Spatial Audio/Dolby Atmos speaker output

[macman4789]

Hi,

A question I’m sure you knowledgeable people will be able to answer:

If I am listening to a form of Spatial Audio e.g. Dolby Atmos via Apple Music through a standard stereo studio monitor setup via an Apple silicon Mac, will the speakers output Spatial Audio at all? If not, will whatever comes out of the speakers be enhanced at all or just regular 256kbps AAC?

Thanks

 

[olavsu1]

If you listen spatial audio in 2.0 system, all channels will mixed back to 2.0 system. Sometimes it can feel better than a standard two-channel mix

You can't heard spatial audio if you don't have these output channels. What the specific project requires.

 

[macman4789]

If you listen spatial audio in 2.0 system, all channels will mixed back to 2.0 system. Sometimes it can feel better than a standard two-channel mix

You can't heard spatial audio if you don't have these output channels. What the specific project requires.

View attachment 2332921

Thanks for your reply. That makes sense. Is that screenshot from Apple Music settings? So if I was listening to music via an Apple Studio Display (which has support for Spatial Audio) would it always have this 7.1.4 surround profile?

 

[olavsu1]

Is that screenshot from Apple Music settings?

No, it's from logic pro when demo spatial audio project is opened.

So if I was listening to music via an Apple Studio Display (which has support for Spatial Audio) would it always have this 7.1.4 surround profile?

Do not believe. How many speakers does it have and are they located around you as the listener? Each number means a separate speaker with a specific location.

The only thing that will happens is deceiving the listener's brain through the location of the listener's device, the device has a sensor that detects the position of the device in the room.

 

[Avatar74]

If I am listening to a form of Spatial Audio e.g. Dolby Atmos via Apple Music through a standard stereo studio monitor setup via an Apple silicon Mac, will the speakers output Spatial Audio at all? If not, will whatever comes out of the speakers be enhanced at all or just regular 256kbps AAC?

Hi, audio engineering guy and Dolby Labs licensee here. There's several questions here, so I'm going to try to address each separately:

First, I think what you are meaning to ask is what is the fidelity of the audio but it's important to understand that all digital audio goes through a digital-to-analog conversion. Atmos is not an AAC format, but even AAC 256 Kbps is basically transparent... that is, it is indiscernible from 16-bit uncompressed stereo linear PCM which is sufficient to reproduce the analog sound wave with zero perceptible difference from the source.

The Atmos master file is ADM BWF, 24-bit 48 kHz which is greater than the Shannon Nyquist limit (the minimum sampling depth necessary to reproduce analog sound in the spectrum of human hearing, end to end, without distortion).

Atmos for the home is essentially Dolby TrueHD with a metadata layer on top. Dolby TrueHD is uncompressed 24-bits per channel, 44 to 192 kHz sample rate.

Apple can support lossless audio up to 24/192, so which format it delivers in is dependent on whether you have Dolby Atmos set to Automatic or Always on:

If you have this turned off then what format it delivers depends on your general settings for playback:

Then it depends on the DAC. If you're using Apple's internal DAC, it supports up to 24/96 so Atmos is supported but understand that Atmos is not discrete channel. It's discrete objects, and those objects will be down mixed to match whatever number of speakers you have.

It does not apply any additional DSP to "fake" spatial audio. What you will hear is a stereo mix unless you use the enhancer to create some arbitrary phase effects that give a sense of bigger sound... but this is a feature of Apple Music's Sound Enhancer and not Dolby Atmos.

 

[macman4789]

Hi, audio engineering guy and Dolby Labs licensee here. There's several questions here, so I'm going to try to address each separately:

First, I think what you are meaning to ask is what is the fidelity of the audio but it's important to understand that all digital audio goes through a digital-to-analog conversion. Atmos is not an AAC format, but even AAC 256 Kbps is basically transparent... that is, it is indiscernible from 16-bit uncompressed stereo linear PCM which is sufficient to reproduce the analog sound wave with zero perceptible difference from the source.

The Atmos master file is ADM BWF, 24-bit 48 kHz which is greater than the Shannon Nyquist limit (the minimum sampling depth necessary to reproduce analog sound in the spectrum of human hearing, end to end, without distortion).

Atmos for the home is essentially Dolby TrueHD with a metadata layer on top. Dolby TrueHD is uncompressed 24-bits per channel, 44 to 192 kHz sample rate.

Apple can support lossless audio up to 24/192, so which format it delivers in is dependent on whether you have Dolby Atmos set to Automatic or Always on:

View attachment 2351680

If you have this turned off then what format it delivers depends on your general settings for playback:

View attachment 2351679

Then it depends on the DAC. If you're using Apple's internal DAC, it supports up to 24/96 so Atmos is supported but understand that Atmos is not discrete channel. It's discrete objects, and those objects will be down mixed to match whatever number of speakers you have.

It does not apply any additional DSP to "fake" spatial audio. What you will hear is a stereo mix unless you use the enhancer to create some arbitrary phase effects that give a sense of bigger sound... but this is a feature of Apple Music's Sound Enhancer and not Dolby Atmos.

Thank you for this very detailed response. So in essence, if I was listening to it on standard studio monitors on a desk, it will just output the sound to stereo even if my settings are set to ‘Automatic’ which they are.

However, if played through the Apple Studio Display speakers (which support Spatial Audio), they should output Atmos if the track is supported. If not and it’s standard lossless, it will output in stereo?

Many thanks

 

[Avatar74]

So in essence, if I was listening to it on standard studio monitors on a desk, it will just output the sound to stereo even if my settings are set to ‘Automatic’ which they are.

However, if played through the Apple Studio Display speakers (which support Spatial Audio), they should output Atmos if the track is supported. If not and it’s standard lossless, it will output in stereo?

The short answer to both your questions, I think, is yes. But this warrants a bit more explanation because here I think we are jumbling up a couple of concepts. It's important to first not think of Atmos as Spatial Audio per se... any more than stereo is "Spatial Audio".

As the original Dolby Surround is matrixed audio (two main L-R channels and two additional channels phase-shifted 90 degrees), and Dolby Digital is discrete multichannel, Dolby Atmos at least in its theatrical format is an object-based, rather than channel-based format.

When I think of Spatial Audio, I think of digital signal processing like the original QSound, which uses various acoustic signal tricks like phase shift, etc., to create spatial characteristics where none exist. This category of DSP is important, because it has everything to do with how Apple Studio Display interacts with formats like Atmos.

Atmos can technically be 2.0... If you have an Atmos soundtrack playing over a 2.0 system with an Atmos decoder, then you will get a down mix of all the Atmos elements appropriate for your 2.0 (or 3.0, 4.0, 5.1, 7.2, etc.) speaker setup, where the down mix is adjusted based on the metadata present in the metadata layer atop the TrueHD data.

What Apple Studio Display and other DSP-enabled systems/soundbars/etc. are doing, I suspect, is taking that metadata and applying DSP to it to approximate spatially what the engineers had originally intended. So I don't believe that this is a function of Atmos so much as it is a function of DSP enhancement that could, in principle, be applied to any audio stream though with Atmos having its own spatial metadata the result is less random than, say, using the Sound Enhancer on a normal stereo track that has no spatial metadata.

 

[macman4789]

Thank you again for that detailed reply, very interesting and insightful.

Would there be a way of decoding Atmos tracks in device, say in a Mac and then outputting that to a 2.0 stereo channel?

I suppose I’m thinking of when you buy a standalone DAC that can decode MQA tracks that the likes of Tidal use?

 

[Avatar74]

Thank you again for that detailed reply, very interesting and insightful.

Would there be a way of decoding Atmos tracks in device, say in a Mac and then outputting that to a 2.0 stereo channel?

I suppose I’m thinking of when you buy a standalone DAC that can decode MQA tracks that the likes of Tidal use?

Regarding the proprietary MQA, a DAC is generally not a decoder. But I think what you're envisioning is a device that has a DAC component as well as a decoder. The problem is that with proprietary formats, what do you do when they inevitably are replaced? What are we solving for when everything is getting decoded to PCM anyway to go through the D/A converter?

I don't really see any advantage to carrying extra metadata on an iPhone. It's technically feasible but it would largely be a waste of CPU and battery on a mobile device given that most modern music libraries consisting of tracks from 1997 onward will contain poorly mastered audio, headphones are far from acoustically transparent (the sound of even the best headphones in the world is heavily colored), and AAC 256 Kbps is far more resource efficient for the same result: Remember, "Atmos 2.0" is not "3D Audio". It's just Dolby TrueHD mixed down to 2 channels. Any DSP would have to be wasting CPU, memory and battery on top of that.

 

[macman4789]

Yes, I’ve seen DAC’s with built in MQA decoders that I think I am referring to.

So is the lossless format from Apple Music Dolby TrueHD? I know the lossy format is as you say AAC 256Kbps but what are the lossless formats that they stream? I’ve googled a bit about it but most of the streaming platforms stream Atmos in Dolby Digital Plus presumably to save bandwidth?

 

[Avatar74]

Yes, I’ve seen DAC’s with built in MQA decoders that I think I am referring to.

So is the lossless format from Apple Music Dolby TrueHD? I know the lossy format is as you say AAC 256Kbps but what are the lossless formats that they stream? I’ve googled a bit about it but most of the streaming platforms stream Atmos in Dolby Digital Plus presumably to save bandwidth?

The lossless format from Apple Music is ALAC up to 24/192.

However, Atmos for AppleTV is a package of Dolby TrueHD streams (up to 24/96) with a layer of spatial encoding metadata. Dolby refers to this TrueHD + Metadata package as Dolby MAT (Metadata-enhanced Audio Transmission). There is also an option in Dolby MAT encoding to support lossless PCM instead of TrueHD, but I don't believe that AppleTV employs this option yet. If it did, it would be a step up from TrueHD though the difference would be purely technical, imperceptible to us.

All home theater implementations of Dolby Atmos, whether TrueHD-based or Dolby Digital Plus-based, are different from the theatrical implementation of Atmos which is directly object based, supporting up to 128 discrete and fully assignable objects.