Hi everyone,

We've been users of vMix since 2021 and have appreciated the huge upgrade it was over OBS for our Church streaming service. We have always used our local presentation software, Faithlife Proclaim, to generate slides for in house and "lower 3rd" overlays for streaming lyrics during worship songs. We have never had an issue in the ~4 years we've had this setup until about a month ago.

As of March 16th, the two NDI sources (generated by the same program) appear to be out of sync with the "lower 3rd" always lagging behind.

• vMix and Proclaim are running on the same computer with ~25% max CPU load, typically when playing a video stream embedded in proclaim. HW encoding on our GTX 1660 Ti + resizable bar helps quite a bit.
  
• Studio Monitor (from NDI Tools) shows both sources in sync while the same source in vMix's preview window is lagging
  
• Recording all inputs using MultiCorder shows both inputs in sync for the entire recording (~5 minute test) despite also showing the lagging "lower 3rd" on the recorded output
  
• NDI Analysis shows that the presentation software (Proclaim) is likely only sending NDI frames when the screens are changing:
  18:14.0 16201.37 751.58
  18:25.1 27295.62 11094.25
  18:26.8 28974.42 1678.8
  18:28.5 30714.13 1739.71
  
• "lower 3rd" is set to be 100% transparent except when slide lyrics are displayed, NDI Analysis shows these frames as 1024 bytes each
  
• When "lower 3rd"s are lagging behind, it is often by a fixed number of slides (i.e. 17 slides) in the Proclaim software. I find this particularly odd, because it appears to be completely independent of real time (i.e. always 17 clicks behind, even when a 30 min sermon exists between two slides)
  
• Occasionally, Proclaim seems to trigger a vMix buffer flush to advance several slides/frames at a time and "catchup"(https://youtu.be/c-N46gFiwSg?t=1235)
  
• Initially we were on vMix 26.0.0.45, but upgraded to 28.0.0.39 was not sufficient to fix the issue
  
• Proclaim's last prior update was on Feb 25th. The only other thing I would suspect of changing is windows updates

My working theory is Proclaim is doing "out out spec" or "bad" things to the NDI stream in the name of saving bandwidth/CPU cycles. vMix appears to have a fixed number of frames buffered (hence the odd coupling of '17 slides off') and isn't handling the gaps very well. The combination of the two creates the lag effect in real time but vMix multicording flushes the buffers frequently enough to not show the lag.

Any tips on how to troubleshoot this issue further or possibly work around it? Proclaim can send more frames if add "transitions". It's not enough to bridge the gaps, but it might be able to flush a vMix buffer if I had more information to fine tune it. Can I reduce the number of NDI frames vMix buffers? Can I help vMix dump what it has to the rendered output faster like it appears to be able to do with the recorder or with specific "catchup" frames from Proclaim?

Thanks in advance for any help/tip/suggestions!

Sync is always at the input side. You "normalize" the inputs so that you can proceed with mixing. What should vMix do if you put two shots in a PIP. What sync to apply then at the output. Impossible. The only way to do it is at the ingest.