FFmpeg is changing the frame rate to Variable when copying video?

[kkiller23]

I tried lossless-cut, FFmpeg Batch Converter and the results are the same. Mediainfo is saying the output frame rate is variable instead of constant like the source file. I thought it might just be a meta data issue or something, but it messes up the frame rate in Adobe Premiere as well when importing the file from lossless-cut and FBC.

Most of the time it's just a straight copy from mkv to mp4 and other times it's just encoding the audio. It shouldn't really be messing with the frame rate. Anyway to make it properly output the correct data so other programs read it properly as the source frame rate?

[jagabo]

The problem probably is the default time base of 1/90000 seconds. The duration of frames for a 23.976 (24000/1001) fps video cannot be precisely represented as N/90000 seconds per frame, where N must be an integer. 1/(24000/1001) = 3753.75/90000. If the program used 3754/90000 for each frame (CBR) the video would run a little long, if it used 3753/90000 it would run short. So it alternates between the two values (VFR), 3754 for three frames, then 3753 for one frame, repeating that over and over, giving an average of 3753.75/90000.

I thought you might be able to work around this by specifying "-time_base 1001" or "-time_base 90090" but it doesn't appear to be that simple. In any case, it's generally not something you need to worry about.

[poisondeathray]

you can "fix" it with mp4fpsmod

[Sharc]

you can "fix" it with mp4fpsmod

mp4fpsmod works for .mp4 files only, right? Does the problem with the 1/90000 time base exist for .mp4 only, or (similarly) for other formats as well?

[jagabo]

I've only seen it with MP4 and MOV. If you remux such a file to MKV it becomes CFR again.

[Selur]

@kkiller23: make sure you specify the input frame rate before and after the input ('-i ....'). I ran into this a few days ago and that is how I fixed it.