[FFmpeg-devel] Matroska and output_ts_offset

[Peter Zebühr]

Hello,

I noticed the other day that if I try to mux opus in webm with a timestamp offset set I don't get the expected timestamps out on my packets. Example:

	ffmpeg -f lavfi -i "sine=frequency=1000:duration=60" -output_ts_offset 200ms -c:a libopus sine_200ms_offset.webm

Now if I inspect the packets with ffprobe on the produced output the first packet has a PTS of 186. My expectation would be that it would be: 200 - 6.5 (opus encoder delay) ~= 193.
Looking at the output with mkvinfo also indicates that it starts at 193ms (186 + encoder delay I assume here).

At first I thought it looked like the encoder delay gets added twice, but after some more pondering I think what is happening here is:

1. Opus outputs packets starting with the priming samples, that ends up with a negative PTS (rounded to -7)
2. In mux.c the output_ts_offset takes effect before the call to "handle_avoid_negative_ts"
3. PTS -7 gets shifter by the requested ts_offset to 193
4. Matroska muxer writes the side data about encoder delay. 
5. The produced stream now effectively starts one encoder delay distance too early.

I played around locally with moving the call to "handle_avoid_negative_ts" in mux.c/write_packet to right before the ts_offset handling instead of right after and that seems to fix this particular problem. But, I am unsure what other potential consequences that would have. Would appreciate if someone more familiar with this can help fix this issue.

Regards,
Peter