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Problem with too low bitrate in vmix rtmp streams - what does strict cbr mean?
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T68  
#1 Posted : Thursday, January 2, 2020 7:52:39 PM(UTC)
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Hi!
We have some production teams that use vmix and encode rtmp streams that are pushed to our control room recievers.
Lately we have encountered problems when they display still image graphics in that the bitrate goes so low that our gear interpret the stream as corrupt.
As soon as they start showing real moving video the bitrate goes up and all is well.

I´m not familiar with vmix at all but have looked at the online documentation to try to understand how the built in encoder in vmix operates.
There I see a checkbox in the streaming setting that toggles something called "strict cbr" on and off.

But then below in the text there´s a (not good enough) explanation that this will "try" to keep the set bitrate.

I would be greatful if someone could explain how vmix handles adaptive and constant bitrates. And if activating the "strict cbr" actually is cbr or some type of adaptive bitrate still (that "try to" wording seems a bit unclear).

I also wonder if there is any way to define an interval for adaptive bitrate so that vmix never goes below a minimum set value?

Thanks in advance for any input on this matter.
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Jiii  
#2 Posted : Thursday, January 2, 2020 11:47:49 PM(UTC)
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T68  
#3 Posted : Friday, January 3, 2020 12:09:59 AM(UTC)
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Yes, looks like the same kind of issue as we are having. Thanks for the pointer!

But it would be interesting to know more about how the vmix encoder works regarding adaptive bitrate vs strict cbr. We could still need adaptive and it would be nice if strict cbr had some kind of adaptive in it to avoid framedrops and stuff if team has bandwith issues.
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mjgraves  
#4 Posted : Friday, January 3, 2020 1:41:15 AM(UTC)
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Originally Posted by: T68 Go to Quoted Post
But it would be interesting to know more about how the vmix encoder works regarding adaptive bitrate vs strict cbr. We could still need adaptive and it would be nice if strict cbr had some kind of adaptive in it to avoid framedrops and stuff if team has bandwith issues.


You can't have your cake and eat it, too. The are cases that require an absolutely constant bitrate (CBR.) Thus CBR must be constant. Not kinda constant.

VRB is something else entirely. The logic of VBR can be adaptive to the video content (bitrate varies to sustain constant quality) and/or network circumstance. Things like SRT address this.
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T68  
#5 Posted : Friday, January 3, 2020 1:53:51 AM(UTC)
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Originally Posted by: mjgraves Go to Quoted Post
Originally Posted by: T68 Go to Quoted Post
But it would be interesting to know more about how the vmix encoder works regarding adaptive bitrate vs strict cbr. We could still need adaptive and it would be nice if strict cbr had some kind of adaptive in it to avoid framedrops and stuff if team has bandwith issues.


You can't have your cake and eat it, too. The are cases that require an absolutely constant bitrate (CBR.) Thus CBR must be constant. Not kinda constant.

VRB is something else entirely. The logic of VBR can be adaptive to the video content (bitrate varies to sustain constant quality) and/or network circumstance. Things like SRT address this.



Ok, understand that. But I still wonder about the documentation that says:

"Strict CBR

When this is enabled, vMix will try to keep the streaming speed as close to the selected bit rates as possible.
Note that this option may reduce the quality of the stream."

What does it mean by "try to keep the streaming speed"? To me "try" means that it will do something else if it fails, ie drop bitrate.
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admin  
#6 Posted : Friday, January 3, 2020 3:03:50 AM(UTC)
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Hi,

In vMix, "Strict CBR" is about controlling the maximum bandwidth of the stream much more closely to ensure it doesn't
exceed the set bitrate as much as it can under VBR. (such as when a scene suddenly changes)

It is not designed to prevent lower bitrates due to static scenes, as these are already encoded to the maximum quality possible and can't go higher.

Some CBR encoders will add blank padding data into the stream in to ensure the bitrate is kept, but for streaming purposes however this is not necessary and just results in wasted bandwidth, so we don't do this.

(For what it is worth, messages such as on YouTube about "low bitrate" are warnings only and do not mean anything is wrong with the stream if you are only sending static images)

Hope this helps.

Regards,

Martin
vMix
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mjgraves on 1/4/2020(UTC)
e.belyshev  
#7 Posted : Friday, January 3, 2020 4:22:25 PM(UTC)
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CBR stands for Constant Bit Rate vs VBR Variable Bit Rate, kind of like driving a car at same speed limit vs variable speed depending on turns or straight lane. Switching to CBR with high bit rate of 20MBp/s or above will yield slightly noticeable quality during some scenes but make sure your network switches can handle the increased bandwidth if you have multiple streams. YouTube does not care for Strict CBR but at the moment Facebook does.
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T68  
#8 Posted : Friday, January 3, 2020 5:18:15 PM(UTC)
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Thanks for your response!
I was a bit unclear in the problem description. We not only get warnings but also have issues with that our recievers get problems taking in video from the vmix signals when they display static images and the bitrate drops to 500kbps or below. We get interrupts, freezes and drops.
The reciever is of brand ”Intinor direkt router” and the issue has been replicated on different units sitting on differend isp, cdn and such. We do not see this behaviour on any other signals, adaptive or not.
I have of course reported these problems to Intinor and our other partners that also are looking into the issue trying to find a solution.
We´re going to try strict cbr but as described above it seems as if it will not solve the problem with too low bitrates with static images.
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