Can't get TV to recognise 7.1 sound

The output channels for optical will always be 2.0 as that is all it can do. You can only get multichannel from optical if it is encoded (like Dolby Digital), which is when you would need passthrough.

I would suggest connecting the Pi through the receiver via HDMI and then connect the TV to the receiver via HDMI. It's a better setup if you can run it that way.

It reads as if you are looping the audio through the TV then to the receiver. Especially with your HDMI arc comment. Connect it as such:

RPi 4 (Kodi) -> HDMI -> Receiver HDMI input -> HDMI -> TV

Use the ARC inputs if you have other things connected direct to the TV that you want audio pushed to the receiver.

Disconnect optical from TV as it won't be needed.

Make sure your receiver is on before you power up the RPi and start Kodi.

Set passthrough to HDMI on RPi Kodi. See the Kodi audio quickstart guide for how to configure the settings.

Don't loop it from the RPi to the Tv then to the receiver. The above will get it direct to the receiver and play it. The limitation is the TV, not the signal.

The model receiver you quoted does indicate it can handle 4k input/output through the HDMI repeater block, but at max 8bit. At least the manual I found and read. I couldn't find a manual or listing for your TV.

I would say try forcing everything down to 1080p just for testing through the receiver.

I have no experience with raspberry Pi, but I thought I read that audio output might be dependent on which HDMI port you are using (I understand there to be 2x on a Pi 4).

Someone else will have to tag in here. Good luck.

What 7.1 DD+ sources on your Pi are you playing?

You won't get PCM 7.1/FLAC 7.1/Dolby True HD 7.1/DTS HD MA 7.1 audio from the Pi to your AVR over an optical cable.

The only 7.1 codec that is just about potentially supported via that route is DD+ 7.1 (and possibly 6.1 DTS ES which is very rare) - which isn't that widespread. Kodi will transcode other codecs to 5.1 DD, but not 7.1 DD+.

Also - what other 7.1 HDMI sources have you successfully passed through via HDMI to your TV and then via optical to your AVR? Internal streaming apps on your TV don't count in the same way - as they aren't using the HDMI route.

Quote from caned_monkey

I am playing .mkv files. Some are DD+ others Dolby True Hd and as I've already said it doesn't work through HDMI either.

I have a gaming pc plugged directly into the TV which displays in 4k and gives 7.1 sound.

If i plug the pi into the receiver, I get 7.1 sound but no 4k. If I plug it into the TV, I get 4k but no 7.1.

Sorry - I thought in your earlier post you said you were using Optical from your TV to your AVR - not ARC?

However ARC can't carry True HD, PCM 7.1 etc. (it's limited to PCM 2.0, DD, DTS and in some implementations DD+) - only eARC (introduced much later than your AVR) can do HD Audio.

Can you clarify how you are getting True HD?