The Fundamentals of Atmos (1 Viewer)

[Doug2000]

I’m looking to confirm my understanding of how Dolby Atmos works fundamentally and then ask a few questions.

“Dolby Atmos is an Object Based Surround Sound system”. This is the first thing you read when you research Atmos – But when I first heard this phrase I had no idea what it meant. After some digging I realize what it means. Its basically just a mapping exercise.

Object Position) When the sound track is mixed the sound from a physical object (like an actor) is given a position in space using X,Y,Z coordinates.

Speaker Position) Additionally, when the Atmos system is setup for a specific room the position of each speaker is entered into the system using X,Y,Z coordinates.

When the movie is played on an Atmos system the sound comes across the wire (or off the disk) - not as a continuous stream by channel - but attached to software objects (for example an object representing an actor) with a position (x,y,z) in space representing the position of the actor within the scene. All the sounds the actor makes are attached to “his” object and its position in space.

With the above, the system knows both the Object Position and the Speaker Positions. With this and some porcessing power - in real time as the movie is played the Atmos playback system calculates which combination of speakers the sound for each object (the actor in my example) should be emitted from in order to place the sound (from the actor) in the theater room correctly. If the actor is front/left its going to calculate that the sound should come from a combination of the center and the left front speaker and it’s going to calculate the relative volume for each of these speakers to place it correctly. If the actor is closer to the audience perhaps a faint sound is also emitted from the rear left speaker to pull the actor's voice closer to the audience in the room.

In addition to the x,y coordinates Atmos also uses a z coordinate for height. So the mapping is in 3D. And in a real movie there are many object placed in different position at the same time so the playback system is (in real time) calculating the speakers/volumes to be used for all these objects. And, of course in many cases like a fly buzzing around the room (or a jet flying past), objects are moving and these calculations (and the speakers being used for a given object) are therefore constantly being updated as we watch.

This video was very helpful - especially starting at the 5:00 mark.


Questions

• Does my above narrative accurately represent the way Atmos works?
• If this is how Atmos works when I setup my Atmos HT receiver – why aren’t I required to enter the position coordinates of all my speakers?
• Is the setup different when a top end commercial Atmos system is setup in a commercial theater with many more speakers (do they enter speaker positions)?

 

[JohnRice]

Notice when Atmos is discussed, it’s usually conceptual, and refers to theatrical Atmos. Well……. HOME Atmos is limited to 16 objects (channels) and 12 of those are dedicated to the “core” (bed) (7.1.4) objects, which doesn’t leave much to enhance it, and some studios (Disney) never use any of the remaining four objects.

There is a dark secret to home Atmos. The bottom line is, it‘s not remotely the same as theatrical Atmos, and its debatable whether there’s a real benefit to setting up a home system larger than 7.1.4.

I can provide plenty more, but I’m on my iPad at the moment. I’ll come back later.

 

[JohnRice]

...also, on that video, he says 7.1.4 is eight channels. I think he needs to check his math.

 

[Keith Cobby]

Many members seem to be very keen on Atmos, but here's my question: if I enter a room with a film playing, how would I know if the sound was Atmos.

 

[Josh Dial]

Notice when Atmos is discussed, it’s usually conceptual, and refers to theatrical Atmos. Well……. HOME Atmos is limited to 16 objects (channels) and 12 of those are dedicated to the “core” (bed) (7.1.4) objects, which doesn’t leave much to enhance it, and some studios (Disney) never use any of the remaining four objects.

There is a dark secret to home Atmos. The bottom line is, it‘s not remotely the same as theatrical Atmos, and its debatable whether there’s a real benefit to setting up a home system larger than 7.1.4.

I can provide plenty more, but I’m on my iPad at the moment. I’ll come back later.

In addition to what John wrote, I would add that many studios (though it can sometimes be more granular than studios) don't even use Atmos correctly. I think far too many releases "cheat" in the way they have sounds effects seem like they are discrete objects when in fact they are not discrete objects.

It's a bit complicated to explain, but many Atmos tracks don't actually have discrete objects at all, beyond the 12 bed objects. You can really only see this if you have a Trinnov because of its object viewer software. Instead, many tracks have sound effects "flattened" into the height channels. Sure, you might hear a helicopter "move" across your two or four height channels, but the helicopter isn't actually it's own discrete object: it's simply part of the audiotrack (no different than the music or any other sound) and an engineer has "moved" the effect across the soundscape. That's partly why moving to six height channels (for example) doesn't do much. In this example, adding two more height channels wouldn't result in the helicopter moving across all six in a very obvious and designed manner. Instead, four of the six would contain the same information OR two of the six (likely the middle) would be completely silent. In a "proper" Atmos track, the helicopter would be its own discrete object and it would ask your system "hey how many speakers y'all got? Oh, you have whopping eight height channels? Wow! Okay, I'll move across all eight of them! here I go...wheeeeee!"

That all said, I think Atmos is terrific when done right. Frankly, I find myself more impressed with a good Atmos track than I do with a good 4k transfer.

I agree with John that there is likely little benefit for most people beyond a 7.x.4 system. Though I would suggest two subs in almost every setup.

 

[John Dirk]

My take on home Atmos has been "If I build it, it will come." Based on that philosophy I built myself a 7.2.4 system and started ordering titles I thought would best exploit Atmos' potential. Some titles are objectively better than others today but I'm hoping future releases will begin to more consistently implement the format in a way that better exploits its true potential. If and when that happens, at least I know I'll be ready.

 

[Doug2000]

Many members seem to be very keen on Atmos, but here's my question: if I enter a room with a film playing, how would I know if the sound was Atmos.

Of course my question is on a fundamental/theoretical level. Once I understand the theoretical goal I can more easily spot the compromises and infer the practical advantages.

OTH your question (a good one) jumps straight to the practical. To that end I would guess the thing that would be most noticeable with Atmos (vs most others) is the sound coming from the ceiling - if you happen to walk into an Atmos movie/scene that uses height.

 

[Doug2000]

In a "proper" Atmos track, the helicopter would be its own discrete object and it would ask your system "hey how many speakers y'all got? Oh, you have whopping eight height channels? Wow! Okay, I'll move across all eight of them! here I go...wheeeeee!"

Yes - agreed. But to do that the system needs to know where the speakers are physically located in the room. If its a home theater and you just have 4 on the ceiling then assuming where they are located seems practical.

But if you are in a commercial theater equipped with Atmos and you have 16 or say 20 ceiling speakers, the systems need to know exactly where those speakers are located in space to pull off the "fly across the speakers/wheeee" effect. So my assumption is in the setup phase for commercial implementations each speaker is mapped using x,y,z coordinates. Wonder if anyone in this forum could verify this?

 

[John Dirk]

So my assumption is in the setup phase for commercial implementations each speaker is mapped using x,y,z coordinates. Wonder if anyone in this forum could verify this?

I'm no expert but I do not believe it needs to be done this way as the coordinates are used to place a particular "object" in 3D space using a possible combination of different speakers to do so. Dolby Labs provides recommended placement guidelines for both home and commercial setups of varying sizes.

 

[JohnRice]

But if you are in a commercial theater equipped with Atmos and you have 16 or say 20 ceiling speakers, the systems need to know exactly where those speakers are located in space to pull off the "fly across the speakers/wheeee" effect. So my assumption is in the setup phase for commercial implementations each speaker is mapped using x,y,z coordinates. Wonder if anyone in this forum could verify this?

Yes. The setup for a commercial theater is completely different, and much more complicated. No automation involved. Everything is mapped, if it's done correctly. Also, commercial Atmos has the capability for (as I recall) 168 objects, vs 16 for home.

So, as Josh also dug into, home Atmos soundtracks have 12 "bed" channels. That is the basic 7.1.4 configuration. Those channels (objects) are not object oriented. They are no different than the 5.1 or 7.1 discrete channels we've had for 30 years. There's just more of them. Then there is the capability for an additional four "object based" channels, which are not tethered to any individual speaker, but can roam independently, such as the "joystick" example in the video. A lot of home Atmos soundtracks actually don't use these object based channels. Instead, the sound engineer just hard codes the entire soundtrack into the 7.1.4 bed channels. In reality, if you have a 7.1.4 or lesser system, it makes zero difference, so it's not a negative at all. The real down side (I see it almost as a scam) is, as Josh pointed out, you expand beyond 7.1.4, any additional (middle) surround and overhead speakers are only sent audio from the four object based channels, which often don't even exist. Now, this can be overcome with additional processing, but that involves synthesizing virtual channels like with Dolby Pro Logic.

Anyway, the takeaway is that anyone wanting to spend the significant $ to expand beyond 7.1.4 really needs to know the serious limitations they will face. Manufacturers do NOT make this clear, since they want to sell you more stuff.

BTW, regarding subwoofers, I've decided to always talk about one subwoofer channel, because there is only one subwoofer channel in soundtracks. You could have 20 subwoofers, but there's still only one subwoofer channel in the soundtrack. I have dual subs, but that doesn't technically means it's 7.2.4, since it's running both subs off one channel from the preamp. I'm just trying to be accurate., Yeah, there are receivers and processors that have independent sub outputs, but the soundtrack still only has one sub channel. There can be more to it, since the subs also play audio that's crossed over from the other channels, but, again, the soundtrack only has one sub channel.

 

[Carlo_M]

I have a slightly different interpretation of how I use the speaker numbering convention. I use it to describe my current system so that if I'm reviewing a soundtrack, people know what system I have. So currently I use 7.2.4 so people know I have seven floor speakers (L/C/R/SL/SR/SBR/SBL) 2 subs and 4 overhead speakers.

Because not everything is in Atmos. So if I'm reviewing an old mono or stereo film, I would hope people understand I'm not spreading it out to all channels.

The reason I think it's important for me to point out the dual subs, is that before I had two subs I would have had different reviews of a soundtrack's sub quality. When I had one sub, in order to get good low bass extension in my room I would have to run it at a level where the mid-bass would be a tad too hot (because of natural dB drop off of subs at the lowest frequencies). Now that I have dual subs dialed in, I can get the deep 20hz playback without inappropriately boosting the 50-80hz range.

 

[Tom McA]

Notice when Atmos is discussed, it’s usually conceptual, and refers to theatrical Atmos. Well……. HOME Atmos is limited to 16 objects (channels) and 12 of those are dedicated to the “core” (bed) (7.1.4) objects, which doesn’t leave much to enhance it, and some studios (Disney) never use any of the remaining four objects.

There is a dark secret to home Atmos. The bottom line is, it‘s not remotely the same as theatrical Atmos, and its debatable whether there’s a real benefit to setting up a home system larger than 7.1.4.

I can provide plenty more, but I’m on my iPad at the moment. I’ll come back later.

There's no dark secret about it. It's just the reality of mainstream consumer device capabilities. There's no way that most home processors can handle a cinema object routing load, so objects are clustered toward their most logical speaker location in a process called Spatial Coding. Spatial Coding is dynamic (i.e. objects can hop from one cluster to another), and is automatically applied during encoding. So "limited to 16 objects" is false. Clusters, yes, but all objects (up to 118) are still represented in the mix, unlike certain competing "object based" codecs for home. And the premise about Disney's present-day object usage is also wrong, or at very least, outdated.

 

[JohnRice]

Current home Atmos is limited to 16 channels, which in the lingo of Atmos are also called "Objects". Nothing is false in that statement.

Ignoring the bulk of my actual explanations to nitpick the meaning of that one word only causes more confusion.

My goal was to explain the complexities and limitations of home Atmos in systems beyond 7.1.4.

 

[JohnRice]

Just to try to clarify again...

A movie can really have an unlimited number of individual sounds in them, and all of those sounds can be hard coded into the 7.1.4 "bed channels" of the Atmos soundtrack. It's the independent, "object based" part of home Atmos that is heavily advertised, but quite misleading, since there are only four channels left over from the total of 16 to be free-roaming "objects". Many studios never even use any of those four channels, and leave it to only the 12 hard coded "bed channels".

That is my point. That is fact.

 

[Tom McA]

I'm not ignoring your explanation - I'm disagreeing with it. Clustered objects can roam across all 16 "channels", and it's not true that there are only four channels for free-roaming objects. And "many studios never even use any of those four channels" is also untrue. It's up to individual rerecording mixers and post facilities. There are no studio-wide mandates (including at Disney, which you're also off-base about...) to use or not use objects or those "four channels" you keep referring to. If you want to examine a "dark secret" (your term), then look at the competing immersive audio codec, where most released content is absolutely nothing but channel-based 7.1.4.

 

[JohnRice]

What I'm trying to explain is being misinterpreted in order to win an argument. I have no interest in winning an argument. My only interest is to convey factual information to the best of my ability.

So I'll just try to figure out with a better way to explain it for the next time the subject comes up.

Plus, none of this makes any difference with systems of 7.x.4 or less. Only when more surround or overhead speakers beyond that are used.

 

[Doug2000]

Current home Atmos is limited to 16 channels, which in the lingo of Atmos are also called "Objects". Nothing is false in that statement.

Ignoring the bulk of my actual explanations to nitpick the meaning of that one word only causes more confusion.

My goal was to explain the complexities and limitations of home Atmos in systems beyond 7.1.4.

I don't think this is correct. When you first posted this idea I was confused as you seemed to be using the term Object interchangeably with Channel. They are not the same at all.

In Atmos, Objects are the sound elements in the movie. For example, if I have a fly buzzing around in the sound stage the fly's sound is represented by an Atmos sound Object. This object has meta data describing it including its position in 3D space represented by X,Y, Z coordinates. As the fly moves the coordinates change accordingly. The system calculates which speakers (ie Channels) the buzzing from the fly should come from, and at what level, to place the fly in the correct position. (Which seems like it would require a bunch of simple distance formulas computing distance from the objects to each speaker.)

There's no dark secret about it. It's just the reality of mainstream consumer device capabilities. There's no way that most home processors can handle a cinema object routing load, so objects are clustered toward their most logical speaker location in a process called Spatial Coding. Spatial Coding is dynamic (i.e. objects can hop from one cluster to another), and is automatically applied during encoding. So "limited to 16 objects" is false. Clusters, yes, but all objects (up to 118) are still represented in the mix, unlike certain competing "object based" codecs for home. And the premise about Disney's present-day object usage is also wrong, or at very least, outdated.

I believe in a movie theater scenario these calculations are done in real-time. Still not sure in home theater as home Atmos content appears to use pre-determined speaker locations so the calcs could easily be done ahead of time requiring less processing power (essentially converting it to a channel based system). Not sure? On the other hand, Clustering appears to be the grouping of sound objects such that these calculations only have to be done once for all the objects in the cluster thereby reducing the number of calculations required and reducing processing requirements. Is that correct?

 

[JohnRice]

It is so hard to explain. A channel is not merely an object, when it's one of the 12 bed channels, but an object is a channel in the sense that it occupies one of the remaining four channels. Even that isn't quite right, but it gets the concept across.

It's very important to distinguish between theatrical Atmos and home Atmos, which are completely different animals.

 

[Carlo_M]

I have no dog in this race (I just enjoy Atmos as a consumer, I'm no surround audio techie) but maybe an explanation direct from the horse's mouth will help:

Dolby's own PDF from their website about what Atmos is and how it works

See page 4 of that Dolby PDF. First their description of Atmos in general:

Dolby Atmos and sound objects

Dolby Atmos is based on the concept of sound objects. In the cinema, Dolby Atmos relies on a combination of 9.1 ‘‘bed’’ channels and up to 118 simultaneous sound objects to deliver an enveloping sound stage. Every sound in a scene----a child yelling, a helicopter taking off, a car horn blaring----can be a separate sound object. Each of those sounds comes from a specific location in the scene, and in some cases, they move. The car careens from left to right, while the yelling child runs up a set of stairs.

Using sophisticated content creation tools that represent the sound objects in a three-dimensional space, filmmakers can isolate each of the sound objects in a scene and decide exactly where they want them to be and how they want them to move. In the final sound mix, the sound objects are combined with positional metadata----additional data that describes a number of parameters about the sound object, including its location and movement, if any.

And then for Home Theater

Dolby Atmos in home theater

Dolby has developed the technology required to translate the Dolby Atmos experience in cinema to home theaters. In the case of the home theater, every sound in the mix is represented as an audio object. When you set up your Dolby Atmos enabled AVR, you inform your receiver how many speakers you have, what type of speakers they are (large, small, overhead, and/or Dolby Atmos enabled), and where they’re located. Armed with this information, a sophisticated processor in your AVR----the Object Audio Renderer or OAR----analyzes the positional metadata and scales each audio object for optimal playback through the connected speaker system. This process include includes determining in real time exactly which speakers it needs to use from moment to moment in order to reproduce the sounds of the car careening across the screen and the child fleeing up the stairs. That detailed, very specific movement of sound helps your brain suspend disbelief and feel as if what you’re watching is real. 4 Key to the reproduction of this three-dimensional soundfield is the creation of a layer of sound above the listener. This is achieved through the introduction of overhead or Dolby Atmos enabled speakers in the home theater system. Later we will describe how that is achieved.

The flexibility of Dolby Atmos object-based sound makes it incredibly adaptable. A Dolby Atmos movie can be played back on nearly any speaker configuration in the home. You’ll be able to hear the placement and movement of sound in a Dolby Atmos movie whether you have a system with five speakers on the floor and two overhead or 24 speakers on the floor and 10 overhead (the current maximum for a Dolby Atmos supersystem) or any variety between. The more speakers you have, the more precise the audio positioning becomes. And you have lots of flexibility to upgrade your system. Adding more speakers to the system will provide a higher level of object resolution and even more detailed, richer sound.

 

[JohnRice]

I have no dog in this race (I just enjoy Atmos as a consumer, I'm no surround audio techie) but maybe an explanation direct from the horse's mouth will help:

Dolby's own PDF from their website about what Atmos is and how it works

See page 4 of that Dolby PDF. First their description of Atmos in general:


And then for Home Theater

That's a perfect example. Notice that is specifies how many objects (128, though I had gotten the idea it was 168) the cinema version of Atmos can have, but on the home version it only talks about the number of speakers. That's because they don't want to spell out that the total number of objects home Atmos can have is 16. They also play games with wording "every sound in the mix is represented as an audio object". In Atmos, every channel can be referred to as an "object", even if it is one of the locked, 7.1 (or as many as 7.1.4) bed channels.

In late 2021 I got into a conversation with Dave Upton, one of the former owners of HTF, on this topic and he led me to some very detailed information. Dave is an excellent source for reality-based info.

Reading through that info again, I did make one little mistake. I said that there are typically 12 channels locked as bed channels, meaning they are dedicated to a speaker, rather than a location in space. That's not always the case. However, there are virtually always eight bed channels (the core 7.1 soundtrack) that are locked to a standard 7.1 surround system. So, half of the 16 available channels (which can be bed channels or dynamic, moving objects) are used up for the base surround. It's also not unusual for another four to be locked to the four overhead channels, leaving just four dynamic ones.

In reality, it's actually not a bad thing to allow the sound engineer to "finalize" the soundtrack by creating a fixed 7.1.4 soundtrack with zero dynamic objects, since it tends to produce a more reliable surround effect. And yes, it does appear Disney prefers this approach. The only drawback to that is if someone has a system larger than 7.1.4, those additional speakers will never produce sound. The upside is, the other 99% who don't have a system larger than 7.1.4 will probably get a better overall surround effect.

The most important detail is, none of this makes ANY difference for anyone who has a system of 7.1.4 or less. And, how many people have systems beyond 7.1.4? In fact, what got me looking into this 18 months ago was someone who expanded their system to 9.1.6 and then got angry when they discovered the middle surround and overhead speakers rarely if ever produced any sound. That's what got me curious. Not to shoot holes in Atmos, which I think is pretty awesome, but to understand that as things are currently, it doesn't make much sense to spend on a setup larger than 7.x.4. So, anyone who wants to ignore this and invest in a 24.6.10 system, go right ahead, but don't come crying to me when most of it rarely produces any sound. For the rest of us with 7.x.4 or less systems, it's just interesting information.