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.