[FFmpeg-user] getting wall clock time from rtsp stream

[Moritz Barsnick]

On Wed, Mar 16, 2016 at 22:42:09 +0100, Daniel Oberhoff wrote:
> around that there are things like RTCP sender reports and such on the low
> level containing this info,

Without reading the RTP specs, I could see exactly that from Wireshark
dumps.

> and I have seen indication that it is somehow parsed by ffmpeg.

Yes, ffmpeg's source code fits to those findings.

> What is the easiest way to get to this information? Best
> wpuld be command line dump of the time of the first frame, but rendering it
> somewhere, or even hacking ffmpeg would be doable too.

ffmpeg doesn't provide this value, it only uses it for calculating the
PTS (offset) in the demuxer. So you would actually have to edit the
source.

Check for last_rtcp_ntp_time and especially first_rtcp_ntp_time, which
is initialized from the frist packet's last_rtcp_ntp_time initially.

You could either print it out with av_log() for your personal purposes.
Or better, you could try to get that timestamp into the PTS, which
would call for a "-use_source_wallclock_as_timestamps" option to the
demuxer. That assumes this PTS is useful for you (it's easy to extract
from the first frame via ffprobe), but at least it would be flexible.

BTW, there's also already the global demuxer option
"-use_wallclock_as_timestamps", but this uses the wallclock of the
*receiving* end, and is only useful for formats which don't send
timestamps at all.

Moritz