What is Dolby Atmos?

Posted by RosB on 3/17/2016
What is Dolby Atmos?

A Brief History of Film Sound Since the introduction of sound with film in 1927, there has been a steady evolution of technology used to capture the artistic intent of the motion picture soundtrack and to replay it in a cinema environment. In the 1930s, sync sound on disc gave way to variable area sound on film, which was further improved in the 1940s with theatrical acoustic considerations and improved loudspeaker design, along with the introduction of multitrack recording and steerable replay (using control tones to move sounds). In the 1950s and 1960s, magnetic striping of film allowed multichannel playback in theatres, introducing surround channels and up to five screen channels in premium venues. In the 1970s, Dolby introduced noise reduction, both in postproduction and on film, along with a cost-effective means of encoding and distributing mixes with three screen channels and a mono Surround channel, as shown in the following figure. The quality of cinema sound was further improved in the 1980s with Dolby SR noise reduction and certification programs such as THX®.

The 1990s saw the launch of digital sound to the cinema, allowing 5.1 mixing, mastering, and playback providing discrete Left, Center, and Right screen channels, Left Surround and Right Surround arrays, and a Low-Frequency Effects channel played through a subwoofer.

In 2010, the first step in enhancing digital cinema sound was undertaken with the introduction of Dolby Surround 7.1. This format continues the pattern of increasing the number of surround channels by splitting the existing Left Surround and Right Surround channels into four “zones,” as shown in the following figure.

How Dolby Atmos Works Its Magic

Picture above with various colors indicating speakers that are  strategically placed throughout the theater.  Solid Red indicates existing Surround Speakers, Red with speaker insignia indicates existing Screen Loudspeaker,  rectangular Blue indicates additional Top Surround, square Blue is additional Side Surround, Purple with speaker insignia is Surround Subwoofer and lastly in Green indicates optional Screen Loudspeaker.

A More Effective Speaker Setup

The most immediately noticeable difference in a Dolby Atmos system is the use of overhead speakers, but that's just part of the story.

A typical surround sound system consists of left, center, and right discrete channels with the speakers behind the screen. The surround channels are handled by wall-mounted arrays of speakers, divided acoustically into two or four zones. All speakers within a zone receive the same audio information.

In a Dolby Atmos theatre, every speaker—as many as 64 total—is powered independently and gets its own separate audio feed. In effect, each speaker is its own zone. In addition to the overhead speakers, Dolby Atmos typically adds more surround speakers and screen speakers.

Three critical elements significantly improve the audience experience over 5.1 and 7.1 systems:

• Sounds originating overhead

• Improved audio quality and timbre matching.

• Greater spatial control and resolution improvements of Dolby Atmos.

Sounds Gain Their Independence

Imagine sitting in a restaurant. There's a general buzz of conversation and music all around, yet you can pick out an individual voice behind you or a clink of silverware from the terrace above you—and you can tell exactly where each sound is coming from.

Now you're watching that same restaurant scene in a movie. With conventional surround sound, you'll get the ambience, but the voice and clink come from vague locations—if you can pick them out at all. That's because channel-based sounds—particularly surround effects—have to be assigned to a general zone, not a specific location. And because there are no overhead speakers, the sounds cannot move above you.

In Dolby Atmos, each of those sounds can be created as an independent entity—an audio object. Put all the objects together, and you'll feel like you're actually in the restaurant, not just watching a scene.

Any sound can be a single audio object, placed and moved independently anywhere in the theatre. The filmmaker decides exactly where the sound should come from and where it should move. So you hear the roar of a plane flying overhead from above you, or a door closing to the left. Sounds can originate from a single speaker or sequence of speakers, or from any number of speakers simultaneously.

Audio objects empower filmmakers to focus on the story and put the sounds where they belong, rather than compromise the artistic impact to fit a fixed channel or zone.

Making the Bed

Some elements of a movie soundtrack, however, still benefit from a channel-based approach—for instance, ambient effects and music backgrounds. So a Dolby Atmos soundtrack also includes a more conventional channel-based "bed," together with the audio objects. Dolby Atmos packages up to 128 audio tracks—a 9.1 bed and up to 118 audio objects.

Putting It Together

The Dolby Atmos processor in the theatre intelligently assigns each audio track. It maps the bed channels to screen channels or surround arrays, and positions objects within the room. It's all reproduced in real time based on where the loudspeakers are. Dolby Atmos scales to the specific speaker complement of a theatre, so the effects will be the same regardless of the auditorium's size.

Sound placement is consistent throughout the audience. Thanks to audio objects originating from specific locations rather than general areas, you'll hear the exact same effect no matter where you sit in the theatre—every seat is the "sweet spot."

Even More Audio Improvements

Having independently powered speakers improves the sound in other ways as well. For instance, tonal quality sometimes suffers when a sound is reproduced by an array of loudspeakers. Being able to direct that sound to single speakers makes the reproduction much more accurate and realistic.

Also, in traditional surround setups, a sound moved from the screen to the surround zones drops in volume. Dolby Atmos, using improved room equalization and better bass management along with the independently powered speakers, avoids this problem. Sounds maintain the right volume as they move, adding to the realism.

The audience can now enjoy a completely new listening experience with enveloping sound that brings the stories on screen more fully to life.