HDMI audio passthrough support

[Zep]

The Steam Deck is a pretty good media player with software such as Kodi, especially in Desktop Mode (where Kodi has free rein over the HDMI video output). However, it seems that 'raw' output of compressed audio streams (Dolby Digital (AC3)/Digital Plus (E-AC3)/TrueHD, DTS (HD), Dolby Atmos, etc.) is not supported over HDMI. At least, Kodi does not show this as being supported.

Supporting HDMI audio passthrough is desirable in situations where the Steam Deck is connected to an HDMI receiver which is able to process compressed audio streams.

Of course, 'HDMI' can be replaced with DisplayPort/DP for those situations in which it is appropriate. I simply used the term HDMI since it is the most common connection/protocol used when compressed audio streams are involved.

[Zep]

HDMI audio passthrough seems to be supported for AC3 and DTS, but not E-AC3 ('Dolby Digital Plus'), nor HD audio formats. E-AC3 support seems to be lacking due to the Deck not supporting it. When the relevant options are enabled in both PulseAudio volume control and Kodi, a DD+ stream is not passed to the HDMI port. HD audio format support is missing due to PulseAudio limitations.

In Kodi I tested both Passthrough Mode (AC3 only, assuming DTS works as well) and Multichannel Mode. Note the specific HDMI setting that is required in PulseAudio volume control for Passthrough Mode (Stereo) and Multichannel Mode (5.1/7.1, depending on the number of speakers you have), and that Kodi needs to be properly configured. More information:

https://kodi.wiki/view/PulseAudio

Passthrough Mode should work with 5.1 audio from any source to any digital audio destination if AC3 re-encoding is enabled in Kodi. Multi-channel audio that is not AC3 or DTS is decoded, and re-encoded to 5.1 AC3 (Dolby Digital). AC3 and DTS streams should be directly passed through without any decoding by Kodi. In particular, AC3 should be supported by all 5.1 digital audio receivers, including those connected to an S/PDIF output (e.g. a TOSLINK output on a TV or an HDMI audio extractor). In other words, a +20 year old digital receiver should work perfectly, assuming that the used method to extract the audio from the HDMI signal supports 5.1 AC3 audio.

Multichannel Mode lets Kodi decode all audio in software and output it as LPCM, which should be supported by any decent HDMI receiver.

[Zep]

Other formats (E-AC3, HD audio formats, even Atmos) work in Passthrough Mode when these instructions are followed to bypass Pulseaudio:
https://forum.kodi.tv/showthread.php?tid=356360

Of course, replace the 'kodi' command with the full command to run Kodi in its flatpak. Use something like:

systemctl --user stop pipewire{.socket,.service}
KODI_AE_SINK=ALSA /usr/bin/flatpak run --branch=stable --arch=x86_64 --command=kodi tv.kodi.Kodi
systemctl --user start pipewire{.socket,.service}

This can be tested in Konsole, or put in a shell script.