Would this connect a 4th Gen Apple TV HDMI out to USB-C in on an LG UltraFine 4K display?

USB-C Digital AV Multiport Adapter

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    No. This adapter does not convert HDMI to USB-C DisplayPort alt mode.

    This adapter takes an HDMI signal from a PC's USB-C port (probably using USB-C's HDMI Alt Mode or HDMI from DisplayPort dual mode from USB-C DisplayPort Alt Mode). HDMI alt mode is limited to 4K 30 Hz.

    Since HDMI Alt Mode requires three of the four USB-C Super Speed+ lanes, there are not enough lanes for USB 3.0 (two lanes: one for receive, and one for transmit), therefore this adapter only supports High Speed USB (USB 2.0). It's sad that Apple doesn't mention what speed of USB is supported by this adapter.

    There are adapters that can convert HDMI 1.4 to DisplayPort 1.2 but HDMI 1.4 only allows 4K at 30 Hz. I'm not sure if the LG UltraFine 4K supports 4K at 30 Hz. Those adapters require a USB connection for extra power, but the Apple TV doesn't have any USB ports, so you'll need another USB source.

    There might be new adapters coming later in 2018 that can convert HDMI 2.0 to DisplayPort 1.2.

    Then you can add a DisplayPort to USB-C cable to connect the adapter to the LG UltraFine 4K. However, the DisplayPort connection does not include USB, so any USB functions of the Display will not work (speakers, brightness control, USB ports). The display only supports USB 2.0, because all four USB-C Super Speed+ lanes are required for 4K at 60 Hz. Even if DisplayPort could pass USB (using the AUX lines), the HDMI 1.4 to DisplayPort 1.2 adapter would not include USB.

    The LG UltraFine 5K display can support 5K at 60 Hz and USB 3.0 because Thunderbolt uses the USB-C cable's lanes at twice the speed of USB 3.1 gen 2. However, the LK UltraFine 5K display requires a Thunderbolt 3 connection.

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