Multi channel PCM issues

    I would like to start off by saying that I have been using Libreelec for many years and very rarely have any issues whatsoever. It's a fantastic piece of software and I would like to thank everyone who contributes to the development.

    My setup for the last few years was only capable of playing DD 5.1 so I was setting Kodi to output 2.0 and then selecting to re-encode everything to DD 5.1. This was fantastic and meant that I could play all of my media regardless of the sound format.

    Recently, I have upgraded my soundbar to one that supports Dolby Atmos and Multi Channel PCM via eARC. It sounds SO much better and I'm really pleased with it. I changed the audio settings within Kodi to output 5.1 and selected TrueHD capable receiver but left all the DTS options disabled. This way I can get TrueHD or Dolby Atmos when available and if the source is DTS-HDMA or AAC then it outputs Multi Channel PCM. Perfect! That's exactly what I was hoping it would do.

    This was working when I first set it up and tested a few different files to ensure everything played OK. Since then I have noticed a random problem. Sometimes when I play a file with DTS-HDMA or AAC 6 channel audio, Kodi will only output the FR and FL channels. There is no surround sound and crucially there is no centre channel so there is no dialogue! This doesn't happen every time - it seems to be random. I have to reboot my Intel NUC once, twice or sometimes three times before it will output the full 6 channels. When it starts working, everything is fine for the duration of that particular file. Dolby TrueHD, EAC3 and Dolby Atmos always play correctly.

    I assume it's not a common problem so it may be a problem with my hardware or my particular installation of Libreelec. I don't have any additional add-ons installed, I like to keep things as simple as possible.

    When I first upgraded the soundbar I was running version 12.0.2 but I have since updated to 12.2.1 but my particular issue remains.

    I don't think any of my settings are wrong because when it eventually decides to work, it's perfect.

    Obviously this is a little frustrating so I would be very grateful for any suggestions of what may be going wrong here.

    I have a hunch the random problem is related to bandwidth on the HDMI connection. HD audio formats require more than non-HD media and can cause problems. This can be a problem with cables (not using HDMI certified cables). It can also be a problem with the Intel GPU drivers prioritising video bit-depth (they don't account for audio needs). It can also be an issue with firmware if the system outputs HDMI through an LSPCON chip and some users end up using an external DP-to-HDMI dongle (which also has firmware, but different to the onboard one) to mitigate issues. It can also be a combination of "all of the above" resulting in no clear obvious solution.

    The main suggestion and starting point would be to bump to a current LE13 nightly so that newer drivers and a patch that clamps video bit-depth to 10-bit output (the max used with modern media) are used.

    Thanks very much for your reply - that makes a lot of sense. I just checked and my NUC is a NUC8ieBEK that does indeed use an LSPCON chip to achieve HDMI2.0b.

    I think it's probably time for an upgrade so I was looking at the Asus NUC14 Essentials as that has HDMI 2.1.

    I've checked a few threads here to see if if will be compatible with Libreelec but I'm not 100% clear on the current status. It sounds like there may be issues with wired network and possibly HDR. I believe the chipset requires Linux kernel 6.14?

    Can anyone confirm if this model is supported with the latest nightly builds?

    I've been running LE13 nightlies on an assortment of hardware for the last ~2 years and aside from self-inflicted issues, there have been few to none, so go experiment as it might resolve the issue and avoid the need for new hardware. I've no specific knowledge on NUC14 support but LE13 is on Linux 7.1 so it probably just works. The N150 box I have here for tests works without problems.

    Thanks again for you suggestions.

    I have installed the latest nightly build but it made no difference to my particular issue.

    Is there any way to confirm if it is an HDMI bandwidth limitation? I would rather be sure before I'm forced to upgrade.

    Any logs or debug mode or similar?

    Please share a couple of debug logs using the LE13 image and the pastekodi command so we see kernel and other infos alongside the Kodi log; one with successful playback as desired, the other where things are not working. Share the URLs generated.

    Quote from duffbeer

    Never done this before so please bare with me if I make a mistake.

    Enable 'debug logging' at Settings → System → Logging. Neither of the logs are Kodi debug logs, so lots of info missing.

    Also leave media to play for a full minute before running the command so the playing configuration has settled.

    I have a better example of the issue that may be a little easier to analyse.

    The following logs are from the same file on the same hardware. The source is 2160p with HDR and DTS-HDMA.

    The first log is with everything working correctly - the audio is output as Multi Channel PCM.

    https://paste.libreelec.tv/versicolor-addison.log

    The second log is where I only get FL and FR PCM channels.

    https://paste.libreelec.tv/unmonopolized-lilyanna.log


    The thing that concerns me is the inconsistency. No settings were changed but I can get different results each time I watch something.


    Thanks for your help and advice so far. I hope these logs are more useful.