[Cin] Audio out of sync when rendering MP4s

[Matteo Bini]

Hello Cinelerra GG developers,
I've found a strange behaviour in this great editing software.

When I render my project, sometimes the audio is out of sync. Not all
the audio, but from a certain point, from half of the video the audio is
out of sync and it gets worst the more we get to the end of the video.
It's like latency piles up. I've only done MP4 renderings so far.

My idea, but I don't know how to demonstrate it, is that ffmpeg doesn't
know how to render audio and video in sync, when there's some audio that
is not aligned on frames. Since I do a lot of precise audio editing in
my project, there are some audio cuts that are not aligned on frames.

It's not a big problem, I just wanted to let you know. The solution is
very simple: render only the video track, then render the audio track in
WAV and then mux them together with ffmpeg, copying the video track and
encoding the audio one. This way the video is perfectly in sync with the
audio.

The strange thing is that, when I watch my project from the compositor
window, audio and video are in sync.

Thank you for making Cinelerra GG a really great free software.

--
Matteo Bini