How to send multichannel PCM audio from a Mac to an AVR receiver with HDMI?

Question

I’ve been investigating if (or when) there might be a way to send 12 channels of PCM audio from a Mac to an AVR without the need for a hardware audio interface, and specifically via HDMI. While I’ve yet to be able to test it, what I’ve learned has led me to believe 8 channels may be currently possible, but that things are capped at 8 channels, as is the max sample rate (which I don’t recall, but think I heard it might be 48kHz).

That said, I’ve also heard there are updates to Monterey (still in beta perhaps?) that might make 12 channels possible. (Note: my consideration extends beyond making use of the speaker configuration utility in macOS Audio MIDI Setup. My primary interest is just in the ability to send the 12 channels).

Additionally, my understanding is that HDMI 2.1 has added considerable extra bandwidth with the potential to transmit up to 32 channels of PCM (though I’m not sure what the sample-rate-to-track-count ratio is, i.e., when/where the sample potential gets capped relative to track count).

But I’m not clear about where the bottleneck(s) currently exist to make use of this potential.

• Assuming one has a capable HDMI 2.x port and cable, is there something native to macOS that would prevent such a scenario? If so, is that limitation likely to be addressed in the foreseeable future?

• Is what I’m describing already possible and I just don’t know it? Specifically, on Big Sur with a 2019 MacBook Pro?

• Clearly, the receiving AVR would need to be able to make use of such a signal, and/or presumably present that many path options in the audio device dialogue in macOS Audio-MIDI setup. And while maybe outside the scope of this site, if anyone has any insight into what would be needed on the AVR end of the equation, I’d be curious to know. Specifically, if an AVR already has an HDMI 2.1 port, could this potential be unlocked with a firmware update on the AVR side of the things?

Ideal use case:

• DAW outputs 12 discrete channels of PCM audio (a 7.1.4 decoded Atmos mix, NOT a Dolby stream) to Core Audio.
• ideally up to at least 96 kHz per channel
• Core Audio sends these 12 signals to an AVR via HDMI
• AVR routes to the speaker array.
• Ideally using a 2019 MacBook Pro, Big Sur, with a TB3 to HDMI 2.x adapter.
• Or if above the above machine and/or OS doesn’t work, but another configuration would, would be curious about that.
• Or some other scenario that might exist whereby the PCM could maybe be embedded into a 12 channel stream (bit rate?) that a modern AVR might be able to make easier use of, again via an HDMI pathway. In this scenario, open to a simple (read: not costing thousands of dollars) “interface” type solution that is virtual and/or hardware-based, to act as a go-between. For example: DAWs PCM signals > virtual audio device and/or physical interface > embedded multichannel audio > HDMI > AVR.

Any insight into any or all of the above considerations would be greatly appreciated!

Answer 1

Check System Information on your Mac by going to the Apple menu, select About this Mac, then click the System Report button. In the Audio section you should see the output device you're connecting to. If it says "Transport: DisplayPort" there's the cause. DisplayPort supports a max of 8 channels.

DisplayPort can be used to transmit audio and video simultaneously, although each is optional and can be transmitted without the other. The video signal path can range from six to sixteen bits per color channel, and the audio path can have up to eight channels of 24-bit, 192 kHz PCM audio that is uncompressed.

I'm running into this 8 channel cap using USB-C > HDMI from a Macbook Pro running Big Sur to a receiver which is configured for 7.1.4. The receiver auto-detects the input signal and is showing 7.1 on its front display when connected to the Macbook Pro. The receiver only registers in the Audio Midi Setup utility as being 7.1. I'm still investigating but will report back if I find a way to get more than 8 channels to output. Hopefully, there's a way to use a different protocol to get around DisplayPort's 8 channel max.