I have been trying --without success-- to get my HP Spectre x360 15t notebook (with Intel Isis 540 graphics and the I7 6560 processor) to work reliably in the UHD Color mode (4:4:4 chroma), at 60 Hz (60p) with a Samsung UN40KU6300 TV. This is a UHD (3840x2160) TV, with the UHD Color capability on HDMI port 1.
I have, in fact, been able to get the TV to recognize the laptop output in the correct mode (3840x2160 60p UHD Color) several times, which required the workaround whereby the HDMI cable had to be unplugged while the computer was booting, and plugged in only after the boot was finished. The TV has the UHD Color mode enabled on port 1. When the cable is finally plugged in, the display SOMETIMES recognizes the connection and clicks into the right mode. Other times, it'll just flash the computer desktop image briefly (for half a second), and then the signal is lost (and the message appears on the TV to that effect). Even when the correct connection is established, it is easily lost if I replug the cable, or reboot the computer WITHOUT unplugging the cable. When replugging the cable live, sometimes the TV displays the message that the resolution is incorrect on source. I have verified that the display configuration is correct in the Intel Graphics control panel, where the external display does, in fact, appear as "Digital Television Samsung", at the 3840x2160 resolution and at 60Hz.
The TV has a lot LESS trouble recognizing a 3840x2160 60p connection WITHOUT UHD Color. So, when I connect the laptop to the TV on the HDMI 2 port (which does not have the UHD Color capability) or even the same HDMI 1 port, but with UHD Color mode disabled, the TV recognizes the signal EVERY TIME, although it still requires that the cable be unplugged while the computer is booting and plugged in only when the boot is complete. Replugging the cable live in this mode also still causes the loss of signal and/or the "incorrect resolution" message.
One other piece of information is that on the few occasions that I WAS able to establish the correct 3840x2160 60p UHD Color connection with the TV, HALF of the time the HDMI Audio driver disappeared from the computer (or at least the "Samsung Digital TV" option had vanished from the Playback Devices window), and the only way to get it back was to reinstall the Intel Isis 540 graphics driver, after which the Samsung audio device would reappear, but the TV would again be unable to connect in the right mode.
I have completely ruled out the possibility that the problem is caused by an incorrect or faulty HDMI cable (by trying it with THREE different cables, all rated specifically for High Speed, HDMI 2.0 and UHD Deep Color). Also, to rule out a faulty TV, I have tried this with another Samsung TV of the same model, as well as an LG 43UH6100 TV (which is also rated for UHD Deep Color connections), and exactly the same problems occurred.
I have explored just about every option and workaround I could think of, and there is just no reliable way to get this connection to work. I need to figure out where the problem lies -- is it in the graphics chip, the motherboard, the Intel Isis 540 graphics driver? The laptop is still under warranty, so I should be able to get it replaced if this is a hardware issue. But, I am still hoping that this may be a driver problem. The driver version I have installed is 126.96.36.199.4377.