Thanks for the answer, but the problem is not resolved. I got exactly the same exported video. If you don't see the problem by yourself, it means that perhaps you can't help my so much. In the exported video, the slow motion is not smooth at all and the frame rate might be something like 5 fps. With a frame rate of 25 fps, the motion should be smooth and not jerky as in the exported file. I've tried to slow the clip with the media player (VLC), it works perfectly. I tried with another software, it works perfectly. I tries with other files in Videopad, it works perfectly. The preview of the slowed part in Videopad is smooth, it works perfectly. Now there's this problem and it looks like Videopad is recognizing the file as a 25 fps file when exporting the file. The problem is in the exportation of the file.
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Actually I tried with another software (Openshot video editor) and the result is exactly the same, a jerky clip. 😕
I think there might be a problem with the gopro file (all aother files from other cameras work pefectly) or the codec used by go pro, who knows... The preview in videopad works, as well as the preview in Openshot video editor, so really the problem comes when exporting the file.
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Compare with this exported file (shotcut) and see the difernce by yourself, no need to explain https://content.invisioncic.com/r33241/emoticons/default\_smile.png : https://drive.google.com/file/d/13Td2-oUJmO5qHaMRXxz4Hkzr2HmTHLpR/view?usp=sharing