Working with 60p in Apple Final Cut Pro
Dec 7, 2010 12:00 PM, By Jan Ozer
Converting 60p to 24p
The other problem that the client faced was converting 60p to 24p for DVD. The spacial quality was uniformly good, but the results weren't smooth, with jerkiness again most notable during panning sequences. The client knew that there was an inherent issue converting 60p to 24p, but wanted some help identifying alternatives.
The "inherent" conversion issue is shown in Figure 5that is, because 24p doesn't divide evenly into 60p, the pattern of dropping frames is uneven. I'm not sure I have the starting point exactly right in the figure, but the cadence is correct: To create a 24p file from a 60p source, you include a single frame, then drop two frames, include a single frame, drop a single frame, include a single frame, drop two, and repeat as necessary until the end of the file. While this sounds awful, the result isn't really distracting unless you're panning around an image. Even during a fast spinning motion, the dropped cadence is tough to see.
What are the alternatives if jerkiness is noticeable? Again, there are multiple techniques to smooth out the video, and the best resource I found was a great article produced by Jack Daniel Stanley, with a video illustrating the techniques here and a descriptive article here.
Most of the techniques use some form of frame interpolationin other words, combining data from multiple frames to produce 24 discrete frames per second that preserve motion more smoothly than you get by dropping frames as shown in Figure 5. You can see an example of this on the left in Figure 6.
To explain, the original 60p video is in the middle, the video created by dropping frames as shown in Figure 5 is on the right, while the interpolated frame is shown on the left. The three circled areas show regions that appear to be composed of data from different frames. You can watch the video file here.
If you play the video, note that it has two distinct segments. The first 3.5 seconds compares 29.97i and 24p files produced from the 60p clip in the middle. If you page through the file frame by frame, you'll see the drop frame cadence shown in Figure 5, and also that the fields in the 29.97i file map directly to all the frames from the 60p file. Then there's a short break in the video, and the next clip contains the three videos as shown in Figure 6.
Again, Stanley's article documents four techniques. I used the simplest for my tests (the one Stanley recommended and, as it turned out, the one that the client was using before I got there), which involved retiming the clip in Compressor. Here are the relevant steps.
1. Adjusting the frame rate to 23.976. In Compressor's Encoder pane, click the Video Settings button, and change the frame rate to 23.976.
2. To make sure that you don't process the audio, change the audio setting to pass-through.
3. In the Frame Controls pane, you have to tell Compressor which technique to use when converting the frame rates. I used Best in the Rate Conversion list box when creating the clip shown in Figure 6.
Again, you'll almost certainly want to use some flavor of ProRes as the format for the file to minimize any compression-related loss and maximize downstream editing compatibility. Render as normal, and then author with the interpolated 24p file.
Let me say for the record that irrespective of the technique used, interpolation never produces as smooth a result as you would have gotten had you shot in 24p. In the case of my client, they were using 24p to try to fix an idiosyncratic problem in the video, and we ultimately found a better solution that didn't involve using 24p. Overall, if you want to produce in 24p, shoot in 24, and if you want the smoothest possible result on DVD from 60p footage, use 29.97i.
That's it. I'm sure there are other techniques out there to accomplish both objectives discussed above. If there are any that I absolutely need to know about, contact me at email@example.com.
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