Since 65eb3c0815 we tried to get as close
a sync between audio and video tracks as we could before starting to
send frames to the output.
But in be717dbb2c when multi-track audio
was considered for synchronization it continued to try and line ALL
audio packets up with one video packet. While audio and video packets
are similar size this will work out. But once video packets duration is
smaller than 1/2 audio packet duration this may fail forever. In
practice with AAC's 20-23ms frame duration we can often get tracks
producing frames 5-10ms out of phase. For 60fps video this mostly fine
as 16ms frame duration will cover the gap, but for framerates as low as
100 its possible to fail to synchronize. In practice this is ~50%
for 6 audio tracks and 120fps video on my system, or 100% at 240fps.
Kurt Kartaltepe
b7f29ca767