25fps clips became longer when inserted in a 24 fps project

[Joe Marler]

[Giuseppe Mangione] “Can someone explain me why if I insert a 25pfs clip long 10 sec. in a 24pfs timeline the clip cover 10 second and 10 frames of the timeline?

And If I insert a 120pfs, or a 29.97fps, clip long 10 sec in the same timeline it still cover 10 second?”

The best way to rate conform 25 to 24 fps is via a speed change, IOW to slow the 25 fps material by 4 %. That is what FCPX does and I think it automatically does audio pitch correction. The other option is discard a frame occasionally which would cause a motion cadence issue.

120 and 29.97 are essentially evenly-divisible multiples (within 0.1%) so discarding 75% of the 120 fps frames produces a perfect frame-rate match to 29.97 (essentially 30 fps).

Larry Jordan article on frame rate conversion: https://larryjordan.com/articles/frame-rates-are-tricky-beasts/

[Giuseppe Mangione]

[Joe Marler] ” The best way to rate conform 25 to 24 fps is via a speed change, IOW to slow the 25 fps material by 4 %. That is what FCPX does and I think it automatically does audio pitch correction. The other option is discard a frame occasionally which would cause a motion cadence issue.”

Hi Joe that’s what i was thinking, I thought to change the speed of the clip. But I didn’t expect that inserting a 25fps clip in a 24fps timeline the clip speed change automatically. I notice that in the bottom of the video inspector, it says “25 converted to 24”

And the speed of the clip is changed (from 10 second to 10:10) In the retime editor it should be 96%, not 100%! This is what surprises me. And If you make “automatic speed” it reduce the speed of another 96%. The clip of 10 second became 10:20!

I personally think this is a bug, I haven’t find any explanation.

thanks.

[Joe Marler]

[Giuseppe Mangione] “I thought to change the speed of the clip. But I didn’t expect that inserting a 25fps clip in a 24fps timeline the clip speed change automatically. I notice that in the bottom of the video inspector, it says “25 converted to 24”

Anytime a differing frame rate is added to an existing timeline, something must be done to “rate conform” the added footage. There are various methods depending on the frame rates involved. E.g, when adding 30 fps footage to a 24 fps timeline, frames are usually discarded. Even in that case the rate-conformed clip length may be slightly altered, because the “pulldown” or other conforming method is done in small groups of frames. I think this happens if the clip isn’t an exact multiple of that group size. That would create border cases where they’d have to discard an end frame, which should be avoided.

In the 30>24 case picking “automatic speed” on the 30 fps clip will cause a 20% slowdown.

For 25>24 fps, the rates are so close that the main method to rate conform them is slow down the material by 4% and correct audio pitch. While this is internally achieved via a speed change I suppose it’s not classified as such, as other algorithms are theoretically possible but rarely used. E.g, you could hypothetically analyze and reconstruct every frame using optical flow methods which discards some frames but blends the difference across the entire clip to produce a frame count matching the 24 fps. There might be highly specialized rate conforming software or hardware which does this for major film companies, but to my knowledge no NLE does it.

[Giuseppe Mangione] …And the speed of the clip is changed (from 10 second to 10:10) In the retime editor it should be 96%, not 100%! This is what surprises me. And If you make “automatic speed” it reduce the speed of another 96%. The clip of 10 second became 10:20!”

The FCPX UI seems to separately classify retiming and rate conforming – even if sometimes the underlying technique is similar. E.g, if you add a 25 fps clip to a 24 fps timeline, then in the video inspector under Rate Conform pick “optical flow”, the Modify>Retime>Video Quality>Optical Flow shows a greyed out option. Then if you slow the clip by 50%, then optical flow retiming becomes available.

You could probably argue maybe the UI should more clearly indicate what is happening under the covers but it’s widely known that 25/24 rate conforming almost always uses retiming with most NLEs. If the UI clearly showed a colorful retiming bar on the clip this would probably confuse people.

[Giuseppe Mangione]

Thanks Joe,

your explanation was very clear. It tooks time for me to understand all the process, but now it’s pretty clear.

I knew that changing the speed is the best way to convert the 25 to 24 FPS but I didn’t expect that FCPX was doing this automatically when a 25fps clip was inserted in the 24fps. I’m sure I have done it in the past, but never notice the change of speed until now.

[Joe Marler] “If the UI clearly showed a colorful retiming bar on the clip this would probably confuse people.”
this was what confused me.

Now I’m waiting the movie distributor answer. They say that nowadays mostly movie theaters accept 25p. But I asked the percentage of movie they still accept only 24p… Then I will decide if doing a conversion or not. If yes I will also do a test to convert the audio speed in Pro Tools and check if it will sound better!

Thanks again to all for the suggestion!
beppe