No HDMI audio when using Displayport MST/4k on SLI GTX 780s
I'm having quite a specific problem that might need a bit of explanation of the configuration first.
I have a 4k display (Panasonic wt600) that I can drive at 60hz through displayport with full 4:4:4 colour from my GTX 780s. I recently upgraded from my logitech z5500s which were driven by multichannel analogue to a proper AV-receiver (Yamaha RXV-677). I can use HDMI to get 4k60 video but only at 4:2:0 colour which looks pretty terrible for text. Using just the HDMI output like this I can get audio to work fine with the receiver.
However, what I want to do is use displayport to get the video signal with 4k 60hz 4:4:4 goodness and then use HDMI to send audio to the receiver
. The problem is that when the displayport cable is connected to the TV and using MST, the HDMI audio output to the receiver just won't work. Windows and the Nvidia control panel detect that the receiver is connected via HDMI and the receiver detects an HDMI signal. The receiver is set as the default audio device and using the test tone from Windows just gets no audio even though the bar moves for that device showing audio should be playing. If I switch the displayport to use SST (limited to 1080p or 4k30) then the audio device starts actually working.
I'm using Nvidia 340.43 drivers at the moment and have Geforce experience installed. I have a few more ideas to test but I'm at work at the moment. I want to connect a second TV instead of the receiver to see if HDMI audio will work in that case. As far as I know the HDMI outputs on the second GPU are disabled during SLI so I want to try disabling SLI and using the second card to output audio. This isn't the solution I want for gaming though. I'm considering trying different driver versions. I'm not sure if this is a hardware limitation or a bug.
In summary, HDMI audio won't work when using MST mode from displayport. How can I fix it?