Multichannel PCM 5.1 dropping out since 14.10 update

Oh. Too bad then. Our older Sony soundbar has since been paired with our older Sony TV and now enjoys simpler use cases. But when we used it with our LG for a number of months, had no issue with Stereo, PCM 5.1, PCM 7.1, DD, DD+, TrueHD, Atmos, Dolby MAT, volume control, and power sync.

For more general eARC issues, I would be willing to do a coinflip on who to blame. But for this very very specific situation where only PCM 5.1 is affected and the Sonos Arc works fine, I am still inclined to believe it’s a Beam issue.

I’m done researching and testing for this issue myself though. Already did everything everyone posted awhile back to no avail. Figured why waste more of my time and provide free labor when Sonos has done jack squat. Still would be nice if it get fixed though as I’d be doing more of a sidegrade than an upgrade at this point if I’m forced to buy a new soundbar.

Does feel that way, although I’ve failed to reproduce the behaviour on my A1 so far (I guess that’s a different HDMI chipset to the C1 given the lack of 2.1 features). Could be one of those cases where there’s a bug on both sides.

It is possible that LG TVs are quirky, considering that the Bose 600 only had the no sound issue with them. However, we’ve also seen that these issues can be addressed from the soundbar end as Bose did. Heck, Sonos Arc already works so why can’t they check and do the same with the Beam? This apparent laziness 8 months in is my main problem with Sonos.

I should add that this bug completely nullifies the possibility of playing Nintendo switch with surround sound sound together with the beam 2 and an LG tv because unlike the PS5 the switch has no option to change the surround sound output to anything that’s not 5.1 Multichannel PCM.

The fact that such a long time has passed without any update from Sonos with regards to whether they’re even looking into it is infuriating.

LG is one of the most popular TVs out there, and the first choice for many people who want OLED TV. Odds are high that people who use Sonos also use LG OLED since they’re both premium products.

How come the priority of this isn’t high?

Sharing borked scenarios that I am aware of as well as possible workarounds if any. If any prospective buyer comes across this and has a similar use case, just turn around and run:

Switch - You’re stuck with Stereo as PCM 5.1 surround is borked

PS5 - You are forced to set it to PCM 7.1

Bluray - For DTS, you’re stuck with Stereo if your player only converts DTS to PCM 5.1 (are there even bluray players who can convert DTS to Dolby?)

Apple TV - The ATV doesn’t do DD/DD+ passthrough and outputs them as PCM 5.1. So non-atmos and non-stereo contents are borked. Workaround is either force the ATV to output as DD (DD only, not DD+) which is a downgrade or use the LG webOS app

PC - Using Dolby Atmos for Home Theatre is sometimes bugged esp for non-Atmos content (I think it’s an nVidia/Microsoft issue) so in those cases you’re forced to set it to PCM 7.1

With the popularity of LG OLEDs, there is something for everyone lol.

What’s funny about this issue is Sonos completely screws over customers who actually spent on building a 5.0/5.1 system. If I had used all this money for an actual surround system, I sure as hell would want to set my devices exactly to PCM 5.1 and avoid downmixing from 7.1.

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