Originally Posted by donkrx
What exactly makes you say LG is maintaining 4:4:4 and downgrading 10-bit to 8-bit? Are you sure it's not the source material to blame (some HDR games may have this problem)?
I also question the statement "the TV knows it can't transmit the signal". The TV isn't transmitting anything other than HDCP handshaking ... the signal is coming from the player. The player should be checking in with the display for capabilities, and enforcing any limitations. If the signal is greater than the HDMI 2.0 bandwidth, a good image simply isn't going to ... get there. Any "mode" the TV is placed into will make assumptions about the data being sent - for example if in PC, it assumes 4:4:4. It is possible Panasonic is dynamically determining chroma and bit depth, while LG is static, but we need a controlled test to say that definitively.
Also you mention PC mode. In HDR mode I think Picture mode = Game has to be selected, no? At least, I do notice differences. For example in HDR Picture = Game I am not able to use TruMotion, but I can with HDR Picture = Standard (both using Input = PC).
Just now I confirmed in SDR that Input = PC will replicate 10-bit depth when sent a 4K 60fps 4:2:0 signal, so the issue suggested by some others appears not true in my experience: PC mode does not downgrade a 10-bit signal to 8-bit.
My setup is GTX 1070 --> AVR --> TV in PC mode. I changed the Nvidia settings to output 4K 60fps 4:2:0, and confirmed I was getting 4K 60fps 4:2:0 with a chroma test pattern. I also checked that 4:2:2 and 4:4:4 work correctly in PC mode, and that 4:4:4 is not achievable in Game Console mode. I then used Rtings 16-bit gradient to compare performances in bit depth. When I swap my Nvidia settings between 8-bit and 12-bit, I see an obvious improvement in the color banding. The exact same thing happens when I do this in Game Console mode.
It would be really weird for PC mode to force 8-bit, because to do this, the 10-bit color data would have to be on-the-fly scaled down to 8-bit (converting a 1024 range to a 255 range). This would hurt the overall input lag, the very thing PC mode is designed for. Instead it seems like the other modes are doing the processing/limiting.
I will have to check HDR another day... it should be the same, but there's always a chance it's doing something odd (intentionally or unintentionally).