[FFmpeg-devel] [PATCH] Sync the NTP timestamps for RTP streams

[Martin Storsjö]

On Mon, 15 Mar 2010, Michael Niedermayer wrote:

> On Mon, Mar 15, 2010 at 11:27:59AM +0200, Martin Storsj? wrote:
> > On Thu, 11 Mar 2010, Martin Storsj? wrote:
> > 
> > > On Thu, 11 Mar 2010, Michael Niedermayer wrote:
> > > 
> > > > On Thu, Mar 11, 2010 at 11:51:08AM +0200, Martin Storsj? wrote:
> > > > > 
> > > > > diff --git a/libavformat/avformat.h b/libavformat/avformat.h
> > > > > index 10371a4..7c74951 100644
> > > > > --- a/libavformat/avformat.h
> > > > > +++ b/libavformat/avformat.h
> > > > > @@ -526,6 +526,14 @@ typedef struct AVStream {
> > > > >       * Number of frames that have been demuxed during av_find_stream_info()
> > > > >       */
> > > > >      int codec_info_nb_frames;
> > > > > +
> > > > > +    /**
> > > > > +     * Start time of the stream in real world time, in microseconds
> > > > > +     * since the unix epoch.
> > > > > +     * - encoding: Set by user.
> > > > > +     * - decoding: Unused.
> > > > > +     */
> > > > > +    int64_t start_time_realtime;
> > > > 
> > > > this is not completely clear, we have
> > > > time of transmission of first packet
> > > > time of decoding of first packet
> > > > time of presentation of first packet
> > > > time corresponding to timestamp=0 instead of first packet
> > > > 
> > > > and i think you should write out what the unix epoch is
> > > 
> > > Ok, I'll clarify it. In this case, it is the real-world time corresponding 
> > > to timestamp = 0 in the stream, that is, when the data was captured.
> > > 
> > > > that said, i didnt follow the thread very much, could you explain in a few
> > > > lines why this is needed?
> > > 
> > > Syncing of multiple RTP streams is done through sender reports, where the 
> > > sender says that timestamp X in the RTP stream corresponds to real-world 
> > > time Y.
> > > 
> > > When sending multiple RTP streams, currently the timestamp=0 <-> realworld 
> > > time mapping is made when the RTP muxer is opened, which makes the 
> > > timestamp=0 map to different realworld times in the muxers, since the RTP 
> > > muxers can't be opened at the exact same time. (In practice, one can get a 
> > > 5-30 ms difference between them, when doing RTSP commands inbetween.)
> > > 
> > > So when writing multiple RTP streams, we need to make sure timestamp=0 
> > > maps to the exact same realworld time, so that the receiver can sync the 
> > > streams back together.
> > > 
> > > In the RTP scenarios, the actual value of the NTP timestamps doesn't 
> > > matter all that much, only the difference between them is used for syncing 
> > > the streams together.
> > 
> > Updated patch attached, clarified the details of this field.
> > 
> > Also, I realized that there's no need to keep it separate for each 
> > AVStream, so moved it to AVFormatContext instead.
> > 
> > Is this ok to be added?
> > 
> > // Martin
> >  avformat.h |    9 +++++++++
> >  1 file changed, 9 insertions(+)
> > 8532797ce5adc0f97261d7e970b9d0904f29b1bc  start_time_realtime.patch
> > commit 68520d720168df043013d48a8f6d578b9cea1aac
> > Author: Martin Storsjo <martin at martin.st>
> > Date:   Thu Mar 11 11:11:43 2010 +0200
> > 
> >     Add a field to AVFormatContext for specifying a real world start time for the stream
> 
> looks ok, dont forget minor ver bump and apichanges update though

Thanks, applied and bumped minor version and added apichanges entry.

// Martin