Output of FFmpeg has a "slow motion" part - what is it and how to avoid?

Question

Source video

I have a screencast recorded from iPad with QuickTime Player on macOS.

When I open the original recording in QuickTime Player for playback, I can see the playback controls as below:

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Processing

I processed the video with the following command (full log):

ffmpeg -i test-source.mov test-result.mov

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Resulting video in QuickTime Player

Then, when I open the result in QuickTime Player, I see the controls as below:

The part with more sparse "dots" plays in slow motion, I can drag the bars to change the start and end of this part and minimise it to zero-length (the overall length changes from 03:06 to 01:00 then and full video plays at normal speed).

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Resulting video in iMovie 10

Similarly, when add the video in iMovie with Import Media, I see "Speed"-control already applied on the source:

I can turn it off before adding to the timeline, but if I don't, the clip shows with the following decorators indicating normal speed (left and right) and slow (the middle section with the turtle):

^{(for this screenshot I added a black overlay to hide the video)}

Both the limits of the speed sections and slowdown rate differ between QuickTime Player (1 min. movie expanded to 3:06) and iMovie (expanded to 1:30).

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VLC does not notice it, plays the video at normal speed.

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Questions

• What is this feature called?

• How to avoid this slow motion part in the output of FFmpeg?

Answer 1

Your source is variable frame-rate with a peak rate of 120 and an average rate of close to 60. Editors tend to want constant frame rate streams. If you can interpret your whole transcoded file as 120 fps, then you should get the expected result. Alternatively, you can preserve the original variable frame rate,

ffmpeg -i test-source.mov -vsync vfr test-result.mov

or transcode to a constant 60 fps,

ffmpeg -i test-source.mov -r 60 test-result.mov