HDMI 2.0 (4k) has been with us for less than five years before it was updated with with HDMI 2.0a (HDR)and now HDMI 2.1 (8k).
I have an old Onkyo 875 which has HDMI 1.3a (not 3D compliant) which I" cheated" 3D by using a blu-ray play that had two HDMI outs - one directly to the TV and one to audio in the amp. I think you can do this to get 4K working with my HDMI 1.3a Onkyo Amp - plug the blu-ray player via HDMI directly to the TV and another HDMI to the receiver. What are the detriments to this?
Now, I am thinking that if I upgrade soon to a HDMI 2.0a or 2.1 amp, I sho