[FFmpeg-user] AV sync issues with transcoding and DASH output

Benjamin SOMERS benjamin.somers at telecom-bretagne.eu
Fri Sep 25 12:44:22 EEST 2020


I'm trying to convert DTT signals to DASH and am facing an issue with AV synchronization. The (simple) architecture isĀ :
DVB-T cards -> MuMuDVB -> FFmpeg -> DASH player

MuMuDVB grabs the signals from the DVB cards and streams them in multicast. By checking in VLC, the raw output is ok. The video signal is in h264 and does not need to be transcoded (anyway, my machine is not beefy enough to do the transcoding, as I'm dealing with ~20 channels) but the audio signal does. So I'm running FFmpeg like this:
ffmpeg -i udp://@ -map 0 -ignore_unknown -c:v copy -tag:v avc1 -sn -dn -seg_duration 5 -window_size 20 -remove_at_exit 1 -f dash /var/tmp/index.mpd
The -ignore_unknown, -sn and -dn options help to clean the streams, as DTT signals carry a lot of additional data.

In Firefox, everything is good, the audio and video streams are in sync, but in Chromium and VLC (which IĀ consider my reference player), the audio has ~1s of delay. The fact that it works in Firefox convinces me that I can make this work elsewhere. If I let FFmpeg transcode the video, the streams are in sync in all the players, but as I said, that is unfortunately not an option.

Should you need an FFmpeg output, you can find it here -> https://pastebin.com/wzTkvU0Y
I zipped my /var/tmp directory which contains the files generated by FFmpeg here -> https://0x0.st/iUoM.zip
If you only need the manifest (or fear a zipbomb), it is here -> https://pastebin.com/A895JTUW

Is there anything I can do without resorting to a video transcoding?

Thanks for your help.

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