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

[Martin Storsjö]

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
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