Multichannel PCM 5.1 dropping out since 14.10 update

Same issue here. I have the latest Apple TV 4K (3rd Gen), LG C2 and Sonos Beam Gen 2 (all on the latest software). I get a momentary audio drop every minute or so with ‘Multistream PCM 5.1’ audio only. Every other audio type is fine. I was using my Apple TV on HDMI 4 but changed to HDMI 3 with no improvement. I have good HDMI cables and have swapped them around with no improvement. As suggest above, I’m trying HDMI 1 which seems good (currently testing).

However, as per the linked thread above, it is not the Beam causing this. If you go into one of the hidden menus on the LG by going to Settings > General > Programmes. Then highlight ‘Programme Tuning and Settings’. Then press 11111 (1 five times). Info will be displayed. You will see to 2 ‘more’ links with the second under ‘DVI/HDMI Status’. Click this. In the new info page in the top left you will see it displays the audio format (5.1 is shown as 6CH). Keep this menu displayed when playing 5.1 audio and when an audio drop comes, ‘6CH’ drops to ‘2CH’ momentarily before coming back (I managed to get this on video). Hit the up button on the remote every minute to stop the menu timing out while you do this.

So it is not the sound bar. It is either the source (although no issue on my other TVs) or the TV. I’m guessing TV as why would it work on HDMI 1 if it was the source? I also tried an earlier Apple TV 4K box (2nd Gen) and this bizarrely sends 5.1 audio as 7.1 - so I couldn’t use it to test with.

At this point I’m fairly certain it is an issue with the TV. My LG C2 is only 5 days old so I’m considering exchanging it just in case it is a hardware issue, not a software issue. If this was a software issue I’m sure there will be many many posts about this online.

Good spot, this is new behaviour.  I’ve recently seen the TV flick between 6ch and 2ch on this menu when I’ve confused it by making too many changes, but it’s the first time I’ve noticed the TV show any reaction at the time of a drop out. In my case it has just flicked between 6ch → 4ch → 6ch during the drop. It didn’t do this when I started looking at this months ago (when I’m certain HDMI 1 was also buggy). Perhaps this indicates LG is looking into this bug after all (or that I just have a bad memory...)

There’s no issue on my alternative eARC devices either (Marantz in my case or Sonos ARC according to this thread), so that logic doesn’t quite stack up. Whilst the Beam 2 may not turn out to be buggy it isn’t blameless in this issue.

This is confirmed by a few posts on this thread - could be a h/w revision change between gen 1&2 and 3, or just that some manufacturers have no interest I back porting s/w fixes. I’m guessing Beam 3 won’t trigger this issue, will be interesting to see how C3 behaves (if not solved one way or the other by then).

Contributing factors to the bug seem to be:

• Beam 2 reboots
• CX-C2 (and G family) TVs
• HDMI 3 (and 4?)
• TV cache of HDMI devices (disconnecting the Beam 2 and reconnecting doesn’t clear the bug but fiddling with some of the TV audio/CEC settings can do)

Can confirm both HDMI 3 and 4 are affected.

And I don’t think it’s up to a specific faulty TV, it’s affecting all of them with this specific setup.

Sure it might be caused by the TV, but since it only affects the beam 2 then Sonos definitely has a hand in it too.

Fair enough. My thinking was that the hidden details state HDMI 3 (or 4) drops to 4CH (correction from above, it drops to 4CH not 2CH, but not every time). The TV is set to pass through audio, so it shouldn’t change the number of channels, but I also guess there is some kind of handshake going on with the Beam that could feed back different source requirements, causing the TV to stumble……? Either way, it sounds like exchanging the TV won’t help and everything now seem to work with Apple TV on HDMI 1. The devices on HDMI 3 and 4 don’t seem to output ‘Multistream PCM 5.1’, so that isn’t an issue.

See video below of the audio drop out and the channel number dropping to 4.

That’s my guess too. eARC has it’s own handshake: the Beam presents the TV with a Capabilities Data Structure (CDS), independently but similar to how the TV presents the AppleTV (and Beam) with an EDID. The CDS received by the TV on HDMI2 would drive the logic in the TV on what audio sink to present to the Apple TV in the EDID on HDMI3.

Without knowing what the audio stream on HDMI3 looks like during the dropout it’s difficult to know if the AppleTV is involved (maybe an HDFury device can “sniff” this) in dropping the audio. Having read a little about the HDMI port implementations it’s also difficult to know which point the 11111 menu is picking to determine the audio format. Is it really what’s in the TMDS stream, or is it examining a transposed stream on an internal I2S link or ARM bus - in which case the Beam 2 may already have had chance to interfere indirectly via the HDMI3 control registers (via a CDS handshake every 59s) whilst the AppleTV keeps streaming Audio/Video happily to HDMI3 as if everything is stable. We do know that the video stream is not interrupted.

I haven’t yet thought of a way to prove/dismiss the above with any of the equipment I have to hand though. Introducing any kind of passthrough sniffer device may even alter the signalling enough to fix the problem enough so that it can’t be observed :(

I’m sure I checked the HDMI status to see if it changed during the dropouts but didn’t see it previously - possibly a tweak in the firmware since then? The app never shows a change from PCM 5.1 when it drops out that I’ve seen.

If it was TV only then why would the Arc be unaffected? Regardless it feels that we’re homing in on the issue here 

I have contacted LG UK (since I just assumed the local LG Israel here will be completely useless in addressing the matter) and after a good 4 days they responded to my email. I will update if I come up with anything useful.

If the audio is actually dropping at the source (AppleTV, PS5, etc) then maybe streaming PCM 5.1 out of a Linux device may yield some debug info in the system logs. May plug a raspberry pi into HDMI 3 when I have a couple of hours to experiment.

Also encouraging that recently more unique posters are finding this thread and describing similar experiences

Sometimes it didn’t do it. It was about 1 in 3 change of showing. In the app, when you start playing something it takes a second or so for the app to show the format. I’m guessing the dropout is so quick that the app doesn’t have time to react.

Update on HDMI 1 - I’ve watched my Apple TV 4K Gen 3 for maybe 6 hours and it seems good so far.

Also - my previous TV was a LG SM9000 which also has eARC. I had no issues with this at all running the same Apple TV 4K Gen 3 and the Beam Gen 2.

I contacted Sonos support so they know there is another user with the same issue.