Multichannel LPCM over HDMI 2.0 Down Mixing

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Level 1

4 points

Multichannel LPCM over HDMI 2.0 Down Mixing

When trying to send multichannel LPCM audio via HDMI 2.0 from my 2021 M1 MacBook Pro to my AVR (Denon AVR-S750H), the audio is somehow getting downmixed to Stereo.

In the MIDI Setup application, I am able to select from a range of 2-8 ch, 16/20/24 bit, (48 kHz only) audio formats, but all other formats including all Digital Encoded Audio options are grayed out. I have confirmed the AVR supports (in LPCM) a wide range of audio formats that includes most of the grayed-out options.

Currently set to 6ch, 24 bit, 48 kHz, and the AVR is correctly showing "Multi In - Dig. / PCM" on the front display.

Unfortunately, no settings in the AVR seem to enable these grayed-out formats or stop this Multichannel LPCM to Stereo downmixing. Note that Dolby Digital files successfully play isolated channels as it should.

The AVR was able to decode multichannel PCM without issue when a Windows machine was used, so I wonder if this is a MacOS issue. Apple Support was not able to help. Any thoughts on this?

MacBook Pro 16″, macOS 12.1

Posted on Jan 23, 2022 4:09 PM

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Answer 1

Jan 25, 2022 4:38 PM in response to HOSKINGJ

According to an internet article, specifying Bitstream results in the audio decoding being performed at the endpoint (e.g. a surround-sound receiver). On the other hand, when PCM is specified, the decoding is performed at the originating device (e.g. a Mac mini or Blu-ray player) so no decoding is needed at the endpoint.

The article Set up external speakers for stereo or surround sound in Audio MIDI Setup on Mac states:

Some macOS apps render audio output in stereo while others use multichannel, depending on the media content. You may want to set up your speakers for both configurations to customize your audio experience when using apps.

It seems to me, if the content is intrinsically multi-channel, it should not matter whether the receiver is configured as Bitstream or PCM. The exception, however, would be if the receiver doesn't support the encoded content format, in which case setting the receiver to PCM would be necessary.

The above doesn't explain why a multi-channel source would be sent as stereo over HDMI 2.0. I suspect that it has more to do with the app/content rather than the capabilities of macOS CoreAudio.

[extracted from hdmi-cable-types]

- Pie Lover

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Answer 2

Jan 23, 2022 4:18 PM in response to Ridagstran

Have a look at Set up external speakers for stereo or surround sound in Audio MIDI Setup on Mac.

Could it possible be a cabling issue? Also, check System Report for the HDMI interface to see if there any clues as to why might be happening. Last, try reaching out to folks in the GarageBand community for troubleshooting advice.

Hope this helps.

- Pie Lover

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Answer 3

Ridagstran Author

Level 1

4 points

Jan 24, 2022 3:18 PM in response to BlueberryLover

Hey Pie Lover,

Thanks for your response. Unfortunately the setup guide did not help, and the System Report did not show any useful information either. Will try another HDMI cable when I have access to one later. If that does not work, I'll try reaching out to the GarageBand community or contacting Denon.

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Answer 4

Jan 24, 2022 4:41 PM in response to Ridagstran

You're welcome. Unfortunately, I don't have any multi-channel equipment to test with. Mac minis are often used as media servers so it wouldn't surprise me if they are used to drive multi-channel surround-sound receivers. Perhaps a web search with those terms could be useful.

Good luck.

- Pie Lover

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Answer 5

HOSKINGJ

Level 1

8 points

Jan 25, 2022 1:28 PM in response to Ridagstran

Apparent success by switching the receiver (or TV?) to Bitstream rather than PCM - https://forums.macrumors.com/threads/m1-how-to-output-multichannel-audio-hdmi-to-tv.2290001/

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Multichannel LPCM over HDMI 2.0 Down Mixing

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