Yet another audio passthrough question

Firstly, thanks so much for all your hard work in creating such an incredible project.

I have a few quick questions about NON-passthrough audio mode in CoreELEC/Kodi.

a) I assume that surround audio signals are decoded/decompressed and then passed over HDMI as some kind of portable format like PCM?

b) And that Kodi itself performs a downmix to whatever channel layout you specify in the settings?

c) Is it true that DTS-X and Atmos will have their object metadata discarded entirely (leaving vanilla 5.1 or 7.1 surround)?

d) And are DTS-HD and Dolby TrueHD preserved at their original full lossless bitrate? Or do we just end up with the core/lossy/degraded signals (e.g. TrueHD becomes DD+/E-AC3)?

e) Or does that depend on the specific sampling/bitrate of the HD track?

And just curiously, does anybody chose to feed audio output directly into an amplifier? Or do you always go through a receiver or TV? I’m wondering if something like a MiniDSP product could technically be used instead of an A/V Receiver (for 2.1 or 3.1 etc.).

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Correct, LPCM to be exact.

Yes, the LPCM format does not have support for that metadata, and more than that, Atmos and DTS-X require knowledge about your exact speaker placement, which Kodi does not normally have. LPCM are discrete channels, meaning there’s a 1:1 mapping between an audio stream and a speaker. With object-based surround, the receiver can decide where the sound for a given “object” exists in the space around you, and mix it into the respective speaker channels on the fly.

That depends on the version of HDMI you use (and thus, the maximum bandwidth available for audio). Because LPCM is uncompressed, you will always get less capabilities than with any of the compressed (lossless or not) formats. Best scenario, you will get the same audio as original (which also greatly differs in channel count and bit- and sampling rates, even within the same “format”). But with the current HDMI version, you can’t go beyond 7.1 at 192kHz.

You will generally get the best results (especially with object-based audio) with passing the original stream through to a receiver / amplifier capable of decoding that stream. To replicate that within the PC/STB, you’d need a multi-channel (however many you use) amplifier and somehow let the software DTS / DD decoder the exact placement and properties of your speakers. That’s what the microphone calibration does in modern AVRs.

Hope this clarifies some things and I didn’t mislead you anywhere.

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Thanks so much for your clear response. I couldn’t find that info anywhere on the Kodi forums. I had read that HDMI Wiki but must have missed the audio spec.

Out of curiosity, is there a way, how to find out which HDMI protocol is used through the command line for example?

I know cable could limit this, but how we can check, that cable is ok and what version is used?

Btw., for OP, I am using passthrough to the AVR and also think its best option (although older AVR limits new 4k/HDR capabilities, and therefore my first part of this post)

There might be some Kodi debugging or verbose logging option that would tell you this.

But generally it’s going to negotiate the fastest option available to both sides of the connection (e.g. if one side is 2.0b and the other 1.4 - you’re always going to get 1.4). Sometimes the HDMI cable itself can limit your speed also (especially with HDMI 2.1).

FYI if anybody is looking for a way to split/passthrough the HDMI audio from an A/V signal (so you can pass 4K video to the display and encoded audio to an old receiver) then this is the cheapest option I have found. You can get them for $70 on Newegg/Amazon - so they are a lot cheaper than the HDFury etc. A normal HDMI splitter/duplicator won’t work for this.

XOLORspace 23421 HDMI 2.0 4x2 4K 60HZ HDR HDMI Matrix Switch Splitter w/ HDMI audio output