HDMI to digital active speakers. Why is this so hard?

[kfrancis]

If you want to get annoyed then complain about Dolby rather than calling things ‘jank’.

Yeah, sorry, I didn't mean to imply the jankness is by Apple's choice. Entirely Dolby's doing. And all I really meant by jank is that TrueHD is not possible. I do not see how this benefits Dolby, but alas.

 

[kfrancis]

macOS can:
- Output discrete Atmos to speakers from Apple Music *only*.

Out of curiosity, I just opened the Apple TV app and started one of their own shows, and it definitely seems to be outputting Atmos to the audio interface too, for what it's worth:

 

[chelgrian]

There is not a blu-ray player out there that can decode Dolby Atmos or DTS:X and output PCM while also retaining metatdata. They must be bitstreamed.

Both the Apple TV and many Blu-ray players output Dolby MAT2. One of the modes for transporting Atmos over MAT2 used uncompressed audio streams. The Apple TV uses that mode. There are some Blu-ray players which use that mode.

You obviously cannot output to LPCM8 both due to not enough channels and due to licensing.

 

[Tre2023]

I am simply not familiar with any disc player using the same mode as the Apple TV 4K.

Getting back to the Apple TV 4K, those using it with a Flex HT must turn Atmos OFF in the sound settings. This will result in a signal recognized as multichannel PCM 7.1 when playing Atmos tracks rather than Atmos or Atmos/PCM when using an AVP/AVR with a Dolby Atmos decoder. Non Atmos tracks would be recognized as PCM 2.0 or PCM 5.1 whatever the case may be.

Again, those with blu-ray players could set audio output to PCM and Dolby Atmos and DTS:X tracks would be recognized as multichannel PCM 7.1. If using a universal player supporting SACD, multichannel DSD is output as PCM 5.1 24/176.

Things get more complex with macOS. Setting it to 8 channel output(7.1) in Audio MIDI settings will result in a multichannel PCM signal recognized over HDMI. This is also the case using Thunderbolt with a USB-C to HDMI adapter fitted with special firmware to make the mac think it is connected using DisplayPort. I had to work around the M1’s 4K @60Hz over HDMI limitation to get 4K @120Hz output.

Anyway, using 8Ch output over HDMI means all signals are recognized as multichannel PCM 7.1B and up mixing in the AVP/AVR will not work correctly. Two channel signals will have dialogue remain in the Front channels even when applying the Dolby Surround upmixer. The Audio MIDI would need to be set to 2CH output in this case for the upmixer to work properly. But, multichannel Dolby tracks will be fine using 8CH setting as long as Passthrough is OFF in apps that support it and multichannel PCM is output to a TV connected to a Flex HT or similar device.

There is much to consider when going the route of multichannel PCM from a source device into a TV and then out eARC to Flex HT to avoid an analog connection from a Dolby decoder equipped AVP/AVR pre outs to powered speakers.

 

[wingfeather]

I am worried about the audio latency that would be induced by having a general-purpose computer in the middle of the chain, which is not normally designed for reliable real-time low-latency DSP.

I think you might be over-indexing on this a little; general-purpose MacOS machines are used all the time in studios for live performance and recording with significantly stricter latency requirements than A/V sync.

A linux machine with the Ravenna kernel module has configurable latency (which is itself highly predictable because AoIP uses PTP), and works reliably down below 1ms with the right configuration. If you set the latency too low, of course you get dropouts, and then you just set it to something higher. But you'll always know what it is, and A/V is very tolerant of the sorts of latencies you can get from a system like this.

Since this option is so significantly cheaper than the alternatives, I'd encourage you to at least give it a shot.