Figuring out: Dolby Atmos

Figuring out: About this series
They say the best way to really learn about something is to force yourself to explain it to someone. That is the goal of this series. 
I will delve into a topic that I feel don't know enough about and explain my findings. Hopefully, we would both learn something useful!

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More than a gimmick?

Up until some months ago, Dolby Atmos was to me mostly about having speakers on the ceiling in the hope of attracting people back to the cinemas. After getting to know Atmos a little better, I wanted to see what it has to offer and if it is really going to be the new standard in professional audio. Consider this a 101 introduction on Dolby Atmos.

Surround Systems

Before Atmos, let´s start with something familiar. Surround systems have been used for decades to offer a more interesting audio experience for the listener. 5.1 and 7.1 are the more used formats for both cinemas and home setups.

Something important to understand about these systems is that they are channel-based. For example, a 7.1 system would offer us the following channels:

As you can see, these channels can be composed of just one speaker (like the central channel) or by several of them (like the left surround channel). We can send audio to any channel independently but we would have no control on how much is sent to each of the individual speakers that form a channel.

That is basically how all surround systems work, the only thing that varies is the amount of channels.

Dolby Atmos introduces two innovation to the table. Firstly, it uses an object-based approach on top of the previous channel-based system. Secondly, it expands the surround feel by adding speakers to the ceiling and unlocking 3D sound. Let´s look at both of these features:

Object-based

Dolby Atmos allows for 128 channels in total. We can use a certain amount of those for traditional channel-based stems and the rest for the new sound objects. 

Think about these sound objects as individual mono sounds that you can place and move around the room. If you place a sound object on a specific location, Dolby Atmos will play the sound on that location, addressing the nearby speakers individually as needed, regardless on how big the room is or how many speakers there are.

In other words, you are telling Atmos the coordinates of the sound instead of how much the sound is feeding each of the channels. It allows you to place sounds with great precision in big rooms but at the same time, the mix will translate well into smaller rooms or even headphones since Atmos is just using the coordinates of each sound object in 3D space.

3D Sound

The second innovation is probably the flashiest.

If you think about it, stereo is one dimensional, sound moves in a horizontal line. Surround audio is 2D, the soundscape is around you, on a horizontal plane. 3D is the next step: sound would be on a cube or a sphere.

Before Atmos, some surround 9.1 systems tried to achieve this by placing two speakers on top of the front speakers in order to give some "height" to some elements of the mix.

Dolby Atmos goes one step beyond adding speakers to the ceiling itself. Elements like ambiences, FX or music can now be placed overhead, opening the third dimension for the listener.

In theatres, these ceiling speakers usually go in two rows. There are also some extra surround speakers on the walls to make panning smoother when transitioning sounds between onscreen and offscreen. In total, up to 64 individual speakers are allowed on a theatrical Atmos installation.

At home, usually two or four overhead speakers are used, so you'll see configurations like 5.1.2 or 7.1.4. Note how the third set of numbers denotes the number of ceiling speakers. Up to 22 speakers are allowed on home setups.

Since installing ceiling speakers may not always be very practical on a home setting, sometimes sound is "fired" to the ceiling so that it bounces back to the listener giving the impression that it comes from above.

Crafting a soundscape with Atmos in mind

Knowing that a project will be mixed in Atmos changes the approach in terms of sound design and mixing, giving us more tools and challenges to achieve a compelling soundtrack.

For example, building ambiences now has an additional dimension. Imagine a scene inside a car while is raining. You could have different layers of the car engine and the city exterior and then the sound of the rain falling into the roof featured on the overhead speakers. A forest ambience could have discreet mono birds chirping above and around you, some of them static, some of them moving throughout the 3D space.

It's also worth noting that Atmos setups usually include one or more extra subwoofers close to the surrounds and overhead speakers. Although low frequencies are not very directional, it sill makes a difference in terms of sound placement to use the surround subwoofer instead of the one behind the screen.

Additionally, the Atmos standard makes sure that all surround speakers offer the same sound pressure level and frequency response as the onscreen ones. This means that while designing sound objects with a wide frequency range like a fighter jet going by overhead we have the whole spectrum at our disposal. This wasn't the case with previous systems, since the surround speakers did not have enough power and were best suited for simple atmospheric and background sounds.

Atmos makes you think more on where you want the audio to be in a 3D space rather than thinking about which channels and speakers to feed the audio to. It turns the mix into a full frequency canvas to position your elements.

Encoding for Dolby Atmos.

When preparing audio for Atmos, there are two distincts uses we can give to each of the available 128 channels. We can have sound objects as discussed above and we can also have channel-based submixes (beds). These beds can be created in any traditional channel-based configuration like 5.1 or 7.1 and are mapped to individual speakers or arrays of speakers the old fashioned way. In contrast, objects are not mapped to any speaker but saved with metadata that describes their coordinates over time.

This double approach (beds + objects) makes Atmos backwards compatible since we are also creating a traditional channel-based version when creating the masters.

To put all this information together we use a renderer. I won't go into a too much detail here, but Dolby basically offers two ways of doing this:

Dolby Mastering Suite + RMU:
This is the most advanced option, it is used for theatrical applications and Dolby certified rooms. It combines the Dolby Mastering Suite software with the Dolby Rendering and Mastering Unit (RMU), a dedicated Dell server computer that communicates with Pro Tools via MADI and processes all the Atmos information while compensating for any delays in the system. 

The RMU can be used for monitoring, authoring and recording Dolby Atmos print masters. It is also used for creating and loading room calibrations and configurations.

Note that the Dolby Mastering Suite software runs only on dedicated hardware (the RMU), while we would still need a different software package for any Pro Tools systems involved in the Atmos workflow. This would be the Dolby Production Suite, which I'm explaining below. The Dolby Mastering Suite includes three Dolby Production Suite copies but you can also buy the latter separately.

The mighty RMU

Dolby Production Suite:
This is the package that should be installed on the Pro Tools machines. It basically includes the renderer itself, a monitoring application and all the necessary Pro Tools plugins. In case you are using an RMU, this package will allow you to connect with it. If you are not, it will allow you to play, edit and record any Atmos mixes all within the same Pro Tools system.

While the Dolby Atmos Production Suite includes the ability to render Atmos objects, just like you can using the RMU, it has significant limitations. The software is an "in the box" renderer that runs on the same system as your Pro Tools session so if your project is large you may not be able to run it. Also, the software won't be able to compensate for any delays produced in the system.

Having said that, the Dolby Production Suite may be powerful enough for Blue-ray, streaming and VR projects with a limitation of up to 22 monitor outputs. For larger and/or theatrical projects an RMU is necessary, being capable of up to 64 outputs.

Dolby Atmos Everywhere

Atmos in home theatres is not rendered the same way as in cinemas because of limited bandwidth and lack of processing power. Close objects and speakers are clustered together conserving any relevant panning metadata. This simplified Atmos mix can be played through a home Atmos setup, like a 7.1.2.

Since ceiling speakers are cumbersome, home setups are becoming more accessible with the inclusion of sound bars and upward-firing speakers.

Blu-rays can carry an Atmos soundtrack and some broadcasting and streaming companies like Sky or Netflix are starting to offer Atmos content. The 2018 winter olympics was the first live event offered in Atmos.

In the world of video games, Dolby Atmos could be specially promising, enhancing the player's experience with immersive and expressive 3D audio. Currently, Xbox One, the PC and somewhat the PS4 offer dolby Atmos options via either an AV receiver or headphones (behind a paywall). There are a handful of titles ready for Atmos like Overwatch, Battlefield 1 or Star Wars: Battlefront.

Any Atmos mix can be scaled down into a pair of headphones. You don't need surround headphones for this, the Dolby algorithms convert all the Atmos channels into a stereo binaural signal that sounds around you in 360°. Some phones and tablets are starting to support this already.

Final Thoughts

It seems like Dolby Atmos is here to stay and become the new standard the same way stereo and surround sound replaced their older counterparts.

In my opinion, The key quality about Atmos is its object-based technology and scalability. Overhead 3D audio is very cool, but it may not be game changing enough and/or very accessible for the average user. It is still to be seen if binaural headphone technology and upward-firing speakers are going to be good enough to recreate the 3D feel that currently theatres can provide.