I'm new here and it's the first time I am using FCPX for a big project so I need some help!
I shot a music video and now I am editing it. Some clips are 24fps, some other ones are 30fps.
I set the general project rate at 24fps, because I had read that it's easier to go from 30fps to 24fps than the other way around.
After I imported all the clips into the project, I noticed that the 30fps clips have not been converted to 24fps (if I click on them, it still says 30fps in the inspector). Do I need to do anything to avoid that when I export the video, people can notice the difference of time frames/clips used? Do they get converted automatically?
All I want is a music video that plays smooth and clear.
You want all cameras to be using the same frame rate or an even multiple of that, such as 30/60/90 fps or 24/48/96 fps. The timeline should generally be either 24 or 30, matching the clip frame rate or an even multiple. For brevity I mean 23.98 and 29.97 fps.
FCPX will automatically use "rate conforming" to combine differing frame rates, but it is an imperfect process. You will often see jerky motion. You can attempt using optical flow rate conforming to smooth this, but it doesn't work perfectly and entails high CPU cost.
This usually works better for 23.98 clips in a 29.97 timeline. To try it, select a 23.98 clip in a 29.97 timeline, then in the FCPX video inspector, scroll to bottom and under Frame Sampling select Optical Flow. Then select that clip in the timeline, render with CTRL+R, and examine the playback smoothness. If that works, do all the clips with frame rates that differ from the timeline rate.
This normally improves smoothness at the cost of significant CPU time. But in some cases it can cause morphing artifacts so you must check for those on every clip.
Gerald Undone discusses using 29.97 in a 23.98 timeline at 9:15 into this video "Mixing Frame Rates":
The video by Gerald Undone that you linked is great.
After watching it, I was expecting to see a bad result when playing the 30fps clips of my project into the 24fps timeline. However, I cannot detect any bad results or judder. Maybe it's because I did not export it yet and I have been working with proxy files instead of original media?
How bad the judder is depends on subject motion and camera motion. If subject is a talking head interview you may not notice it. If a car in the background passes behind the subject, it may appear to jerk. Panning shots, drone shots and gimbal shots are highly susceptible to that. Any significant subject motion, such as dancers, people walking left/right in the frame may show it.