4k audio

Any response from support about AC-4?

It would appear your problem is fixed,  that Auto works for both types of content.  But, one question, on the non-Atmos 5.1 PCM input, is there actually sound in all the channels?   In some circumstances, my XG1v4 sends 5.1 with the only sound in the L/R pair.  

Getting down in the weeds..

PCM is the digital audio format that home theater receivers use to process sound.   That's how sound tracks are mastered.  But, multichannel PCM files are quite large.  That's where codecs like Dolby and DTS come in.  They are not audio formats.   They are audio zip files whose sole purpose is saving space in storage and transmission.  The PCM track is fed into the encoder to shrink it down and the decoder in your receiver unpacks it back into PCM.

In the current case, the Xi6 box is doing the decoding to turn the track back into PCM and that's what gets sent to the AVR for processing.  So, no, Atmos PCM is not Dolby Digital.  It started out as DD+ or AC-4 Atmos and was decoded by the Xi6.  It's already 5.1 when it gets to the AVR.  If your system is Atmos capable or has a 7.1 layout, the AVR will do the Atmos processing or upmix 5.1 to 7.1.  But, in your case with a 5.1 layout, it will simply play the PCM channels coming from the Xi6.

@XfinityJustinM Please don't close this out until we hear from the backend team about what audio the Xi6 is processing.  Are the 4K channels taking AC-4 audio rather than DD+ and outputting it as Dolby MAT?  And, as mentioned, why is the output only 2.0 PCM in Expert mode?  Audio from the Xi6 is completely different from the outputs of the XG1v4.  Thanks.   

Since, BIslander has been so helpful with this,  lets keep this open as they requested.