Apple Broke Dolby Digital 5.1 Output on Apple TV 4K

On my 4k Rotel preamp, while I don’t see DTS or Dolby anymore, multi-channel content is detected, but shows up now as Multichannel PCM.

This is as intended. The output is one of Dolby Atmos MAT (when not from TrueHD sources), Multichannel PCM (Auto), Dolby Digital 5.1 (custom), Stereo (custom). The Apple TV box never supported DTS, although select apps may do their own conversion.

It's getting weirder. In my case, Dolby does not work in any application except the Youtube application, where Dolby sound is emitted even from materials that do not have surround sound.

Apple TV 4K 2017 connected directly to Polk MagniFi 2. Soundbar connected to the TV via HDMI-ARC.

Audio output set to AUTO (but changing this setting doesn’t change anything).

You must try it in DIRECT MODE, your AVR may call it differently. Mine takes any signal even PCM 2.0 and will convert it into 7.4.2 and will indicate that it is playing through all channels. But the real test is putting the AVR in DIRECT/STRAIGHT/PASS-THROUGH so it doesn't colorized the source. When doing that mine shows PCM 2.0. Please find out what yours say.

I thought the same thing you did, until I realized there was a huge sound quality difference between playing same moving, same content, multiple of same streaming services between the APPLE TV 4K box vs the Apple TV application on the LG TV. Then I realized I was not testing it in DIRECT/STRAIGHT/PASS-THROUGH so my AVR kept upconverting it and indicating it was running thru all channels. All it did was split the sounds as it thought fit, but quality was terrible as it was taking 2.0 and pumping it out to 7.4.2. Once I realized that, I put it in DIRECT, and sure enough, it shows 2.0 is coming from the original source of the APPLETV box, while the correct amount of channels came out of the APPLE TV application on the LG TV.

My Apple TV 4K is running tvOS 18, connected to my AVR.

When using the Apple TV 4K, with the AVR in "Straight" mode and watching some simple show that doesn't use multi-channels like say the news or some show that is just in stereo, my AVR display will indicate stereo two front channels only. If I watch a movie in whatever app I am using on Apple TV 4K, Amazon Prime, Hulu, etc. My AVR display will indicate multi-channel mode in my case 5.1, this is all while in "Straight" mode.

I would do the same but I have a whole lot of other issues using the apple TV app on fire stick and also my LG TV, the issue is some movies ( usually universal movies) only play in HD HDR for some reason.. when you're watching a movie and press down in the remote it gives you the info of the movie, it says HD HDR and not UHD HDR.. I'm in the UK so maybe it's just an issue here and probably a conversation for a different post

Are folks with modern receivers also having this problem? I spoke with a friendly customer service rep with no immediate fix. It is strange that the Youtube app works properly while none of my streaming services such as Netflix works.

I have a Marantz that can receive 5.1 from my Apple TV 4K if I set the audio to Auto, but even that won't receive 5.1 if I designate Dolby Digital 5.1 for the audio. My Apple TV HD is connected to a simpler system, and can't receive 5.1 whether Dolby Digital 5.1 or Auto is selected. This was not the case before the tvOS 18.0 update. Before the update, I could receive 5.1 through either system when Dolby Digital 5.1 was selected.

Yes, there seem to be a lot of interpretations for the constraints on using the Community, so searching for “Best 5.1 Surround Sound Test - By DTS” at youtube.com is probably the best workaround. Anyway, the YouTube streaming app as well as Home Sharing streaming, which appears on Apple TV as a nameless orange icon containing a white outline of a laptop with a triangular "play" button shape , are apparently immune to the detrimental effects of the tvOS 18 upgrade to which I and several others appear to have fallen victim. Thanks for your comment.

I've purchased an S/PDIF adapter from Amazon that allows me to receive 5.1 output while converting the digital to a stereo analog system that can be received by an analog stereo amp or standard stereo headphones. It includes a volume control. Cost me about $30 and prevents me from needing to switch off the Apple TV 5.1 output when my surround sound system is otherwise occupied. I used it before the 18.0 update. The only need for it now is to convert audio output while listening to 5.1 content through Home Sharing or the YouTube streaming app. It's called PROZOR 192kHz Digital to Analog Audio Converter Support Dolby AC-3 DTS 5.1CH with Volume Adjustable, Optical to L/R DAC Decoder, Digital DAC Converter SPDIF TOSLINK to Stereo L/R & 3.5mm.

Is your Apple TV's Audio Output set to Auto or Change Format: On with Dolby Digital 5.1 listed next to New Format? If it's set to Auto, then the Apple TV is able to recognize the 5.1 capability of your AVR. The issue is that there appear to be 5.1 compatible AVR or audio amplification systems which are capable of receiving and playing 5.1, but only if the Apple TV setting is Change Format: On, and Dolby Digital 5.1 listed next to New Format. That is the problem that didn't exist before the tvOS 18.0 upgrade. The fact that the setting exists appears to indicate that Apple was aware of the contingency, or else the option wouldn't exist. My complaint is if the option exists, it should work. It did before the 18.0 update. If it can't be supported, it would have been more considerate for Apple to have warned us that the 18.0 update could potentially make some audio systems obsolete for use with Apple TV.

Yes, that's exactly what I'm seeing and hearing. I had to use the change format setting to get DD5.1 in the past with tvOS17 but can't be exactly sure of the version numbers. But since tvOS the change format option does not work. Maybe that's what tvOS18 has broken i.e. the change format option.