Problem with video frame rate

[corleone88]

Hello,

This is my first post and I don't find a solution to my tricky problem.
I have a video ripped from a Blu-ray which has 23.976216 fps (info displayed from VLC) and I would like to add an audio stream which is at 25 fps. I managed to convert the audio stream to 23.976. I muxed the video and audio. It is synchronized from the beginning to the middle part but it started to look desynchronized after that. I suspect that it is the slight difference in framerate.
I was not able to convert the video to the exact 23.976216 fps as I didn't find any software which has this kind of accuracy for framerate. So I thought about changing the audio stream to 23.976216 fps. I used the following option with besweet command line, but I don't know if it is understood by the software: -ota (-r 25000 23976.216.
Thanks for your help

[manono]

I suspect that it is the slight difference in framerate.

That can't be it as the amount it might get out of synch would be way too little for you to notice. Look for some other reason for the problem.

And I find your thread title somewhat misleading as the problem isn't with the video framerate which would be 23.976024fps. Whatever VLC says is slightly off. Divide 24 by 1.001 and see what you get.

[smrpix]

Audio doesn't have a frame rate. It has a sample rate, a bit rate and a bit depth.

Try opening your original track and your new track stacked in either an nle or something like audacity and see if the waveforms help you figure out what's going on.

Once it starts to go out of sync is it constant or does it drift?

[hello_hello]

If the audio stays in sync till half way through then suddenly loses sync (rather than drift out slowly) then the problem's not the frame rate. The problem is the video from which the 25fps audio was taken has a different number of frames than the 23.976fps version.

I've seen it lots of times, video which seems the same but is actually cut slightly differently (comparing PAL and NTSC versions). Sometimes it's just a cut to black between scenes which is a few frames longer in one version that the other (for example) or sometimes there's even a scene here and another there which is one frame shorter in one version than the other.... enough of those and it almost appears the audio is slowly drifting out of sync when it's actually not, it's happening in distinct steps according to the slight frame count differences.

The first thing to do would be to find a way to determine the exact number o frames in each version. If the frame numbers of each version match according to the video at the beginning but the last frame of video is a totally different frame number in each version then you may do your head in trying to sync the audio by stretching it by different amounts and never get it in sync.

[hello_hello]

Audio doesn't have a frame rate. It has a sample rate, a bit rate and a bit depth.

True, but many audio converters call it frame rate conversion for the sake of convenience and have standard conversion options such as 23.976fps to 25fps etc.