5.1 Audio from Stereo Sources

@XfinityAmandaB  Thanks for the reply. 

Yes, expanding stereo to 5.1 is a feature of many sound systems.   I guess the question is how Xfinity is doing the expansion.   Is it using a tool like ProLogic or Dolby Surround?  Is it duplicating the LR channels to the surrounds?  Or, is it sending the stereo signal in a 5.1 wrapper where the other channels are silent? 

Also, is there a way on the X1 to send stereo in stereo?  I can change the audio output to PCM, but doing that means genuine 5.1 and Atmos content is downmixed to stereo.  So, that's no good. 

@XfinityMartyR  Thanks.  I'm quite familiar with DD 5 1, ProLogic, and DD+, which you you didn't mention. ProLogic is a 2.0 output with the extra channels folded into the signal for extraction by the home audio processor.  It is used when the source content is multichannel.   But, with apps like Spotify and YouTube, the source content is only 2.0 to begin with, so how would ProLogic be used? 

Let's drill down a little more.  I asked which type of expansion to 5.1 the X1 is doing and your reply says i should be able to switch between DD 5.1 and PL depending on my set up and preferences.  I don't see a way to do that on my XG1v4 boxes or to even identify which format the X1 is applying.  As noted in my first post, Expert mode sends 5.1 PCM while Auto Detect sends 5.1 DD+.  So, my initial question remains, how is X1 expanding a 2.0 source into a 5.1 output. 

As for a setting to send the original stereo, my post and your reply both note that the current setting for that means actual multichannel content will be downmixed to 2.0, making that setting a non-starter.

Thanks again. 

I will add that as i listen to these third party apps with stereo content output as 5.1, they appear to be LR stereo locked in a 5.1 wrapper.  By that, I mean the only sound is in the two front speakers with the center and surround channels silent.  So, X1 is not upmixing to produce actual 5.1 content and that makes it impossible for the home receiver to upmix since the source is already 5.1.  Not good.

FYI - I posted the same question in the r/Comcast_Xfinity sub on Reddit and got this reply from a Comcast Community Specialist:

Thank you for taking the time to create this post. And great questions! Here's what I've found:

The XG1v4 has a feature where it places stereo (2.0) audio from third-party apps into a 5.1 container.
- In "Auto Detect" mode: It wraps the stereo audio in a Dolby Digital Plus (DD+) 5.1 signal.
- In "Expert" mode: It outputs it as Multichannel PCM 5.1.

In both cases, the X1 box is essentially "telling" your AV receiver or soundbar that it is sending a full surround signal, but it only populates the Left and Right channels with data. Because your receiver sees a 5.1 signal coming in, it assumes the center and surround channels are intentionally silent (for effect) and refuses to apply "Upmixing" logic (like Dolby Surround or DTS Neural:X). On linear cable channels (QAM), the box typically respects the native format (2.0 or 5.1), which is why those channels still work correctly for you.

There is no "secret setting" to force the apps to output native 2.0. However, here are your best workarounds:

1. The "Stereo" Toggle (The Manual Fix)
As you noted, setting the X1 to Stereo forces the box to output a 2.0 signal, which allows your receiver to upmix (using Dolby Pro Logic II or similar). The downside: You have to manually toggle it back to Expert/Auto for movies or sports in 5.1/Atmos.

2. Use Your TV's Internal Apps (The Recommended Fix) 
If you have a Smart TV, using the YouTube or Spotify apps built into the TV (or on a dedicated streamer like a Roku, Apple TV, or Fire Stick) will almost always solve this. These devices typically output a "clean" 2.0 signal for stereo content, allowing your receiver to handle the upmixing properly.

3. Switch to a Different X1 Box
Community reports suggest this "5.1 wrapping" is most prevalent on the XG1v4. Older boxes like the XG1v3 or the smaller wireless Xi6 clients often handle 2.0-to-5.1 transitions more gracefully, though you may lose 4K DVR capabilities or local storage by switching.

4. Check for Receiver-Specific "All Channel Stereo" 
Some AV receivers have a "Multi-Channel Stereo" or "All Channel Stereo" mode that ignores the 5.1 mapping and just duplicates the LR audio to all speakers. It’s not a "surround" experience, but it fills the room and solves the "silent center" issue for music.

Please let me know if this helps, and what other questions you have. We remain here, happy to help in any way we can :)