Atmos and TrueHD 7.1 playback on 5.1 systems - Tests, Results, questions, experiences

[artwwweb]

I do not have an Atmos system, but I understand about 'objects' from a computer programming perspective. So this is what I think should/ does happen. Please let me know if I've misunderstood or you have something to add.

'Objects' means that the mixer can place sounds in a 'virtual' position - i.e. an ideal location that does not have to exactly match the location of a speaker. This is the crucial difference between an old 5.1 or quad mix and an Atmos mix. In the old mixes you assign a sound channel to a particular speaker. The Atmos decoder on playback decides the best speaker(s) to play this sound from. This is similar to playing a 5.1 mix on an AVR set to 4 quad speakers - the AVR knows to create a 'phantom centre' which is actually coming from both front speakers. It's just that on Atmos the decision is not hard-coded, but is calculated depending on your speaker layout.

So... in theory the Atmos mix should closely approximate the original mix. I would therefore have expected that if you had no side speakers, the sound would come from both front and rears, to create a 'phantom side'. If 'side' sounds are always played from the fronts, this sounds like a hard-coded translation from the full Atmos mix, which does not fit in with my understanding of how Atmos should work. I wonder if the problem might be that some mix engineers do not understand Atmos. My understanding is that although you can mix to objects, you can also mix to 'beds', which seem to be much more like the old-fashioned channel assignments. Maybe the beds are only ever downmixed in a fixed fashion? I do not know if this is true, but it feels like a possibility. (I have heard/ read at least one mix engineer say they usually use beds not objects.)

 

[AYanguas]

I do not have an Atmos system, but I understand about 'objects' from a computer programming perspective. So this is what I think should/ does happen. Please let me know if I've misunderstood or you have something to add.

'Objects' means that the mixer can place sounds in a 'virtual' position - i.e. an ideal location that does not have to exactly match the location of a speaker. This is the crucial difference between an old 5.1 or quad mix and an Atmos mix. In the old mixes you assign a sound channel to a particular speaker. The Atmos decoder on playback decides the best speaker(s) to play this sound from. This is similar to playing a 5.1 mix on an AVR set to 4 quad speakers - the AVR knows to create a 'phantom centre' which is actually coming from both front speakers. It's just that on Atmos the decision is not hard-coded, but is calculated depending on your speaker layout.

So... in theory the Atmos mix should closely approximate the original mix. I would therefore have expected that if you had no side speakers, the sound would come from both front and rears, to create a 'phantom side'. If 'side' sounds are always played from the fronts, this sounds like a hard-coded translation from the full Atmos mix, which does not fit in with my understanding of how Atmos should work. I wonder if the problem might be that some mix engineers do not understand Atmos. My understanding is that although you can mix to objects, you can also mix to 'beds', which seem to be much more like the old-fashioned channel assignments. Maybe the beds are only ever downmixed in a fixed fashion? I do not know if this is true, but it feels like a possibility. (I have heard/ read at least one mix engineer say they usually use beds not objects.)

I think it is exactly as you say.

It should be that the render of an Atmos object location would use the existing speakers to image that location. Several speakers if needed. This is the technology that Dolby sells, and I think it would be correctly implemented by the AVR manufacturers.

But it looks that when the mix use a bed channel (associated to a physical speaker), it is not imaged using several speakers if the bed channel speaker is missing in the instalation, but redirected to the 'nearer' speaker. My previous tests with the Atmos speakers test file seem to prove that. The only "imaged" results work with Top Middles (x.x.6) when you have only 4 Heights. And this should be because the test mix use Atmos 'objects' for the Top Middlkes instead of bed channel. I think that there is only Two bed channels for the Atmos Heights. Someone could confirm this?

In my 9.1.4 (with Wides) I find myself many Atmos mixes that does not use the Wides at all (neither discrete or for any panning). Looks like a 'fixed' mix to 7.1.4. On the other hand I find also some good mixes that use the Wides and I feel more 'spaciality'. They would use 'objects' in that case.

It is also known some film Atmos mixes from Disney that were mixed in a 'fixed' 7.1.4. So people with several seats rows that had 6 heights, would not get any sound from the Top Middle speakers. Or even worse, if the mix is 'fixed' to 7.1.2 bed channels, there is only a single stereo pair of heights, going to the Top Middles and not going to the Top Fronts and Top Rears.

The use of beds instead of objects could be chosed by some mixer engineers for any number of reasons. Some of them could be:
- Easy to manipulate the tools or other sound settings, when applied to beds vs to objects.
- Final result bigger with Objects with TrueHD or more lossy sound quality when using DD+ due to the restricted bandwith.
- Learning curve of the mixer engineer skills.
- The quality of the calibration mix studio speaker installation used to test by the mixer engineer.

 

[artwwweb]

I found this article which confirms there are only 2 height beds and that beds are channel-based. This would still not be a problem if the translations to a lower number of speakers was handled better. But this puts a lot of responsibility on the engineer to test their mix on layouts with fewer speakers or truly understand how it will be interpreted in different circumstances. If it is being sold to them as "don't worry about it, Atmos will sort it out for you" then it's going to lead to problems and they will have no idea (why) it's happening.

https://www.pro-tools-expert.com/ho...ow-about-dolby-atmos-home-entertainment-r57h5

 

[mandrix]

I put this together to show what a complicated room I have. The reason is because of doors, a queen size bed, a 42" tall computer, another tall pc, a 6 ft table with two speakers and my AVR's, a double dresser, a tall chest of drawers and a desk....well you get the picture. Thank goodness for Dirac room adjustment. It won't compensate for everything but it does a pretty good job.

 

[barfle]

Some titles translate really well, others less so. I think it's worth exploring in any case, as there's so much music available in this format.

Maybe already addressed, but a disc player won’t decode Atmos. The “virtual” objects mean that the overhead speakers need to be calibrated for the room, and that’s done in an AVR or pre-pro.

 

[sjcorne]

Maybe already addressed, but a disc player won’t decode Atmos.

As mentioned earlier in this thread - a Blu-Ray player will internally decode the Dolby TrueHD 7.1 'core' stream on an Atmos Blu-Ray to PCM 7.1, with the height information instead emanating from the corresponding ear-level positions (top fronts -> fronts, top rears -> rears). Nothing is lost, the object metadata is simply ignored.

 

[mandrix]

I realize everyone is not set up for it. But I'll stick with the pc as playback device. The Blu Ray Player is not going to decode Atmos anyway, it's just another piece of hardware in the chain to me that's not necessary.

 

[Sal1950]

I realize everyone is not set up for it. But I'll stick with the pc as playback device. The Blu Ray Player is not going to decode Atmos anyway, it's just another piece of hardware in the chain to me that's not necessary.

I use both. The disc player for Atmos discs and Kodi for mkv encoded Atmos files, both via HDMI into my receiver.

 

[Owen Smith]

No. But that is totally subjective. I've listened to the Atmos down mixed to 7.1 in Dolby True HD. It sounds different not better. The Atmos mix played in Atmos sounds better, a lot better.

Down mixed is not how Atmos works. All sounds are present in the 5.1 or 7.1 bed, they have to be otherwise someone playing it through a Dolby True HD decoder that doesn't understand Atmos would have things missing. The Atmos object data both says what the sound is and how it moves, and how to remove it from the bed. If it didn't work that way every Atmos blu ray would need two True HD tracks, once for the Atmos mix and one to play on devices that don't understand Atmos.

 

[key_wiz]

Down mixed is not how Atmos works. All sounds are present in the 5.1 or 7.1 bed, they have to be otherwise someone playing it through a Dolby True HD decoder that doesn't understand Atmos would have things missing. The Atmos object data both says what the sound is and how it moves, and how to remove it from the bed. If it didn't work that way every Atmos blu ray would need two True HD tracks, once for the Atmos mix and one to play on devices that don't understand Atmos.

Understood, but then why do they have a separate 5.1 mix?

Or don't they and it's just a menu item that directs to the same tracks?

I do know that on many/most of my discs the 5.1 mix sounds different than the Atmos played through 5.1 (I don't have an Atmos setup). Or is that just my imagination?

 

[Owen Smith]

Understood, but then why do they have a separate 5.1 mix?

Why do previous Pink Floyd blu rays have LPCM and DTS HD MA tracks despite the fact they will play bit for bit identically? We don't know, Floyd do strange things.

Or don't they and it's just a menu item that directs to the same tracks?

I do know that on many/most of my discs the 5.1 mix sounds different than the Atmos played through 5.1 (I don't have an Atmos setup). Or is that just my imagination?

I don't have any discs with both a 5.1 mix and a separate Atmos mix, everything I have is one or the other and the Atmos mixes just play as Dolby True HD 5.1 or 7.1 on my Oppo 95.

 

[key_wiz]

Why do previous Pink Floyd blu rays have LPCM and DTS HD MA tracks despite the fact they will play bit for bit identically? We don't know, Floyd do strange things.

I don't have any discs with both a 5.1 mix and a separate Atmos mix, everything I have is one or the other and the Atmos mixes just play as Dolby True HD 5.1 or 7.1 on my Oppo 95.

All the SDE discs I own have both. As does Abbey Road and Let It Be. Off the top of my head.

 

[Owen Smith]

Down mixed is not how Atmos works. All sounds are present in the 5.1 or 7.1 bed, they have to be otherwise someone playing it through a Dolby True HD decoder that doesn't understand Atmos would have things missing. The Atmos object data both says what the sound is and how it moves, and how to remove it from the bed. If it didn't work that way every Atmos blu ray would need two True HD tracks, once for the Atmos mix and one to play on devices that don't understand Atmos.

Here's something I found with a bit more explanation:

https://avgadgets.com/downmixing-dolby-atmos-what-are-you-missing/

 

[Bill B]

Here's something I found with a bit more explanation:

https://avgadgets.com/downmixing-dolby-atmos-what-are-you-missing/

So I have to ask... If there is a new Atmos mix (or bed, or whatever you want to call it) and that new Atmos mix is downmixed to stereo by whatever means is used (this article states that you aren't losing anything), then aren't you ending up with a different stereo mix (than 1973) by default? (call it a happy accident)

 

[rontoon]

So I have to ask... If there is a new Atmos mix (or bed, or whatever you want to call it) and that new Atmos mix is downmixed to stereo by whatever means is used (this article states that you aren't losing anything), then aren't you ending up with a different stereo mix (than 1973) by default? (call it a happy accident)

Not quite. It's just like taking a stereo mix and running it through your system in one of the fake multichannel options. That's different than an actual multichannel mix.

 

[Bill B]

Not quite. It's just like taking a stereo mix and running it through your system in one of the fake multichannel options. That's different than an actual multichannel mix.

OK, but what you would hear out of each speaker, if you compare the 1973 stereo option to the Atmos option on the menu, would be different, right?

 

[Owen Smith]

OK, but what you would hear out of each speaker, if you compare the 1973 stereo option to the Atmos option on the menu, would be different, right?

Depends how much they tried to make (or accidentally ended up with) a 5.1 or 7.1 bed that downmixes to the same stereo as the original stereo mix.

 

[minimumtumbleweed]

Why do previous Pink Floyd blu rays have LPCM and DTS HD MA tracks despite the fact they will play bit for bit identically? We don't know, Floyd do strange things.

I don't have any discs with both a 5.1 mix and a separate Atmos mix, everything I have is one or the other and the Atmos mixes just play as Dolby True HD 5.1 or 7.1 on my Oppo 95.

Really? There are plenty of discs with both. In fact, every disc I own that has an Atmos mix also has a separate, different 5.1 mix. I'd say discs with only one or the other are the exception.

 

[Owen Smith]

Really? There are plenty of discs with both. In fact, every disc I own that has an Atmos mix also has a separate, different 5.1 mix. I'd say discs with only one or the other are the exception.

Given I have a 5.0 setup I've not been looking for Atmos discs, and I've only ended up with one when it's the only option for the release. So I may have an unusual selection, generally films not audio discs.

 

[kfbkfb]

I ordered this yesterday - a person can never have too many mixes of DSotM

One issue about downmixing: out of phase content is lost (this is a problem with stereo to mono and CD-4 type quad to stereo and mono).

I hope the required DD 5.1 (when a Blu-ray title has a Dolby TrueHD/Atmos soundtrack) is at the highest allowed data rate (the ABC SDE DD 5.1 isn't).


Kirk Bayne