Interesting reading those papers about eARC - the ITE paper states that TMDS signalling is limited to 4/8/16 channels. Perhaps that’s why everything was 7.1 (8 channels)?

if the new ATV4K has moved to FRL signalling then perhaps that’s why it now outputs 5.1. Possibly the PS5 has also switched to FRL signalling hence the fix to 5.1 that came a few firmwares ago.

is FRL the culprit somehow? 

No, my understanding is that FRL is not used on the eARC (previously HEC) pins - eARC will run its own differential encoding regardless of the whether the video pins are running FRL or TMDS. HEC was designed for 100Mb/s - 8 channels of 24-bit@192KHz is as much as they can cram into that bandwidth when including the overhead (which takes the 36.8Mb/s audio signal up to 98Mb/s). eARC is using differential signalling, I think it’s TMDS, and it seems the HDMI org didn’t want yet another cable type so worked with whatever the “with Ethernet” configurations would support. 

I guess the beam will just use TMDS video signal for its SONOS picture on the eARC port (the green buttons show my A1 is receiving video from the Beam using “RGB 8b TM”, but then it doesn’t have HDMI 2.1 so will never show FRL, I’ll check again when I move it back to the C1). It’ll be low frame rate regardless of resolution so won’t need 2.1 bandwidth.

I’ve so far failed to reproduce the dropouts with my ATV 4k as the source (gen 1 and gen 2), but I only use ATV+, Disney+, Youtube, Prime and Netflix apps there, nothing like Plex. I did also completely forget that ATV has eARC built-in, but only if you have one of the HomePod devices, apparently doesn’t work with bluetooth headphones. Could have been a cheaper way to test an alternative eARC sink. I don’t have a gen 3 ATV 4k to test with.

I’ve seen PS5 using FRL for video on the C1, but again this is independent of the differential encoding being used on the eARC/HEC pins. This is on the source HDMI cable, not the eARC cable too and I’ve seen the dropouts from a TMDS bluray so think FRL on the source cable can be discounted.

This independence between the eARC and video signals is interesting. I’m used to seeing screen blackouts when HDMI devices resync. Theoretically, it seems eARC could be re-handshaking without disrupting the rest of the HDMI chain signal path, hence the 1 second audio gap without disrupting the video. That’s assuming this is a handshaking issue and the LG isn’t simply setting the audio samples to zero for 1s every minute for whatever reason.

When I first set up our ATV4K gen 3, PCM 5.1 worked fine on it despite PCM 5.1 being bugged on my other devices (I only learned that ATV outputs DD/DD+ as PCM 5.1 when order was already on the way). Tested with Netflix and Disney+ and we were even able to watch a full movie uninterrupted. Couple of days later we played another non-Atmos movie and the bug was on and has been bugged since. I don’t think the gen matters. It’s probably more of the bug just hasn’t triggered for you yet. But inevitably will…

That’s interesting, as I thought ATV used PCM over Dolby MAT. Maybe that’s different with the 4k gen 3, or a change in TVOS 16.x. I’ll re-check mine later.

This is how ATV outputs stuff in my experience:

Stereo - PCM 2.0. No issue

DD - PCM 5.1. Bugged

DD+ - PCM 5.1. Bugged

DD+ Atmos - Dolby MAT Atmos. No issue

I determine if the source is DD/DD+ by comparing with what the native LG webOS app outputs. Would be nice if it can wrap everything in Dolby MAT though without upscaling.

In Windows, previously it’s wrapped in say Dolby MAT 7.1 or 5.1 if you turn on Atmos for Home Theatre and play non-Atmos multichannel PCM. Now, it’s wrapped in Dolby MAT Atmos even for multichannel PCM with a toggle for upscaling.

Edit: DD+ Atmos

Edit 2: Multichannel PCM

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

This bug is an issue with PSVR2 as well when using home theatre audio on my Beam 2 / LG CX setup. PSVR2 ignores the fact that you have Dolby Digital 5.1 selected and outputs in PCM 5.1 (presumably due to the headset audio). You then get the sound drops. Fortunately you can fix it by selecting 7.1 in the PS5 settings with the PSVR2 headset connected.

Remember that we are in this together and Sonos is the enemy lol. One of the most active threads with almost 10k views, almost 200 responses, and multiple tickets filed. They’ve got to address this at some point, right...? I hope the forum mods are merely community volunteers because that’s how it looks like.

@Eruvatar, who just got a refund, only bought their unit Jan 2023 so unlikely that there are already revisions that address the issue. “Repair” could potentially be better if they secretly know how to address the issue but are keeping mum due to costs involved.

Speaking of, if you can still get a refund or if you are a prospective buyer, just drop the Beam and give the Bose Soundbar 600 a look (this may be inappropriate, but I don’t care at this point). Stumbled upon the Bose store earlier and gave it a try. For the same price point, I think it blows the Beam out of the water. Only thing I don’t like is the the forced upmixing which can’t be disabled. But to be fair, without the bug the Beam would still be a fine consolation considering it came out a year earlier. If the bug still isn’t fixed by the next update, I’m getting the Bose on the next sale and sell the Beam.

The unit on display in the store had “2107” stamped on it, like mine. I agree, if there’s a faulty unit with a very recent manufacture date then there doesn’t seem much point exchanging the h/w.

I think the fact that the mods are not engaging with this thread anymore is telling. I think Sonos have reproduced the issue and so don’t need any more diagnostics or support calls coming in.

At this point they either don’t know how long a fix will take, or how it can be done and hence don’t want to set expectations, or they know it’s unfixable and are hoping this just dies away in time.

Beam 3?

Lack of communication and hoping the issue just “dies” on its own are the worst things you can do as a company in the long run, because this basically guarantees you’re not gonna get returning customers.

If you take a look at page 6, the mod’s final interactions with this thread are:

1. Liking a user’s “fix” (user has since confirmed that the issue has returned)

2. Liking a post defending the mods

Pretty telling as you can see. If these guys are actually on the payroll, damn Sonos is getting fleeced.

I've tried a few video modes now but can't get ATV4K Gen 3 to use FRL, it's always TMDS. I doubt FRL is in there, the bandwidth isn't needed for the video modes Gen 3 supports. Apple must have changed something else more fundamental between Gens 2 and 3.

Not sure if the ATV is aware of the Beam 2 during the handshakes, or if the LG just reports it'll accept audio formats that it knows it can "pass through” based on the eARC capabilities received from the Beam2. I did also test that Gen 1 incorrectly negotiates 7.1 with my eARC AVR in place of the Beam2 on the C1, but does get 5.1 correct on the A1

Yeah that’s what really pisses me off. What sets Sonos apart is suppose to be their customer service but they’ve turned a blind eye to this issue. I’m done with this product. I’m selling it and moving on to something that lets me play switch games in surround sound without dropouts every minute. Thanks Sonos.

Btw I’ve noticed the issue occurs whenever a new update either to the app or system becomes available. You don’t even have to download it. I’ll mess around for an hour, unplug various things, somehow it ends up working for a week, then I get a notification an update is available (without downloading it) and like magic it starts dropping out again.