Locutus

speedgarage said:

One reason for Apple to decode audio rather than pass thru would be to mix in UI sounds. I Don’t recall if Apple TV currently generates UI sounds while playing back DD/DD+ content tho.  Other than that, I’m not sure why Apple TV would not just pass thru audio by default (assuming downstream device can decode format)

There are many instances where audio is mixed in while content is playing.  The Apple TV has audio overlays with some accessibility features. These occur while audio is playing.  In addition, Siri lowers the audio when the Siri button is pressed.  This is done so the content audio isn't heard and interpreted by Siri.  In addition to all that, AirPlay 2 multi-room audio requires Apple to decode the audio.  Multi-room audio is likely the reason why Apple plugged the "hole" in the first place that allowed some third party apps to bypass tvOS's audio pipeline and passed through  audio.  The "hole" was plugged in tvOS 11.3.  Multi-room audio was released two months later in 11.4.

Rogue01 said:

Finally an update to AppleTV that is worth getting excited for.  An AV receiver will do a much better job with the sound processing than the AppleTV.  I did not realize the AppleTV wasn't already doing the passthrough over HDMI for a receiver to handle the processing.

Almost no mainstream streaming player fully passes through audio.  The Roku Ultra allows it for DD and DD+ only.  The Shield will for some formats, but it's not really mainstream.

We may be getting ahead of ourselves here.  This is likely just including other platforms in the macOS passthrough API included in macOS last fall.  The API offers limited passthrough support for DD and DD+.  In macOS, it's a per-app preference.  Its not a guarantee.  When set, macOS will passthrough audio as long as you don't have any other audio features in use that requires macOS to decode the audio.  Once you do, macOS falls back to decoding rather than passing through.  macOS doesn't support TrueHD or DTS with this change.  I don't imagine tvOS will either.