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

[Michael Niedermayer]

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

[...]
-- 
Michael     GnuPG fingerprint: 9FF2128B147EF6730BADF133611EC787040B0FAB

Freedom in capitalist society always remains about the same as it was in
ancient Greek republics: Freedom for slave owners. -- Vladimir Lenin
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