[Live-devel] Change Request about rtp timestamping when doing PAUSE/PLAY

David Betrand bidibulle at operamail.com
Tue Oct 24 03:14:27 PDT 2006


Ross,
> > I noted that live555 implementation for this consists of 
> setting > in the rtp-info header a rtptime value corresponding to 
> the last > RTP timestamp sent for this track
>
> I believe this is correct, because the RTP timestamp *does* increase
> appropriately after a PAUSE/PLAY (see below).

The problem here is not the timestamps present in the RTP packets, 
they are correct. It is the "rtptime" value present in the PLAY 
Response which is incorrect because it equals to the last rtp 
timestamp sent BEFORE the Pause. (see usage of rtpSink()->currentTimestamp() in OnDemandServerMediaSubsession::startStream()) 


Following RFC2326bis, the 
definition of "rtptime" is the following :
rtptime: SHALL indicate the RTP timestamp value corresponding to
              the start time value in the Range response header, or if
              not explicitly given the implied start point. The client
              uses this value to calculate the mapping of RTP time to NPT
              or other media timescale. This parameter SHOULD be present
              to ensure inter-media synchronization is achieved. There
              exist no requirement that any received RTP packet will have
              the same RTP timestamp value as the one in the parameter
              used to establish synchronization.

You cannot use an old value for this rtptime otherwise players will 
think the packets they receive are in the future. That's exactly 
what VLC think  (see VLC output message in attachment, starting 
from the time I resumed the stream).
In the example sent in my previous email (extracted from the RFC2326bis) you can see that this rtptime value is coherent with the start time value in the Range response header (20 seconds elapsed between the PAUSE and the PLAY, so the rtptime jumps from 2400 to 164400).

To summarize, I would say that -at least in this case- you cannot use fCurrentTimestamp as rtptime value.
David


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main debug: control type=1
main debug: control type=1
mpeg_audio debug: emulated startcode (no startcode on following frame)
main warning: computed PTS is out of range (7649957), clearing out
main warning: timing screwed, stopping resampling
main warning: PTS is out of range (7687639), dropping buffer
main warning: output PTS is out of range (7675728), clearing out
main warning: vout synchro warning: pts != current_date (-8861183)
main warning: late picture skipped (7769560)
main warning: backward_pts != current_pts (8827815)
main warning: input PTS is out of range (8376350), trashing
main warning: received buffer in the future (9314190)
main warning: received buffer in the future (9340122)
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main warning: received buffer in the future (9573540)
main warning: vout synchro warning: pts != current_date (-17521672)
main warning: late picture skipped (21730324785)
main warning: computed PTS is out of range (17452792), clearing out
main debug: audio output is starving (411517), playing silence
main debug: decoded 104/107 pictures
main warning: received buffer in the future (5698582)
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main debug: control type=0
main debug: control: stopping input
main debug: closing input
main debug: removing module "livedotcom"
main debug: removing module "libmpeg2"
main debug: thread times: real 0m36.112287s, kernel 0m0.030043s, user 0m0.680979s
main debug: thread 3952 joined (input/decoder.c:191)
main debug: killing decoder fourcc `mpgv', 29 PES in FIFO
main debug: removing module "mpeg_audio"
main debug: thread times: real 0m36.102272s, kernel 0m0.010014s, user 0m0.971396s
main debug: thread 4036 joined (input/decoder.c:191)
main debug: killing decoder fourcc `mpga', 0 PES in FIFO
main debug: removing module "mpgatofixed32"
main debug: removing module "bandlimited_resampler"
aout_directx debug: closing audio device
aout_directx debug: DirectSoundThread exiting
main debug: thread times: real 0m36.042185s, kernel 0m0.000000s, user 0m0.000000s
main debug: thread 4668 joined (directx.c:598)
main debug: removing module "aout_directx"
main debug: removing module "float32tos16"
main debug: removing module "trivial_mixer"
main debug: thread times: real 0m36.322591s, kernel 0m0.060086s, user 0m0.140201s
main debug: thread 4108 joined (input/input.c:399)
main warning: refcount is 1, delaying before deletion (id=396,type=-7)
main debug: garbage collector destroys 1 vout
vout_directx debug: DirectXCloseSurface
vout_directx debug: DirectXCloseDisplay
vout_directx debug: DirectXCloseDisplay clipper
vout_directx debug: DirectXCloseDisplay display
vout_directx debug: DirectXCloseDDraw
vout_directx debug: CloseVideo
vout_directx debug: DirectXEventThread terminating
vout_directx debug: DirectXCloseWindow
vout_directx debug: WinProc WM_DESTROY
main debug: thread times: real 0m36.412722s, kernel 0m0.010014s, user 0m0.010014s
main debug: thread 4372 joined (directx.c:494)
main debug: removing module "vout_directx"
main debug: thread times: real 0m36.322591s, kernel 0m0.050072s, user 0m0.420604s
main debug: thread 4632 joined (video_output/video_output.c:461)


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