Sonos Arc (Dolby Atmos) with 4K Projector vs. TV

I don’t know of any projectors that do any version of ARC, either HDMI-ARC, or eARC. 

So, I concur with your second thought, although I’m not familiar enough with that specific TV set to know its capabilities. 

That LG C9 supports Atmos via TrueHD via eArc.  

https://www.rtings.com/tv/reviews/lg/c9-oled

That’s all you would need for Atmos all the way up to Atmos via TrueHD sources.  Note: Atmos via TrueHD is is only available via 4k Blue-Ray.  Streaming sources, if they have Atmos, send it via DD+.

@jgatie Does that mean that streaming devices, like a newer Apple TV, connected to the LG C9 would not be Atmos compatible with the Sonos Arc?

Not sure.  The latest Apple TV 4K does have Atmos support.  The Arc will decode Atmos with TrueHD, DD+, and Dolby MAT.  There seems to be some confusion as to whether Apple TV 4K does these codecs or converts to multi-channel PCM.  I’d check with Apple.  

We've had some discussion in the Arc announcement thread about using it with a projector. The problem is connecting it.

As @Airgetlam pointed out, there are precious few projectors that do ARC. None that I know of do eARC.

There are a few main reasons.

1) people who own projectors tend to use AVRs with dedicated HDMI In/Out, since it is more a space for enthusiasts who would frown at an Atmos soundbar. 

2) In addition, projectors do not have internal tuners or apps that output audio, so another reason they don't need ARC.

3) More generally projectors were never designed to be HDMI hubs like TVs are. They were never intended to handle a lot of different inputs, or route or transcode audio.

4) projectors are currently limited to 4K. There is just no need for the bandwidth of HDMI 2.1 that also brings eARC.

So for now, your best bet is to go with a TV that neatly passes through everything on ARC or eARC.

I’m in same boat with a projector. Is there a AV receiver or HDMI splitter that would work to pass the ARC audio in a setup with ATV 4K?

I have projector, Sonos base and a HDMI spitter which split the video to projector and the audio to playbase via coaxial cable. Is there any such HDMI splitter available to pass through DD+ or Atmos via HDMI arc ?

In the end I’ve gambled somewhat on the HDfury Vertex2 which allows splitting from one source to two outputs, each with separate EDIDs negotiated on the link, and with explicit support for eARC and de-embedding Atmos.

I’ve got a JVC DLA-NX and the following pushed me over the edge into committing to the gamble:

used by JVC projector owners to apply their own gamma curves for HDR content and allow their projector's dynamic iris to work with HDR content

https://www.avforums.com/reviews/hdfury-vertex-review.15094

I’ll try and track down the conversation mentioned by @Sjoop1985 and see if this was a proposed approach.

Last night I contacted HDfury about the vertex2 to see if it would allow me to send an AppleTV signal to an Epson 5050ub projector while At the same time sending Dolby Atmos from the Apple TV to the Sonos arc…

their tech support said:  

No there is no solution in the world for Sonos currently, we will have one by the end of the year.

would an hdmi switcher like this not work?  Not sure of the price and may not be worth it as I know these can be pricey.  But it says it splits hdmi audio and video and “supports audio pass-through up to dolby ATMOS”.   

https://muxlab.com/pro-digital/6x1-2-0-multimedia-presentation-switch