4k audio

It's been more than three weeks since the OP first posted about unexpected 2.0 PCM audio from his Xi6 and almost a week since I got an Xi6 and posted test results confirming what the OP reported.  Xfinity support had both of us open DMs to explore what's going on.  Sadly, not a single substantive response from support in all that time.  Xfinity engineering designed the Xi6 audio settings and must know what the box is doing.  But, after almost a month of silence, I have to assume they do not plan to share any of that information.

So, for any users who would like to know what the Xi6 does with audio, I am posting my test findings.  This is mostly about two significant differences - first, between how the Xi6 and XG1v4 boxes work and second, between the Auto Detect and Expert settings on the Xi6.

The XG1v4 outputs DD+ or DD for everything in both Auto and Expert.  You get whatever the source is providing from Atmos to 2.0 on both settings, with one exception.  In Auto, stereo broadcast content arrives as DD+ 5.1 with the only actual sound in the L/R channels.  That prevents AVRs from upmixing to the center and surround speakers.  Expert sends stereo broadcast content as DD 2.0, which can be upmixed.  So, I use Expert with my XG1v4s.

The Xi6 works very differently, especially when it comes to Atmos audio on the 4K channels.  When set to Auto Detect, the Xi6 sends everything as Dolby MAT.  That means the Xi6 decodes the source audio itself, turning it back into PCM and re-attaching Atmos metadata, if there is any.  That's fine for most audio, except that stereo broadcast content is trapped in 5.1 PCM and can't be upmixed, much the same as what happens with Auto on the XG1v4.

With Expert on the Xi6, regular broadcast content arrives as DD+ 5.1 or 2.0, all good.  But, the 4K channels have a problem.  Atmos arrives as 5.1 PCM, not Dolby MAT, so no Atmos.  Then, sometime quite recently, something changed.  The 4K channels all come in pairs, with the odd numbers in the pairs doing low latency "realtime" transmission.  I just found out that the even numbered versions of those pairs on the Xi6 now output DD+ Atmos instead of 5.1 PCM.  So, you can get 4K Atmos on Expert as long as you don't use the realtime channels.

There's no information from Xfinity as to why the Xi6 uses Dolby MAT in Auto mode.  But, it is my understanding that the realtime 4K channels may be sourced with AC-4 audio instead of DD+ and AC-4 cannot be passed through since no consumer equipment can decode AC-4.  Dolby MAT is the way to output AC-4 Atmos to consumers.  I think this started with the World Cup last summer and that's when users on other sites started posting about changes in audio on the Xi6.

Yes, that's quite recent.  So, you can pretty much use Expert all the time as long as you watch the non-RealTime 4K channels for Atmos.

Followed up in my DM thread today and got this reply.  Finally!

"The Xi6 decodes the incoming source audio (like AC-4) and outputs it as Dolby MAT Atmos to your AVR. You can find more information here." https://www.xfinity.com/sup

That link doesn't actually mention Dolby MAT.  And, in my testing, MAT is only output in Auto Detect.  Expert sends DD+, does DD+ Atmos on the even numbered 4K channels, and regular 5.1 PCM (no Atmos) on the odd numbered 4K RealTime channels.

I followed up to ask about the Auto Detect 5.1 MAT output for 2.0 sources where the center and surround channels are silent.  That can't be the desired output, but it's consistent with Auto output on my XG1v4s.  So, it's probably gonna be that way going forward.  Bottom line:   Auto is fine unless you want to be able to upmix 2.0 sources.   Expert is fine as long as you accept using the non-RealTime 4K channels to get Atmos.

It's taken a month, but I think we have most of the answers now, including that AC-4 source audio on the 4K RealTime channels may be why the Xi6 now outputs Dolby MAT when set to Auto Detect.  

With Expert, on the odd numbered 4K RealTime channels, i get standard 5.1 PCM.  If the source is an actual game, where the originating track is 5.1, I get sound in all 5.1 channels.  But, if the source is the Fox Sports Promo loop, the 5.1 output only has sound in the L/R channels.   I can apply upmixers, but the center and surround channels remain silent since the input is already 5.1.  I assume that's because the promo loop is actually 2.0 and upmixed to Atmos by Xfinity.  But, that's just an assumption.  

@user_x1npns 

Are you saying your AVR reports a 2.0 input?  Mine reports 5.1 PCM.

That's different from what I get.  Unusual.