Audio sync problem when exporting to file

Anything and everything to do with DCP-o-matic.
Post Reply
Antti N
Posts: 8
Joined: Mon Nov 16, 2020 5:03 pm

Audio sync problem when exporting to file

Post by Antti N »

Hi again, guys!

I've noticed that certain video source files seem to trigger weird behaviour in D-o-m: the audio is thrown off sync when exporting to file – but not when creating a DCP from the same file, from the same project, with the same settings.

Case:
  • I've received a short film as an mp4 (h.264/AAC) file
  • I create a new project in D-o-m and import the mp4
  • I trim it to 30 seconds (just to make this test run quicker; all other settings remain at defaults)
  • I create a DCP: audio sync matches the source file exactly
  • I export to a ProRes file: audio is 5 frames late
I've experienced this with several mp4 and ProRes files from various sources. I can't seem to find anything out of the ordinary in these files, but the problem does disappear if I put them through a simple "ffmpeg -i infile.mp4 -c copy outfile.mp4".

The files are copyrighted material, but I can PM a download link on request.
Carsten
Posts: 1723
Joined: Tue Apr 15, 2014 9:11 pm
Location: Germany

Re: Audio sync problem when exporting to file

Post by Carsten »

We had a few audio sync issues recently because of weird audio stream properties (duplicate presentation timestamps).

viewtopic.php?f=2&t=1439&p=6250&hilit=timestamp#p6250

It went back and forth, and maybe some of these issues are solved for the DCP, but not the export. Personally I never had audio sync issues, it is weird that some audio files use these strange capabilities. Wondering wether this could actually be a sick copy protection method.

You could try to reimport the created DCP into DCP-o-matic and export this to Prores. Of course your FFMPEG in-out copy is the better way.
Last edited by Carsten on Wed Mar 17, 2021 10:38 am, edited 2 times in total.
Antti N
Posts: 8
Joined: Mon Nov 16, 2020 5:03 pm

Re: Audio sync problem when exporting to file

Post by Antti N »

OK, so it was a known issue – thanks! I just run a quick test on 2.15.135, and it seems to be fixed all right.
Carsten wrote: Tue Mar 16, 2021 10:49 pmYou could try to reimport the created DCP into DCP-o-matic and export this to Prores.
This wouldn't have worked for me in this case, as it will introduce a color shift. I've always assumed it to be a more-or-less inevitable side effect of RGB –> YUV –> RGB conversion, so I haven't filed a bug report ;)
Carsten
Posts: 1723
Joined: Tue Apr 15, 2014 9:11 pm
Location: Germany

Re: Audio sync problem when exporting to file

Post by Carsten »

Which version did you use that created the sync issue?


Carl - was the fix for the duplicate presentation timestamps also backported to 2.14.x? I guess so?!
Antti N
Posts: 8
Joined: Mon Nov 16, 2020 5:03 pm

Re: Audio sync problem when exporting to file

Post by Antti N »

That was 2.14.47 (Ubuntu 20.04, but I'm pretty sure I've seen the same issue in the Mac version).
Post Reply