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#1 Posted : Friday, June 11, 2021 7:24:48 AM(UTC)

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I use vmix since v12 ou 13 and we did a lot of live streaming. Today was one of my worst vmix day : we had x4 live stream at the same time with 4 vmix and we had a lot of trouble that i can either explain nor understand.
We began at 12:00 and everything was going well with x2 srt h264 on each vmix machine. then we get the first strange problem with a network card and a crash of a vmix computer (msi i9 rtx2080). After a reboot it seems it was ok. But after some time, we loose several srt streams in a very strange way. We choose to change one srt for 1 rtmp on each machine to be secure during the show. So we start with x4 vmix config and on each, x1 srt stream to a server and x1 rtmp stream to another one.
The show start fine, but after 15 minutes we loose again the vmix computer that had stranger network crash, then another one loose rtmp. At the end we had 2 major problem on our 4 streams. With srt and rtmp. And we don’t know what was going wrong.
vmix versions are all 24 (56, 58 and 59)
Any ideas ?

#2 Posted : Wednesday, June 30, 2021 10:06:42 PM(UTC)

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Joined: 6/30/2021(UTC)
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Hi Olivier

First things first, SRT is not a set and forget protocol like RTMP.
You need to measure the RTT and know what bandwidth you will be streaming at.
To give a clue you can use ICMP (Ping), I like it precise and use tools from Haivision or iPerf.
It is better to measure latency with UDP traffic.

The absolute minimum you have to consider on your transmitter side are "latency" and "maxbw".
The "latency" is specified in milliseconds, and if you do not specify this information, the default value of 120 ms is used.
Haivision describes the latency setup in several articles. You can find one here: Haivision
The "maxbw" is specified in bytes per second (not bits) and should be set to your maximum streaming bandwidth (example: 3Mbps * 125000 = 375000).

Now to why this was most likely part of your problem.
If you set the correct 'latency' without 'maxbw', it will lead to bandwidth exhaustion.
I am not aware of any way to provide this parameter in vMix.

Reference: Github

The problem can be solved in many different ways, here are some examples:
  • Restream a local RTMP stream over SRT with FFMPEG. Example:
    ffmpeg.exe -fflags +genpts -listen 1 -re -i rtmp:// -acodec copy -vcodec copy -strict -2 -y -f mpegts srt://host:port?mode=caller&latency=latency&maxbw=maxbw
    and then stream from vMix to rtmp://
  • Use on-premise srt server from todostreaming to receive UDP and restream it with SRT
  • Use nimble streamer to receive NDI and restream it with SRT
  • Use a streamer with reasonable SRT support
  • Use a gateway product from a SaaS provider of your choice

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