[vlc] Problems seeking in MPEG2 TS stream via RTSP?

[Brett Granger]

Following up a bit further on this... it doesn't seem to happen on every 
MPEG2 TS.  I have some streams where the problem happens every time, 
some streams where it happens sometimes, and some streams where it never 
happens...

Any thoughts on whether I might be able to discern differences in the 
streams that would cause this problem?  I am not an MPEG expert by any 
means.

Thanks again,
--Brett

Granger, Brett wrote:
> Hello,
> 
> I am using a VLC client on linux (VLC 0.8.6b default package from Ubuntu
> Feisty repos) to play an MPEG-2 transport stream from a
> live555MediaServer using RTSP and RTP.  I have used the
> MPEG2TransportStreamIndexer application included with the liveMedia code
> to create an index file to go along with the stream.  Normal playback of
> the stream from the beginning works just fine.
> 
> However, if I attempt to use the --start-time=<time> command line
> option, I see strange behavior:  the client puts up a black screen for
> the amount of time in the start-time parameter, and then finally starts
> to display the video from the desired offset into the stream.  Likewise,
> if I launch vlc with the rc interface and let it start playing the
> stream from the beginning, then use the "seek <x>" command in the rc
> interface, the video presentation stalls for an amount of time equal to
> the difference between the requested new presentation time and the
> current presentation time, then resumes from the correct newly-requested
> location in the stream.  Using the -vvv option on the command line, I
> see that the live555 access_demux *always* issues a PLAY request from
> time 0 when starting to play a stream, then follows up almost
> immediately with a PLAY request for the new time passed in via the
> --start-time option, so I believe that both cases are showing the same
> behavior.
> 
> I used wireshark to look at the traffic between client and server.  I
> see that when the seek command is issued via the interface, the RTSP
> server immediately replies and starts sending RTP packets from the new
> location in the stream.  However, the VLC client does not present them
> until the wallclock time has advanced to the point where the next frame
> would have been displayed without the seek request -- e.g. "seek 15"
> from time 0 results in a 15 second delay before video resumes.
> Additionally, the VLC client appears to have buffered all the RTP
> packets that were sent in the meantime.  When I attempt to pause after
> having issued a seek, wireshark shows that the PAUSE request goes to the
> server immediately and the RTP packets stop coming, but VLC continues to
> play video for a time equal to the duration of the seek before pausing.
>   I have --rtsp-caching=200, and yet VLC will play 15 seconds of video
> after the pause if I have done a seek of 15 seconds.
> 
> Has anyone seen this problem, and is there anything I can do either to
> solve it or to get more information about it?  It appears to have
> something to do with the timestamp at which VLC believes it is supposed
> to present the next frame.  VLC just waits until that time and then
> proceeds with the presentation.  I have determined that if I am in the
> rc interface and use the command "goto 0" on a clip which has a
> start-time option, it will immediately start presenting from the correct
> time in the stream, and I think that is because it does not issue a PLAY
> request for time 0 before issuing the PLAY request for the start time.
> However, if I then seek in that stream, I still see the same stalling
> behavior.
> 
> I do not see this behavior when streaming an mp3 stream from the same
> RTSP server, so it appears to be specific to the MPEG2 transport stream
> as well...
> 
> Thanks in advance for any thoughts or help.  Please let me know if this
> issue belongs on a different VLC mailing list.
> 
> --Brett Granger
> 
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