Sorry for the late response. So you are saying even in full passthrough mode the LG C1 sends/shows only the formats it has supported? That doesn't sound correct.
Can you layout the full length of start to finish, I am guessing PC HDMI is the main output? What's next in-line?
I can understand the LG C1 having an EDID with only Dolby formats, its normal to list only the formats the unit can support directly, but not passthrough.
In passthrough mode it should be reading the speakers (soundbar, avr) EDID and either switching to that or some sort of merge.
If it fails to either swap or merge, then the formats from the speakers EDID will not show at the GPU end, meaning they cannot be sent.
The GPU (if fully working as intended) will need EDID data in order to show formats in HDMI, which is third party data.
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If you are trying to do HDMI (PC > TV), then optical out to, for example Z906, the optical portion does not have an EDID to return to the GPU.
However, a correctly programmed device can still inform the GPU via EDID, for example my HDMI to TOSLink converter.
The converter is a small shielded box with HDMI in and HDMI, TOSLink and analogue out (DAC).
The HDMI input is controlled by a virtual monitor inside the box, which comes with an EDID, informing the GPU what to do.
I produced by own EDID override which added HDMI supported formats and a change from 2 ch LPCM to 6 ch.
View attachment 290100
View attachment 290106View attachment 290107
Now the unit sends all that is listed down TOSlink.
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Windows can also be programmed to allow all HDMI formats on SPDIF, although you need Trusted Installer permissions to make the necessary changes.
With Windows 11, if you add the DTS-HD format to SPDIF, you will see DTS:X available in the default format drop down menu.
With Windows 10, you will only see the Dolby Atmos option, because EAC3 can be transmitted via optical.
Atmos has been available since Dolby Digital Plus (EAC3), HRA is I believe DTS's equivalent.
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Note, Sound Unbound will probably spit out any attempts to use DTS:X on SPDIF, as it see's SPDIF form factor, and is programmatically restricted.
View attachment 290111
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Unfortunately both HDMI and DisplayPort are more issue prone than SPDIF, and a GPU can add insult to injury by not allowing EDID override.
https://www.techpowerup.com/forums/...eractive-for-most-devices.283932/post-4978508
Technically speaking, there are actually no issues with SPDIF, other than maybe component quality.
SMI allows for two way 125mbps, it can be used as optical return, other.
View attachment 290101
Not all HDMI-DP devices behave in the same way, or may have non-standard settings.
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As a side note, if your going for a 50 inch+ TV, you want 8K to get more than or around 100 PPI (pixels per inch).
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Utilizing the full 125mbps of TOSLink @ 48k-24b:
View attachment 290113