Strobing 4K Video

[movingpictures]

I have two Barco UDX 4K32 projectors (each at 3840x2160 24Hz) blended for a 7200x2400 image using Arena 6. The Machine is a Late 2013 Mac Pro running Sierra with an Intel Xeon E5 chip (3.5 Ghz, 6-core), 32 GB of RAM, an SSD and dual AMD Firepro GPUs (3GB each).

The footage provided is Apple ProRes 422 HQ, 3840x2160, 23.98fps encoded using DaVinci Resolve.

Every clip strobes intermittently and inconsistently, and after a day of troubleshooting I'm out of ideas. The FPS meter shows a solid ~24fps throughout, with no dips during the strobing. The playhead on the timeline moves smoothly and the timecode doesn't jump (at least as far as I've noticed). I've tried the setting the composition to 23.98fps, 24fps and Auto with no changes. Activity monitor shows a near-constant 34% CPU load with no spikes. Nothing hogging memory or disk usage either, and OS X isn't the least bit sluggish. The videos look fine playing on the desktop in QuickTime, but also strobe when played on the projector in Arena, QLab and Isadora. I disconnected one projector, but the strobing remained in the remaining one, even after it's refresh rate got bumped to 30 Hz. I was given a 1080p H.264 QuickTime version to test with, but still experience strobing.

I tried converting a couple files to DXV via Adobe Media Encoder, but the output looked to be about 4 fps (even in the output monitor), even though it was playing smoothly. I followed the AMC encoding settings instructions in the Arena manual, but no luck. I lost access to the venue earlier, but I just output a test file on my home computer (using After Effects) with the same settings as the show footage. I again converted it to DXV with AMC, and it plays just fine in the output monitor. I'll see how it runs on the show machine tomorrow, but any advice would be greatly appreciated.

[Oaktown]

What are you using for signal distribution (adapter type, cable length, etc..)?

[Zoltán]

The FPS meter shows a solid ~24fps throughout, with no dips during the strobing.

what kind of strobing is this you see ? flashing to black, or slow refresh of the footage?
Do you use SMPTE?
What composition size are you playing these files?
Do you also see this strobing in a 1080p composition?

[movingpictures]

What are you using for signal distribution (adapter type, cable length, etc..)?

Each projector is connected using a 25' HDMI 2 with a Thunderbolt adapter. They're on separate buses with the control monitor connected to the third bus with VGA and a TB adapter.

what kind of strobing is this you see ? flashing to black, or slow refresh of the footage?
Do you use SMPTE?
What composition size are you playing these files?
Do you also see this strobing in a 1080p composition?

It's a slow refresh. It's not chugging and skipping like it's losing frames, and the signal is consistent.

I don't think we're using SMPTE. Correct me if I'm wrong, but my understanding is that SMPTE mode runs from an external clock rather than the footage's timecode.

The composition size is 7200x2400.

[Oaktown]

Couple things:

Are your adapters active adapters? AMD cards are notorious for needing active adapters when you use more than 2 outputs.

25ft HDMI cables at 4K could also be the problem. What brand are you using. HDMI cables are technically not supposed to be more than 15 feet so you might want to introduce an amp on the downstream side.