Dolby Atmos issue with Apple TV 4K and Mac mini on Denon AVR X550BT receiver

The Apple TV box support Dolby Atmos in Dolby Digital Plus streams, not Dolby TrueHD or DTS X or DTS Surround.

Apple’s own apps on Mac support Dolby Atmos in Dolby Digital Plus streams, not Dolby TrueHD or DTS X or DTS Surround.

The Apple TV box can output Dolby Atmos in Dolby MAT format (like PCM), else PCM Multichannel (up to 7.1). [Custom changes to Dolby Digital 5.1 or Stereo are also possible.]

Some third party apps on the Apple TV box may be able to re-sample unsupported formats to PCM Multichannel, if programmed that way. tvOS has disabled unrestricted passthrough (since tvOS 10.x I think).

Apple’s playback apps on Mac [Apple silicon] (macOS Sequoia) may be able to use the HDMI Passthrough option for audio, if programmed that way. This is listed for Dolby formats (DD, DD+, DD+ w/Atmos), not Dolby TrueHD or DTS formats. Licensing might play a role here, but also: Don’t expect Apple to cater to audio format mostly known from Blu-ray discs, as ripping files from such discs, or sharing those files, is likely (IANYL) not legal in the US (DMCA) and other countries. Finding free demo files doesn’t change this.

Not sure what can be available for third party apps; ask their developers. (There is DTS encoding software for Mac, albeit Intel-based. So I would expect that their operators can listen to what they’re doing?)

macOS Sequoia supports HDMI Passthrough for Dolby Atmos content - MacRumors

About HDMI Passthrough - Apple Support

I don’t have an Apple Silicon Mac, so I can’t test myself for how it differs from Intel Macs.

Streams with audio in Dolby Digital or Dolby Digital Plus through the Apple TV box in Output Format: Auto setting would be in PCM Multichannel. This is as intended, and doesn’t affect quality. The stream is a bit bigger, as the audio is now uncompressed. If that isn’t working for you (i.e. not compatible), then try the setting to force output in Dolby Digital 5.1 format (fallback option). This may cause a re-encode (unless the source was DD 5.1).

As Mac is listed to support HDMI passthrough, then when that is active, it should then output Dolby Digital or Dolby Digital Plus if that is in the source, without decoding to PCM, for a direct HDMI connection between Mac and surround sound receiver.

The Denon AVR-X550BT 5.2ch doesn’t support Dolby Atmos, so Dolby Atmos formats don’t apply.

The Denon AVR-X550BT 5.2ch doesn’t support DTS:X, so DTS:X formats don’t apply.

The Denon AVR-X550BT 5.2ch does support Dolby TrueHD and DTS-HD MA (i.e. losslessly compressed surround codecs), but the Apple TV box doesn’t support outputting that using passthrough, instead possibly decoding to PCM Multichannel 48kHz (not high res) for applicable apps. My Mac (Intel) test with IINA pointed to default PCM output over HDMI from DTS-HD MA and Dolby TrueHD. I haven’t figured out if or how passthrough is possible from Mac for these formats.

I have the same set up, except with a Marantz processor. When using the Mac mini only the Apple TV app inside of macOS will allow you to change to audio pass-through. Any other apps it will sound just as you described. But my experience has been that with all of the movies I have on Apple TV. They sound much richer and the true HD really shines versus playing them through the Apple TV 4K. So I’m keeping my Mini right where it is.