot film at 24 fps, when it gets transferred to video for post-production -- at least in the days of videotape -- it was run at 23.976 fps in the telecine.  And if it gets shown on U.S. TV, it gets shown at 23.976 fps even if it was shot at 24 fps. 

So in the days of videotape post for theatrical features shot on film, the movie was 24 fps and so was the audio equivalent speed, but the post was at 23.976 fps and the audio speed was adjusted to match, and then everything was edited at this frame rate. Sound cutters got videotape dubs of the offline cut, which was 23.976. In the final mix, the audio was resolved back to 24 fps to make the optical tracks to match the projected film print speed. 

When film was shot for TV shows, it was easier just to shoot and record sound at 23.976 because it never was going to be shown at 24 fps.

But now we have digital cameras... but post in the U.S. is often still done at 23.976 fps because HDTV broadcast is still 59.94 Hz. Of course, we don't have videotape anymore and Quicktime files, etc. can be true 24 fps throughout the post chain to the sound editors and through the mix, so we could shoot 24 fps and record sound at that speed. But everyone has to know up front. But a lot of post houses are more used to working with 23.976, finishing to 23.976, and then just resolving the master to 24 fps for the DCP's (which still use 24, not 23.976).  But you can choose to go the other way, do everything at 24, make the 24 fps DCP, and then make a 23.976 fps version for broadcast and streaming. All that matters is that everyone agrees.

Personal story: I shot one of the first 24P HD features ever released in theaters, an indie movie called "Jackpot" (2001), shot on the Sony F900. Since it could be set to either 24 fps or 23.976 fps, I shot the movie at 24 fps since it was intended for theatrical and a film-out to 35mm. I figured that since I shot film at 24 fps, I should shoot the HD at 24 fps.  After the sound mix, I asked the editor how everything went and she said "great... except on every reel the sound drifted out of sync so we had to manually adjust the speed to fix it."  I realized then that even though I shot 24 fps, all the sound post had been done at 23.976 fps because this was a video post house used to videotape always being at 59.94 Hz.  Lesson learned.  

Today I just ask the studio and the post people what speed they want, 23.976 or 24 fps. It makes no creative difference to me.  Sound on set is recorded to match whichever one we choose.

dmullenasc
1 month ago

I'm not an audio person but I believe if the camera is recording 23.976 fps then the audio is recording .1% slower to match, so instead of 48 kHz, it gets recorded at 47.952 kHz. I think. So there are no sync problems. Everything is finished at 23.976 and then when the DCP is made, there is a .1% speed up to get to 24 fps.

The Byre
1 month ago

Yes - and the difference drives musicians who have perfect-pitch bonkers!

joshua gallegos
1 month ago

The 172.8 shutter at 24fps would be for 50hz?

dmullenasc
1 month ago

24 fps + 172.8 shutter angle equals 1/50th shutter time.
23.976 fps + 172.627 shutter angle equals 1/50th shutter time.

joshua gallegos
1 month ago

I meant if there was a flicker or pulse of a lamp, isn’t that why he’s shooting 24fps 172.8 shutter, instead of 180, which is (1/48).

dmullenasc
1 month ago

Sure if he’s in a 50 Hz country, he would use 172.8 if he’s shooting at 24 fps or 172.6 if he’s shooting at 23.976 fps to avoid flicker for 50 Hz pulsing AC sources... but that’s sort of unrelated to the question as to why 23.976 fps is used today.

dmullenasc
1 month ago

Now if his post is in a 50 Hz PAL country, maybe he should use 24 fps instead of 23.976 fps, I don’t know, I’ve never done post in Europe. But perhaps he’s shooting for a streaming service that demands 23.976 fps.

joshua gallegos
1 month ago

The camera I rented doesn’t shoot 24fps, so now I have to use 23.976fps with a 1/50 shutter. I’m sure that’s the least of my worries, but I hope there won’t be audio sync issues even if it’s coming from a shotgun mic connected to the camera. Or maybe it only happens with external audio, I wouldn’t know.

Sairaj Batale
1 month ago

Okay. I read through all the comments. In, India, PAL video format is supported. So I guess, 24 fps and 172.8 is good to go.

dmullenasc
1 month ago

In your case, Joshua, you're probably not using 1/50th because you're in a 50 Hz country but because that's the closest your digital still camera can come to 1/48th, which is fine. If you had any flicker problems, you'd have to use 1/60th in most cases anyway in the U.S. -- you don't actually need 1/48th. As for sync, it's only an issue if you record sound to a separate device and the sound recorder is not set for the .1% slower rate to match the 23.976 fps camera speed.

joshua gallegos
1 month ago

Thank you.

Mike
1 month ago

As a matter of interest, a lot of cameras are marketed as 24 Fps for that ‘Cinematic experience‘ when infact they are 23.976 anyway. Your recorder must be set the same as the camera otherwise the sync will wander. Whether you use a shot gun or other type of mic is immaterial, frame rate must be common to both camera and recorder. Any professional recorder will accomondate all frame rates. Ie, Sound Devices all models, Fostex PD6 optical, Nagra V, Zoom F8, even the old Portadats ( best ever portable recorder).
Best ever for sound quality is Sound Devices 3/6, Nagra V. Kashmir Pre-amps are simply stunning.

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