[FFmpeg-user] Some inputs make MP4 output sync incompatible with other applications

[Francois Visagie]

> -----Original Message-----
> From: Francois Visagie [mailto:francois.visagie at gmail.com]
> Sent: 23 August 2013 15:11
> To: ffmpeg-user at ffmpeg.org
> Subject: Some inputs make MP4 output sync incompatible with other
> applications
> 
> Hi All,
> 
> I'm batch encoding our DVDs to H.264 AAC MP4s. Two of these output MP4s
> have audio that start too early in both MPC-HC and Windows Media Player,
> although ffplay syncs them correctly.
> 
> I'm trying to prevent this MP4 desync for other applications in a way that
> safely can be left in the batch process also for better-behaved inputs.
> 
> Through trial-and-error, so far either '-copypts' or the now-
> deprecated '-async 1' prevents desync in other applications.
> 
> For reasons not related to this issue I'd prefer to continue with
'-copypts'

>From some quick tests '-async 1' actually seems to behave better than
'-copypts' with better-behaved inputs.

Put differently, I haven't seen obvious drawbacks so far, but what is the
potential downside to using '-async 1' across the board from a theoretical
point of view?