Meridian Unplugged - Powered by MrTechGuy.co.uk Forums Discussion Meridian - Systems HD621 HDMI problem (5.1 not working)

There's a lot about this on the AVforums Vertex2 official thread.

The Vertex2 has custom code for detecting the HD621 (which is apparently not very compliant with EDID standards) but it only works when the HD621 is plugged into TX1. As in the physical act of plugging it into TX1. You can simulate this with the "force replug" button in the UI.

This leaves you with the problem that if you leave the EDID as dynamically generated, the HD621 part may or may not be included in the blended EDID after a source change.

You need to plug your display into TX0, your HD621 into TX1, select automix EDID video part = TX0, audio part = TX1 and then save it to custom 10. After that you can plug the HD621 into TXAUDIO and you then set your just saved custom 10 as a fixed custom EDID for all the inputs you use.
That will make at least the EDID part of the equation completely stable. Mine's been running for years without a glitch.

The Vertex has no clue about MHR, nor is it an EDID capability to be flagged. The only audio you can get into an HD621 is PCM and the lossy mutichannel bitstreams. MHR applies between the HD621 and the surround controller if you use 96k output on the HD621.