Hello, I have a question: currently, when the SRT in CRB goes black, the bitrate doesn't remain constant, even if you configure it to 5 Mbps in CRB. When the video is static or the screen goes black, the bitrate doesn't stay at 5000 kbps and drops to 425 kbps. Is there a solution?

No, there's no solution. The new update doesn't improve SRT at all; in short, it's more of the same. I just wrote this in another post; let's see what the developers say.



**What is the point of improving SRT if vMix still cannot provide a true strict CBR stream?**

I have been testing SRT output from vMix and I think there is an important issue that still needs attention: **strict Constant Bitrate (CBR).**

For example, I configured my stream at approximately **6000 kbps video + 128 kbps audio**.

When the video contains normal movement, measuring the actual SRT transport gives me around **817 KiB/s (~6.7 Mbps)**, which is what I would expect.

However, when the source becomes a static image, the same SRT output drops to approximately **38.2 KiB/s (~0.31 Mbps)**.

That is a reduction of more than 95%.

So even though a fixed bitrate is configured in vMix, the resulting stream is clearly not maintaining that bitrate when the picture has little or no movement.

This matters in professional broadcast/IPTV environments. Some downstream equipment and distribution systems expect a stable transport bitrate. A static image should not cause a 6 Mbps stream to suddenly collapse to a few hundred kbps.

My question to the vMix developers is:

**Why doesn't vMix provide an option for true strict CBR / constant transport bitrate when using SRT?**

SRT itself keeps receiving improvements and updates, which is welcome, but for broadcast contribution I think having the option to maintain a genuinely constant output bitrate is just as important.

It would be very useful to have something equivalent to a strict CBR mode where, if the encoder doesn't need the entire configured bitrate for the video, the transport stream is padded appropriately so that the final output remains constant.

For IPTV and broadcast distribution, this would make SRT output from vMix much more predictable and reliable.

Thanks for running the test. I've continued testing and found an important detail.

You are correct that **if I load a static image directly into vMix, CBR is maintained correctly**.

The problem occurs while playing a video file, such as a movie, when the content reaches a completely black screen, a long fade to black, or end credits over a black background.

During normal video content, the stream stays around the configured bitrate. However, during these very dark or low-complexity sections, the bitrate can drop dramatically. As soon as normal content returns, the bitrate goes back up.

I've also tested Hardware Encoder, Software Encoder, and Low Power Hardware Encoder, with the same behavior.

So I believe testing only with a static image does not reproduce the specific issue I'm seeing. It would be interesting to test with a video containing several seconds of black screen or credits over a black background and check whether the SRT output continues to maintain the full 10 Mbps.

I will also open a support ticket with vMix including these details.

Thanks again for testing this.