[YouTube] Out of sync audio and video in download (muxing issue)

[Stypox]

Checklist

• I am able to reproduce the bug with the latest version given here: CLICK THIS LINK.
• I made sure that there are no existing issues - open or closed - which I could contribute my information to.
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Affected version

0.28.0

Steps to reproduce the bug

Download https://m.youtube.com/watch?v=sRTPiFzXxYQ in either 480p or 720p. You will see that the resulting video file has out of sync video (i.e. the video lags behind the audio). This is most likely a downlaoder muxing issue, related to #12443.

It would be good if someone who has some experience with the MP4 format (standard ISO/IEC 14496-12) could work on this.

[absurdlylongusername]

A user gave more information about this in the matrix chat. Link to their message so you can scroll down and see what they said

Their primary message:

Hi - I have a small problem with NewPipe downloaded files - they get laggy (as in, rebuffer pause every few seconds) on mpv-android after about 6 minutes, but are fine in VLC.

I kinda found the cause: the MP4 muxer in https://github.com/TeamNewPipe/NewPipe/blob/dev/app/src/main/java/org/schabi/newpipe/streams/Mp4FromDashWriter.java#L333 only seems correct if a sample on both inputs has about the same length. Which isn't the case - for video I commonly get 1/60 s long samples, while for audio I get 1024/44100 s. Remuxing the files with ffmpeg indeed makes mpv happy.

Need any more info for that issue? Running MP4Box -std -dchunk on any downloaded file from YT at least should look rather obvious, as at nearby byte position audio and video dts diverge more and more.

In any case, the general and ideal solution is to find something we can use for muxing, as this is already a solved problem, we just need to find which solution we can use instead of trying to do it ourselves. We currently don't use ffmpeg cus it's huge, but likely there's a way downsize it just for the functionality that we need; likewise also with GPAC.