Multichannel PCM 5.1 dropping out since 14.10 update

There’s definitely a difference with the Gen 3, I had no issue with the Gen 2 but that always showed 7.1 in the Sonos app. It’s also intermittent for me on the Gen 3, I can’t pin it down but a restart of the beam or software update always seems to trigger it

I’ve remembered that incorrectly. I originally discounted the ATV as a buggy source as I thought it offered PCM 2.0, PCM 7.1 or MAT. I must have got a bit confused today about PCM over MAT as that doesn’t seem to be a thing, rather MAT is PCM over the wire but Dolby something at each end. I’m still unsure which device is unpacking the MAT frames, the TV or soundbar (in which case passthrough isn’t passthrough?). 

You should be able to view exactly what the HDMI source audio format is by pressing the 1 key five times whilst “General->Programmes->Programme Tuning & Settings” is highlighted (wish there was a shortcut for this). I’ve managed to see the same YouTube content transmitted as “PCM 6Ch 48000Hz” on ATV4K.2 with eARC Beam2 and “AC3 9Ch 48000Hz” on ATV4K.1 with TV speakers only, but definitely confirms your point that I should be able to find a buggy combination on the ATV4K (actually, seems well reported anyway).

There must be a MAT decoder in the LG TV and another in the Beam2. The TV can decode “MAT 9Ch 192000Hz” from ATV4K to its own speakers. It identifies “MAT_ATMOS 1Ch 192000 Hz” as Atmos, but I think passes through for the Beam2 to decode which displays “Dolby Atmos”. The Beam2 displays “Dolby Atmos (DD+)” in “about my system”  for the same content playing from the LG App.

I think this means the ATV4K is unpacking the source internally, mixing UI sounds and then outputting as PCM over MAT (does exist after all), otherwise the Sonos would display “Dolby Atmos (DD+)” or “Dolby Atmos (True HD)” ? When there’s not Atmos metadata available, ATV4K just pushes out plain old PCM (decoded from whatever source)?

I think a recent LG firmware update may have changed this behaviour, hopefully I’m slightly less confused than I was earlier today as I’m sure I remember seeing 7.1 from ATV4K when this all started:

https://www.avsforum.com/threads/lpcm-passthrough-issue-on-lg-oleds.3202935/#post-61892460

Never had any issue with Atmos content and things always looked correct whenever I checked in the Sonos app:

LG webOS apps - DD+ Atmos

Apple TV 4k - outputted as Dolby MAT Atmos (from DD+ Atmos source)

PC (videos) - DD+ Atmos or TrueHD Atmos via passthrough (depending on source)

PC (games) - Dolby MAT Atmos

I think Apple just disables passthrough and unpacks the DD/DD+ & DD+ Atmos source and outputs them as PCM 5.1 & Dolby MAT Atmos. Common theory I saw back when I researched was Apple probably didn’t want their device to be used as a piracy machine for Plex, Infuse, etc.

Spoke too soon. I put the Beam2 back on my C1 and both ATV4k.1 and ATV4k.2 output LPCM 7.1 for 5.1 source (and therefore no dropouts). The TV displays “PCM 8Ch 48000Hz” on the HDMI input, and this is the same when an eARC AVR is connected instead of the Beam2.

The A1 is showing PCM 6Ch 48000Hz for the same source.

A1 and C1 are set to passthrough. Beam 2 will accept “Dolby Multichannel LPCM 5.1” if set to auto, but I think this is the PCM in MAT (bridged by the TV) so not triggering the dropouts either.

Why the hell are we trying to do Sonos’ work for them here?

This product is bugged. They should be the ones breaking their heads over it, not us. It’s obvious there’s nothing we can do to fix it, we’ve already done more than enough to identify and pinpoint the exact problem for them.

This is because setting it to “Auto” in effect limits the TV. It becomes like the Optical/ARC TVs of old where it can only receive Stereo PCM and uses Dolby for multichannel. Stereo PCM is outputted as such however the TV unpacks AC3 and repacks it into Dolby PCM 5.1 before sending to the soundbar.

With “Auto”, I’m guessing the Apple recognizes that the TV cannot do multichannel PCM so the Apple already sends audio as DD/AC3 even if you had set “change audio format” to off. Tested this before with a PS5 where using DD output results to Dolby PCM 5.1 while all PCM formats (2.0, 5.1, 7.1) only result to PCM 2.0. So sadly this can’t be used as a workaround. Just set to / test for “Passthrough”.

Totally agree. Haven’t troubleshot this issue for a long time just sharing my previous findings. But I’m not exactly being altruistic. If an industrious user wants to find a fix / workaround, I’ll benefit as well. Or if this thread can increase awareness and complaints, maybe Sonos will finally get off their lazy bum.

Edit: That said, I’ve said all I wanted to say and shared everything I could. So I’ll probably go back to lurking while waiting for a good deal on a Beam replacement lol.

I’m not sure my recent ramblings on AppleTV HDMI signalling will help Sonos out at all, I just noted that both @yofalat611 and @ste_ms mention ATV4K Gen 3 can trigger the issue, whereas I’ve never seen it on my ATV4K Gen 1 because of a different bug. What they have done is re-assured me that behaviour that looks so obviously like a bug in the TV can be changed by the upstream/downstream vendor.

I just acquired an ATV4K gen 3 and guess what, it does negotiate LPCM 5.1 correctly when Gen 1 & 2 could only manage LPCM7.1. So if Apple can “fix” an “LG issue” between gen 2 and gen 3, then it’s quite plausible that Sonos are in a position to do the same for the dropouts (but most of us already knew that, right...). What did Apple change? No idea. All my ATVs are on 16.3.2 which leans towards a h/w difference, which goes back to @yofalat611’s conspiracy theory.

Also, I’ve observed that the A1 and C1 behave very differently at the HDMI eARC level (even for non-2.1 features), despite what appears to be otherwise identical system software on both. This may help other users diagnose similarly symptomatic issues but which have a different root cause.

Turns out I am still in warranty after all, but sounds like the retailer would send the Beam 2 for “repair” rather than swap out for a newer unit (the build date is stamped on the exterior of the packaging though).

Sure - I see the same on Auto, wasn’t suggesting it as a workaround (unadjusted audio sync is atrocious too) just wondering if MAT is being generated by the TV in this flow compared to the ATV in Atmos passthrough flow (I don’t get why it would make the effort creating a MAT stream instead of outputting the extracted plain LPCM on eARC), and whether I am interpreting the Sonos audio format labels correctly:

Auto:

5.1 source → ATV4k.x→ “AC3 1Ch 48000Hz” → LG A1 → “Dolby Multichannel PCM 5.1” (PCM MAT?) → Beam2

5.1 source → ATV4k.x→ “AC3 9Ch 48000Hz” → LG C1 → “Dolby Multichannel PCM 5.1”→ Beam2

Passthrough:

5.1 source → ATV4k.1 → “PCM 6Ch 48000Hz” → LG A1 → “Multichannel PCM 5.1” → Beam 2

5.1 source → ATV4k.1 → “PCM 8Ch 48000Hz” → LG C1 → “Multichannel PCM 7.1” → Beam 2

5.1 source → ATV4k.3 → “PCM 6Ch 48000Hz” → LG C1 → “Multichannel PCM 5.1” → Beam 2

Atmos source → ATV4k.x → “MAT 9Ch 192000Hz” → LG C1 → “Dolby Atmos” (PCM with Atmos MAT?) → Beam 2

Atmos source → ATV4k.x → “MAT_ATMOS 1ch 192000Hz” → LG A1 → “Dolby Atmos” → Beam2