sameness

I believe that the Decimator MD-HX "Fixes" the HDCP issue on it's SDI outputs, by being HDCP compliant..

 

SDI is not HDCP compliant as it does not and can not carry the encrypted signal. The Decimator may well allow the Mac to output non-protected content, but if you try and play HDCP protected content it will not pass.

mostly via small dedicated mixers, and two input powered speakers

Standard for FOH and monitors to communicate quickly.

As for the rest...why not utilise a standard coms system? Clearcom, Greengo, Riedel etc?