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Constellation

Does TV control Cable box channel through HDMI or IR signal?

I have a very simple setup. TV is brand new TU7000. Cable box is plugged into TV through HDMI 1. Sound bar is connected through HDMI 2 (ARC). 

 

THe cable box is in a cabinet, and I'd love to keep the cabinet closed. I thought that the TV would communicated with the TV through the HDMI cord, but when the cabinet is closed, the remote won't change the channel. Neither will Alexa or the smartThings app.  When the door is partially closed (no real line of sight between the TV and the cable receiver), I can change channel with the remote, Alexa, and smartthings. So I know there is some communication between the TV and the cable box. Is that communication through some super strength IR signal emitted by the TV, or throught HDMI. If it's IR, is there a way to get it through HDMI?  Plugging it into the HDMI/CEC is not optimal, since then I'll have to go optical cable on the sound bar. 

 

Thanks