24 FPS Footage in a 60 FPS Sequence? 120 FPS Slow-Mo?

[AJ Forum Account]

Hi all,

Brand new to the forum - I'm hoping to learn more about why things work so that I can make better quality videos. First off...

I've been shooting GoPro footage at 60 FPS this year, but I love the look of 24 FPS for speaking segments. Premiere Pro seems to work fone when I start a project and import my 60 FPS footage first. When I drag this to the timeline it sets the "project settings", and then when I add 24 FPS footage up front, or drop 120 FPS footage (slowed down to about 20%) as B-Roll on the top layer, everything seems to export just fine. 

I can't help but think I'm doing several things wrong though...

1. Why does Premiere work when I drop 24 FPS footage into a 60 FPS sequence? WHat is actually happening, and does it lower the quality of my output?

2. Is working with mixed frame rates a more common practice than I realize?

Any insight here would be really appreciated. 

Thank you so much. 

[salvo34]

AJ,

best to maybe start with basics from film days ( before digital ).

film camera = 24fps

film projector = 24fps

Everything looks normal ( film 'look with 180 deg shutter ( 1/48 sec exposure (shutter))).

add 120 fps to film reel in projector looks like slow motion.

Now broadcast TV (ntsc) ..using old CRT tube tv sets with antenna on roof .....= 30 fps interleaved

(two half frames = see one frame )

to put film projection on TV broadcast they had to add 6 frames per second and interleave the frames via electronics.

Some film producers, shooting for tv commercials, shot film at 30fps so it would be one for one frame conversion.

what editing programs do, like when you use interpret footage and so on, to mix fps stuff on same timeline is sorta a secret cause editing programs are made by companies and they compete with each other so some stuff is a secret in terms of EXACTLY what happens in programming. But some people here are really smart and can explain more precisely than me what happens with your specific program.

I always shoot 24 or sometimes in blue moon 120fps for slow motion and put both into 24fps timeline. I never mix resolutions except to put 4k into 1080p cause my computer chokes when editing 4k and my monitors are only full hd anyway. I also proxy the 4k raw to DNxHR SQ but that's another story.

good luck.

[AJ Forum Account]

Thank you for the information, I appreciate the response. 

What do you mean you never mix resolutions?

Are you shooting at what the output will be without any scaling up or down? 

[salvo34]

hiya aj,

I only go from a raw format of 4k to a full HD timeline.

that way I can proxy it and also zoom IN and adjust composition a little bit, without losing quality.

I rarely do that ( adjust it ) but sometimes it makes a big difference in 'focus' of audience eyes on the frame ( edit and action and so on ).

but no, I only do what I have to do with full hd computer setup etc.

[AJ Forum Account]

Thank you for the additional info... I have so much to learn... 🙂

[Steve Griffiths · Accepted solution]

To add to Salvo34's good info.

You ask why (how) does Premiere pro 'work' with 24fps in a 60fps timeline ...

PP repeats frames. In each (60 frame) second it's repeating each 24fps frame at least twice and a few three times. How it repeats these frames is determined by your choice of 'time interpolation'. By default this is set to 'Frame Sampling'. Basically repeating frames to add up to a total of 60fps.

You can make a different choice - right click a 24fps clip in your 60fps timeline and select 'Speed/Duration' from the pop-up menu.

Then under 'Time Interpolation' you can choose from the default or 'Frame Blending' or 'Optical Flow'.

Optical flow will create new frames by analysing the clip and actually tracking pixels to create a 'real'

 60fps clip from your 24fps source. This is not wothout issues (+rendering time) and sometimes you will see weird artifacting when the analysis can't perfectly figure out where pixels are moving.

The downside of this is you will lose that '24fps look' that you like. Stay with Frame Sampling - or try 'Frame Blending'.

Frame blending will help hide any frame judder that you might get from just Frame Sampling.

Mixing frame rates is very common (for me). I'm mostly working at 25fps - TV over here is 25fps (50i) so most everything is shot 25 (or 50i). But I'm constantly using stock footage at 24 and 30fps and footage from countries that use 29.97. Where possible and mostly with 24fps footage I do something different - use > Modify > Interpret Footage and change 24fps footage to 25fps. Not something you want to do as your 'use case' is different.

Putting an interview shot (your speaking segments) that were shot at 24fps into a 60fps timeline AND leaving at the default setting usually looks just fine in a 60fps sequence because there is usually not much movement with this type of footage. It's movement that will show up some problems. If you look carefully - say when someone moves their arm - you might notice the repeated frames. i.e. the movement might look like it has some hesitations. This would be where Premiere Pro has repeated a frame 3 times - so the movement has this (almost imperceptible hesitation) due to that extra repeated frame. If this is not noticiable or does not bother you - don't worry about it.

[AJ Forum Account]

Steve - thank you so much - this is so helpful. I have been concerned because I felt like adding 24 FPS to that 60 FPS timeline would just be... "wrong", for lack of a better term. The exported result (which in all honesty is just getting uploaoded to YouTube) always seemed fine... but I still want to learn to do things as best as I can. 

You explanation of that PPro was doing was exactly what I was hoping to learn more about. THANK YOU!!

All the best.