My understanding is that HDMI is purely for audio visual signal, whereas the USB, if that is what you are referring to, is transferring data to the pc where it is turned in to the image you see in software. I have no idea if Lightroom is able to translate the signal from HDMI in to a format that it is able to use. If you were using the AV cord with phono plugs on the end then I see no reason Lightroom wouldn't work in the same manner as the phono leads also provide an audio visual signal, the only difference being the AV cable is analogue the HDMI is digital.
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