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Discussion Starter #1 (Edited)
I bought Apple's USB-C Digital AV Multiport Adapter thinking I could use it to bitstream HD Audio via HDMI from my new 2017 5k iMac to my Yamaha AVR. Now I know that up to High Sierra (and probably forever) that MacOS doesn't support bitstreaming so I created a Windows 10 partition and have been attempting to monitor Dolby TrueHD and DTS-HD from that. No luck. Either silence or accidentally acquiring a signal only to have it disappear. So I checked the Windows 10 Speaker Setup test and could only get the 7.1 configuration to send test tones out of the Left and Right channels. The other channels animated like they were working but total silence. I disconnected the HDMI from the iMac and retried the 2014 Mac Mini and heard all 8 channels. Then I contacted Microsoft support and let them remotely try to fix the problem for a few hours. Then I called Apple support and they said it was a Windows problem. Microsoft support continued trying but they said I should call Apple again. So I did and this time we looked at the issue from the MacOS side. Going into the Audio MIDI Setup, I selected the 7.1 channel configuration and like Windows, only Left and Right channels worked. We tried 5.1. We tried creating a new account. We tried zapping the PRAM. We tried unplugging the iMac for 15 seconds. Nothing changed. Only Left and Right from this HDMI connection. This was escalated up to another Apple support person, who was then putting me on hold to talk to an Apple tech guy, who's expertise in audio, I was told, stemmed from his DJ-ing experience. And Apple's conclusion was that this was not something they could fix and suggested I look into third-party solutions.

So anybody got any third-party solutions for getting HD Audio out of a 5K iMac?
 

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I know of no media player software on the Mac side that will bitstream HD audio.

Your 5k Mac should have a protected audio path for the audio but you need a player that supports bitstreaming.
 

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Discussion Starter #4
I know of no media player software on the Mac side that will bitstream HD audio.
you will be the first person in the history of mankind to successfully get a mac anything to output HD Audio.
Hilarious! Both of you seemed to have missed the part where I said I was using Windows, the OS which Apple hardware owners who want to bitstream are obliged to use:

Now I know that up to High Sierra (and probably forever) that MacOS doesn't support bitstreaming so I created a Windows 10 partition and have been attempting to monitor Dolby TrueHD and DTS-HD from that.
Anyways I found that the JCA374 USB TYPE-C MULTI-ADAPTER gave me all 7.1 channels where Apple's poorly-reviewed USB-C Digital AV Multiport Adapter (2 out of 5 stars) could only give me 2.0 channels. I posted a video to demonstrate:

https://www.instagram.com/p/Bh6bTEelCwZ/?tagged=jca374
 

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Plex will decode TrueHD and DTS-HD-Ma onboard the mac and send out as PCM 7.1 to the receiver through HDMI. I have been doing this on my 2014 Mac Mini. However, it won't bitstream the TrueHD and DTS-HD-Ma. This was ok so far, but with the advent of Almos, this is not sufficient. So, hopefully Apple will fix their OS.
 

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Plex will decode TrueHD and DTS-HD-Ma onboard the mac and send out as PCM 7.1 to the receiver through HDMI. I have been doing this on my 2014 Mac Mini. However, it won't bitstream the TrueHD and DTS-HD-Ma. This was ok so far, but with the advent of Almos, this is not sufficient. So, hopefully Apple will fix their OS.
Yeah this is one option for Mac users, but I have always been suspicious of Plex's claim that their decoding is bit-for-bit identical to the original HD audio stream. They won't release actual technical documentation that proves this claim, just a basic "trust us, it's the same" in one of their online guides. Also with Plex decoding your AVR won't display the correct HD audio codec that was embedded in the video. Like the original poster (and most Mac users trying to make a HTPC) I went with a Windows partition as well. I'd rather my expensive AVR do all the decoding than my Mac Mini + Plex. Plus for some reason I prefer Plex on Windows than Mac - better peripheral support too (Logitech living room keyboards come to mind).

Anyways, to your point about Apple fixing their OS, I doubt there is enough of a vocal customer base demanding HD bitstreaming from their Macs. Apple is not one to listen to a niche group of users and then develop their OS accordingly. I will come back here and eat my words if they ever add this functionality, but I don't see it happening.

To the OP - thanks for the tip on the adapter, that's good info to store away. Super weird that it's even an issue on a new iMac and not on older Mac Mini hardware.
 

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Discussion Starter #7
To the OP - thanks for the tip on the adapter, that's good info to store away. Super weird that it's even an issue on a new iMac and not on older Mac Mini hardware.
It wasn't an issue on older Mac Mini's because they have HDMI ports while new iMacs need an adapter because everything is USB or USB-C.
 

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It wasn't an issue on older Mac Mini's because they have HDMI ports while new iMacs need an adapter because everything is USB or USB-C.
True that - still weird though, as USB-C is technically capable of doing everything HDMI can do. Guess Apple just didn't spend the R&D dollars on making sure their adapters were up to par.
 
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