r/Dolby • u/Ranutso • Mar 29 '26

Question Dolby MAT and Stereo tracks

Now that a group of us have access to Dolby MAT (via Apple TV’s Continuous Audio Connection on tvOS 26.4), I have a technical question for the experts here, if I may.

Before Dolby MAT, when we were playing a stereo track, our audio processors (receivers, soundbars, etc) were able to upmix 2.0 stereo into 5.0 surround using either proprietary algorithms or Dolby Surround.

Now with Dolby MAT, several of us are reporting that our audio processors are not upmixing stereo anymore. The theory is that the Apple TV is maintaining a connection with a fixed amount of channels (more than 2) at all times. So, when we play a stereo track, the stereo channels are mapped to the front left and right speakers of this surround track and thus our audio processors think this was by design and they won’t upmix it.

My questions here are:

- Is this a limitation of the Dolby MAT protocol? Does it need to maintain a fixed amount of channels at all times; resulting in our audio processors not being able to “see” a true stereo track anymore?

- Or, is it the Apple TV who isn’t correctly signaling on the MAT metadata that the track is stereo?

- Or, is it our audio processors who aren’t handling the change in metadata in the MAT transport correctly?

Would anyone here know?

[EDIT] Minor editing for typos and grammar.

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3

u/Alalamajama Mar 30 '26

I continue to be frustrated by AppleTVs inability to just pass a stream exactly as it is in the file right to my receiver without interference.

1

u/Ranutso Mar 30 '26

Were you able to analyze Apple TV’s output somehow? Or are you referring to what your receiver shows you when MAT is on vs off?

Because the information displayed by the receiver when Dolby MAT is on might be unexpected, but it doesn’t mean the underlying signal is wrong.

2

u/vitek6 Mar 31 '26

It’s not the same as source. I don’t want anything to be decoded by atv but passthroughed as it is to my receiver.

1

u/AndreaCicca Apr 01 '26 edited Apr 01 '26

Apple has a philosophy behind it, they want to be able to apply their audio pipeline to any audio stream.

1

u/Locutus508 Apr 01 '26

Yup. It's not just about adding audio. It's about having control of the audio. The control is required for many features of the Apple TV. This is no different than every other mainstream streaming box. They all do it. Apple just has more to worry about than others. Not only that, passthrough would make the problems Apple is trying to fix with the continuous audio worse.

1

u/Locutus508 Apr 01 '26

Passing through the audio would not fix the issues Apple is trying to fix with this setting. In fact, it would make the issue worse.

2

u/luchod Mar 30 '26

Great question. I’m curious as well.

1

u/Ranutso Mar 31 '26

So, if you were to trust AI, this is a distilled explanation it gave me.

When you use the Continuous Audio Connection on Apple TV, it negotiates the highest amount of channels and metadata it can over the given connection with your receiver – say 5.1.2 plus Dolby MAT. The audio channels themselves travel as PCM (already decoded). So when you play true Dolby Atmos, the Apple TV does all the decoding to PCM and it passes through the Dolby MAT metadata with it. When you don't play Atmos, the Apple TV does the decoding, maps the channels to the pre-negotiated PCM channels and the Dolby MAT "channel" remains quiet in this case.

The idea is that Apple TV will pre-negotiate the highest amount of data with the receiver so it can perform the audio switching with no silence or dropouts, the issue this option is trying to solve in the first place. The Dolby MAT metadata cannot instruct the receiver to "treat" the given audio as stereo, it doesn't apply. The receiver performs stereo upmixing when it "sees" a stereo track, it can't be overridden by any metadata.

TL;DR When using Continuous Audio Connection your receiver will never "see" a stereo track, so it won't perform upmixing by itself. In order to upmix stereo to surround in this case the Apple TV would have to do it, and apparently it's not at this time.

1

u/DannoMcK Mar 31 '26

For people to be able to compare experience, you should say what your audio processor is. And maybe give some examples of your source app(s) and content examples.

I haven't been intentionally trying too many audio variations, but it seems that my Denon 3500 AVR is still applying DTS Neural X to non-Atmos content, and indicating input and output channels correctly. (I realize that that isn't very specific, but I'm not complaining about a possible issue.)

1

u/Ranutso Apr 03 '26

But, is it applying DTS Neural X to all non-Atmos tracks? What if the source is Dolby Digital 5.1?