Low latency streaming over the network, time sync

[Wolfgang Grandegger]

Hello,

Am 04.10.2018 um 17:44 schrieb Nicolas Dufresne:
> Le jeudi 04 octobre 2018 à 11:09 +0200, Wolfgang Grandegger a écrit :
>> Hello,
>>
>> I'm looking into low latency real-time streaming of MJPEG videos over
>> the network with GStreamer. I wonder what module does the frame
>> synchronisation on the receivers side and how. On the sender I start:
>>
>> # gst-launch-1.0 filesrc location=output.avi ! avidemux name=demux
>> demux.video_0 ! vaapipostproc ! vaapijpegenc quality=50 ! rtpjpegpay
>> !
>> udpsink host=192.168.178.52 port=50004
>>
>> and on the receiver:
>>
>> # gst-launch-1.0 udpsrc port=50004 !
>> application/x-rtp,encoding-name=JPEG,payload=26 ! queue !
>> rtpjpegdepay !
>> vaapijpegdec ! vaapipostproc ! vaapisink
> 
> To you first question, none in your pipeline. You'll need a
> jitterbuffer to handle the time, ideally with do-timestamp=1 on the
> udpsrc element.
> 
>>
>> I have some basic questions:
>>
>> - Are the frames displayed as they arrive (in "udpsrc")?
> 
> It will depends on the jitterbuffer "mode" property. By default it will
> try to slave to the remote clock. This mode is not extremely accurate
> though.

I see, I will have a closer look into the various modes. Then also
synchronizing clocks of the participating systems is important. Are
there documents around highlighting that topic?

>>
>> - Are the timestamps of the RTP packets used to  display the
>>   frames at the right time?
> 
> Yes, if you use a jitterbuffer.
> 
>>
>> If I add "sync=false" to the sender's command, the video is displayed
>> faster, even if I replace "queue" with "rtpjitterbuffer" in the
>> receiver's command.
>>
>> Also, with high quality decoding (90%), the video starts jerking. If
>> I
>> play the AVI file directly on the receiver side, it looks good.
> 
> JPEG are big, you'll have to increase the kernel UDP buffer.
> 
>   sysctl net.core.rmem_max=value

Ah, this solves the problem with jerking.

> And then use buffer-size property on udpsrc.

I was already aware of that option.

Thanks,

Wolfgang.

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