I use an ASUS VivoMini (VM65N) as my Roon endpoint (running Roon Bridge, core is on a ROCK), connected to my AVR via HDMI. My AVR is connected to an Optoma projector. As I am playing multichannel music files, my only multichannel output option is HDMI (screen) in windows 10. It works flawless as long as my projector is on. Whenever the projector is turned off, Roon loses contact with the PC´s HDMI output. Returns fine when projector is turned on. Annoying to have to listen to the low whoosh of the projector. Any suggestions?
Sounds as though you have the CEC function turned on? I have it turned off with my Denon AVR > Samsung TV connection and there’s no need to have the TV on when using the Denon to playback multichannel or stereo recordings.
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Not familiar with your gear but you might check to see if the HDMI-CEC settings are affecting things?
CEC is the function where the devices automatically detect each other and either start or shut each other down. In other words, CEC allows for one device to control basic functions of another device, such as turning a device off or change the selected input of a TV.
LOL, @Geoff_Coupe ninjaed me… 
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Thanks. I also suspected CEC, turned off CEC on my AVR and my projector doesn’t have CEC setting, sorry. No CEC setting on PC, neither.
Two ideas. 1. See if there is a setting on your AVR to send only video on HDMI output to the projector. 2. If that doesn’t work, you could try use an HDMI Matrix switch in between the AVR and the ASUS. Set the Matrix switch to send audio only to the AVR and connect the projector to the video output of the matrix switch rather than your AVR. That should enable you to use audio with or without video on.
There may be a setting in the AVR to leave HDMI “on” or something called standby HDMI switching.
The clocks for HDMI follow the video. If there is no video output I assume the HDMI board in your AVR turns itself off or losses the video clock. However, some AVRs have a setting to leave these on in standby without an output in which case the AVR is able to maintain clocks on its own and still extract the audio. Otherwise, you’re going to need to “bury” the HDMI output somewhere which could be a cheap TV or capture adaptor or something that keeps the HDMI board alive.
Thanks. No 2 solved it. Put an Integra 4K on the HDMI between AVR and Projector.
Hi and thanks for advice. Found a simpler solution: A Roon HDMI device can be set in “exclusive” mode within Roon. This seems to keep the HDMI alive even if the display itself is turned off