DTS:X vs Dolby Atmos. Part I: Speaker Layout

Both DTS:X and Dolby Atmos are relatively new audio formats that utilize objects instead of classical channel layouts. The metadata for the objects and their position in three dimensional space tag along either DTS HD/MA or Dolby TrueHD/Dolby Digital Plus. An Atmos or DTS:X processor then uses this metadata to position the sound across the available channels. The idea behind this is that regardless of the theater or home theaters setup the processor can accurately present the location of a sound regardless of both the number of channels and the configuration of those channels. In theory it shouldn’t matter if you are using 5.1.2, 5.1.4, 7.1.4 or 24.1.10, you should get the same relative imaging across the sound field. In the cinema, Atmos offers support for up to 64 individual channels, and up to 34 in a home theater. My guess is the majority of HT enthusiasts will be pushing both budget and space  constraints with a 7.1 channel or 9.1 channel system, let alone a 34 channel system, but, it’s always nice to have the option (assuming AVRs and processors support that many channels). DTS:X supports a maximum of 30.2 channels in the cinema, and 11.2 in the home.

dolby-atmos-speaker-placement-diagram
Atmos Cinema layout example found in the Dolby Atmos White Paper

 

 

One of the things that’s supposed to set DTS:X apart from Atmos is their claim that it can support any channel layout. In theory, you should be set up your system however you want (within the limits of your AVR’s configurations) and it should work regardless, Unlike Atmos’ prescribed layouts. One benefit of this is that it should be compatible with the Auro 3D 10.1 layout, which Atmos is not. Atmos does not support the Voice of God channel (a single overhead speaker) nor does it support Side Heights or Front Top Center. DTS:X should be able to. Dolby doesn’t come right out and say their system supports many different speaker layouts, but it clearly does, considering it can be scaled from 8 channels to 34 in the home. In addition to height/overhead channels, front wides, rear wides, rear centers, and everything in between is supported outside of placing height channels above the surrounds. Atmos requires height channels, while DTS:X does not. This is good, considering not everybody has the ability or desire to add overhead speakers (and dolby enabled speakers really suck in my experience), and object based audio definitely sounds much more precise than a simple 7.1 mix utilizing panning/phase manipulation.

The biggest question I have is does it actually work the way DTS:X claims, and how does the ability to scale to different layouts compare between Atmos and DTS:X? That’s what I set out to discover. Currently I have an Onkyo TX NR-656. It’s a 7.2 receiver with both DTS:X decoding and Atmos decoding capabilities. Obviously, it only allows me to use one pair of height speakers, either Top Front, Top Middle, Top Rear, Rear Height, and Top Height. Dolby recommends using Top Middle speakers for a 5.1.2 config, and that’s what I had originally set my system up with, similar to this:5.1.2

Before we delve into whether or not DTS:X really works with any layout, let’s look at what DTS:X classical layouts look like.

DTS has no recommendations on how to configure your system on their site for DTS:X, obviously, if it’s supposed to be compatible with any layout, you shouldn’t need to follow any prescribed setup. But what about the studios who mix in DTS:X? What setup are they using? I managed to find a few pictures of a studio setup here.

Looks to me like classic 11.1 with 7 bed channels and 4 height (not top) channels.

How will this translate to my 5.1.2 setup with a single pair of Top Middle speakers? To find out, I first used the DTS:X Object Emulator. It’s basically a floating ball that moves around different channel layouts, the sound follows the ball. You can see a preview of it here (in stereo obviously). The demo file can be downloaded from The Digital Theater.

The first thing I noticed, DTS:X does a really good job of making 5.1 sound more like 7.1, Atmos does this too. In Dolby Atmos’ guidelines for home theater, 5.1.x setups specify that the surrounds are placed at 110 degrees vs 90 degrees, and directly at ear level. Originally it was recommended that the surrounds be placed about 2 feet above ear level for traditional 5.1 and 7.1, however, now that overhead speakers have been added, this is no longer necessary and actually detrimental to getting good separation between overhead sounds and floor sounds. With traditional 5.1, a 7.1 track is downmixed to 5.1 by simply folding the rear channels into the surrounds, In DTS:X and in Atmos, things passing through the rear channels are phantomed between the surrounds and the appropriate phase delay is added, so it does sound like it’s coming from behind, similar to how a phantom center channel appears in classic stereo. Off to a good start, we’re getting rear channel/surround channel seperation with two surrounds.

What about overhead sounds though? How does that translate to my Top Middle Atmos configuration? As you can see from the Object Emulator Demo, there is no Top Middle speakers, only Rear Height, and Front Height. dts object.png

Right off the bat I’m assuming that the DTS:X software is going to create a Phantom image for the front and rear heights by mixing it between the fronts/surrounds and top middle speakers. Turns out that wasn’t the case. As the ball passes by the front and rear heights, the top middle speakers remain silent, and the sound is mixed exclusively in the front and surround bed channels. The only time I hear anything coming from overhead is when the ball is directly in the middle above. Not good. Quite a bit of height information is lost this way. What if a helicopter in a movie is approaching from the front or rear? Unless it passes overhead, you’re not going to hear it from the heights at all. With Atmos, I’ve not had any trouble like this. All height information is mixed into the top middle speakers and then phantomed between those and either the front or surrounds, the way that it should be.

This got me thinking, perhaps DTS:X simply doesn’t jive well with a top middle configuration. To see if this was the case, I simply changed my configuration via the setup menu in my receiver to front height and repeated the demo. I immediately noticed a whole lot more sound coming from the height speakers, not just when it passed through the front heights, but also the rear heights. As the ball approached the rear heights, the sound was mixed between the rear speakers and heights, with the heights changing phase as the ball approached the rear. The same thing happened when I reconfigured the heights to rear heights. What about top front and top rear? Same thing, it works.

I set the config back to top middle and decided to try a movie mixed in DTS:X. For this, I picked The Girl on the Train. I watched the entire movie with the top middle configuration, and not once did I ever hear anything come from the height speakers. Changing them to front or rear heights again, solved this problem, with plenty of height information coming from the top.

From this, we can conclude that DTS:X certainly doesn’t just work with all configurations. Perhaps with a greater number of speakers, a top middle pair would work well as an addon, but only with front and rear heights as well. In all configurations, I have yet to see top middle speakers in any studio configuration for DTS:X, and it’s obvious DTS had height speakers, rather than top speakers, in mind when designing the format. This is certainly something to keep in mind when setting up a 5.1.2 system.

It worth noting that Atmos seems to work well no matter what setup I choose, Top front/middle/rear, or Front height/rear. Sounds come from the appropriate location. With this in mind, I would suggest designing an Atmos/DTS:X system that includes only a single pair of height speakers with either Top fronts, or Front heights for maximum compatibility.

Aside from the top middle problem, there is no discernable difference between the way the two work when it comes to panning sounds across a 3D sound stage.

In part II, we will look at the differences and compare between the two Upmixing modes, Dolby Surround and DTS: Neural X.

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