Mono/1.0 PCM (FLAC/AAC) tracks playing back as stereo

  • Anytime I try to playback a video file with a mono PCM track (both FLAC and AAC), it outputs dual mono to my left and right speakers, completely bypassing my (very lovely) center channel. However DD/DTS/TrueHD/DTS-HD MA 1.0 tracks passthrough just fine, and playback on just my center channel like normal. Though if I disable passthrough, it will then output them as 2.0


    I'm not sure if this is an inherent Kodi/ffmpeg issue, or a LibreELEC/Linux/ALSA issue, but I'm using LibreELEC 10 on a Pi 4 connected to an Onkyo TX-NR656

  • The long-standing (since stereo was a new fancy thing) convention for mono output on a multi-speaker system is to send it to FL/FR so that's not unexpected. The HD formats are not mono, they are always multi-channel. So what you describe as "DTS 1.0" is more correctly described as 6-channel DTS where FL/FR/RL/RR/LFE channels have inaudible content and FC channel has audible content. If you disable pass-thru and have set Kodi to downmix to 2.0; that's what Kodi does, it downmixes multiple channels (including the one with audible content) to the default speaker map for 2.0 audio which is FL/FR. So everything working normally and not a bug.


    It's probably possible to use alsa confs to define a multi-channel arrangement that remaps true mono to a multi-channel FC-only arrangement but I don't have any links to share and it's a complete bitch to Google for anything alsa related - all you ever get is a bazillion "not quite what you're looking for" results returned. I'll duck the opportunity to think any further about that idea.


    My $0.02: Claim the sofa sweet-spot equidistant from FL and FR and then it all sounds like it's in the centre anyway ;)

  • It's probably possible to use alsa confs to define a multi-channel arrangement that remaps true mono to a multi-channel FC-only arrangement but I don't have any links to share and it's a complete bitch to Google for anything alsa related - all you ever get is a bazillion "not quite what you're looking for" results returned. I'll duck the opportunity to think any further about that idea.

    I'm not at all familiar with ALSA, outside of being a simple end user, but is that something that could eventually be auto-configed/make it's way into the GUI (assuming a proper arrangement can be figured out/found)?


    I will start poking my nose into more niche areas to see what I can learn about how its config works


    My $0.02: Claim the sofa sweet-spot equidistant from FL and FR and then it all sounds like it's in the centre anyway ;)

    Sadly I spent way too much on my center channel to not use it, lol. Plus it's got a slightly better/larger version of the ribbon tweeter used in my matching FL/FR. I'd much rather just re-encode to a 16-bit TrueHD track and remux, it gets pretty close in size to FLAC

  • I'm not at all familiar with ALSA, outside of being a simple end user, but is that something that could eventually be auto-configed/make it's way into the GUI (assuming a proper arrangement can be figured out/found)?

    Kodi has a GUI option to upmix 2.0 to the current speaker configuration, e.g. 5.1, so IMHO the best approach would be a similar Kodi feature that upmixes mono to either stereo (default) or a centre channel in a multi-channel configuration where FC exists. Fiddling with alsa confs is probably do-able but not something I'd ever want to adopt for distro features as it won't be replicable to all devices (alsa confs are device-specific).