What is Dolby Atmos?
Dolby Atmos is an audio format that expands classic multi-channel sound with a height layer and freely positionable audio objects. In addition to fixed channel groups, individual sound elements can be provided with positional information. The playback device uses this to calculate how the sound is output via the available speakers.
The designation is now found on many televisions, soundbars, headphones, and even smartphones. However, how Dolby Atmos is reproduced in these devices differs significantly. Whether and how strongly the spatial effect is perceptible depends on the speakers, signal processing, and playback environment.
What is Dolby Atmos?
Older surround formats such as Dolby Digital work with fixed channels. In a 5.1 system, for example, there are six of them. During mixing, every sound is assigned to one of these channels. Dolby Atmos builds on this principle and expands it in two aspects.
Channel groups and objects side by side
Dolby describes its format as a combination of two components. The foundation is formed by fixed channel groups, also known in technical jargon as beds. These usually exist in a 7.1.2 arrangement. This is where everything that does not need to move, such as background atmosphere or music, is placed.
This is supplemented by audio objects. These are individual sound elements that can be placed in the room independently of the channel groups. They carry positional data in the form of coordinates, including information on the size of the object. An airplane flying through the frame is a typical example of such an object.
During playback, a so-called renderer takes over the work. It adapts the playback to the existing speaker configuration. An Atmos soundtrack can therefore be played on systems with different equipment, although the spatial effect differs significantly depending on the number and arrangement of the speakers.
The layer above your head
The second difference is height. Classic surround sound distributes sound to the front, back, and sides—essentially around the listener. Dolby Atmos now adds additional channels above the head, so that a further, third layer is added.
This is recognizable by specifications with three numbers. In 5.1.2, the five stands for the speakers all around, the one for the subwoofer, and the two for the height channels. In 7.1.4, it is seven, one, and four respectively.
| Feature | Classic 5.1 or 7.1 sound | Dolby Atmos |
|---|---|---|
| Basic principle | Sound is assigned to fixed channels | Combination of channel beds and audio objects |
| Positioning | Tied to the channel structure | Objects can be placed in space via positional metadata |
| Height layer | Not provided for in classic 5.1 or 7.1 | Additional height layer possible |
| Playback | Mixed for defined channel assignment | Renderer adapts objects to the existing playback system |
Dolby Atmos does not replace classic channels, but supplements them with audio objects and positional information, which the playback device processes to match the set speaker configuration.
Playback in the living room
There are three different options for the height layer. They differ primarily in technical complexity and in how precisely the sound is perceived above the listening position. How well the effect works depends heavily on the respective room and ceiling situation.
- Ceiling speakers are located in or on the ceiling and radiate directly downwards. While this creates the most accurate localization, it requires an AV receiver and cables in the ceiling.
- Upward-firing speakers bounce the sound off the ceiling back to the listening position. They can be found as separate speakers or built into soundbars.
- Virtual methods are used in soundbars without their own height speakers. Depending on the design, several speakers and psychoacoustic methods work together.
Limits of reflection
Upward-firing speakers work particularly well when there is a ceiling that is as smooth as possible and sufficiently reflective at a suitable height. Very high, highly absorbent, or sloped ceilings can weaken the height effect because the reflection reaches the listening position less precisely.
With headphones, the principle works fundamentally differently. There, spatial sound is calculated binaurally for two ears. Acoustic directional cues are simulated so that the sound can be perceived spatially, even though it physically only comes from the headphones.
Significance of the labeling
The Dolby Atmos labeling indicates that a device also supports the format. How pronounced the spatial and, in particular, the height effect is in practice depends heavily on the system landscape, the speakers, the signal processing, and the room.
The path to the television
For an Atmos soundtrack to work, several things must fit together. It is not enough for the television to recognize the format. The source and the connection to the audio device are also important.
Source and soundtrack
The content itself must be available in Dolby Atmos. This is the case with many streaming services and on 4K media, but by no means with all others. Older films and normal television programs are usually not available in this format. Before setting up the playback chain, you should therefore check which audio format the desired content is actually available in.
It should be noted that the information on the packaging often refers to the original version. A localized soundtrack may be in a different format, even if Dolby Atmos is printed on it.
Transmission to the audio device
HDMI is the standard way to connect a television and a soundbar or AV receiver. Dolby Atmos can, for example, be transmitted compressed via Dolby Digital Plus. With suitable devices, this is also possible via HDMI ARC. For high-bitrate and lossless formats such as Dolby TrueHD with Dolby Atmos, HDMI eARC is the appropriate connection.
The optical output is not suitable for this purpose. It was designed for stereo and compressed multi-channel sound and has too little capacity for the formats mentioned here. Anyone setting up an Atmos chain should therefore connect the television and audio device via HDMI.
Significance for dialogue
The question remains as to what the format does for the intelligibility of speech. The answer is twofold. Technically, object-based audio theoretically opens up several possibilities. In practice, however, much depends on how a film or series was produced.
Possibilities of object-based transmission
Object-based audio fundamentally offers the technical possibility of treating individual components of a mix separately. If the speech is available as a separate component and the playback system supports corresponding personalization, the dialogue level can be adjusted independently of music and effects.
Limits of the format
This does not mean that Dolby Atmos automatically offers an adjustable dialogue track. Whether dialogue can be adjusted separately depends on how the content was produced and transmitted and what functions the playback device provides.
Furthermore, the ratio between speech and the rest of the sound remains decisive for intelligibility. This ratio is determined by the mix. The playback format does not change this. A film with dialogue mixed at a low volume will remain mixed at a low volume even in Dolby Atmos.
Depending on the device, Dolby Atmos can be virtually processed via built-in TV speakers. However, the spatial effect differs significantly from a system with separate surround and height speakers.
Frequently asked questions
Dolby Atmos is an immersive audio format. It combines fixed channel groups, known as beds, with individual audio objects that can be freely arranged in space via positional data. In addition, there is a height layer so that sound can also come from above. A renderer in the playback device distributes the result to the available speakers.
The first number indicates the speakers around the listening position, the second the subwoofer, and the third the height channels. In 5.1.2, this means five speakers, one subwoofer, and two channels for sound from above. The third number identifies the height channels, as they typically occur in immersive setups with Dolby Atmos.
eARC is not strictly necessary. Dolby Atmos can, for example, also be transmitted compressed via Dolby Digital Plus, which is also possible via HDMI ARC on compatible devices. HDMI eARC becomes particularly important when high-bitrate or lossless Atmos soundtracks such as Dolby TrueHD are to be transmitted. It is also crucial that the television and audio device support the respective format.
Because the labeling initially only shows that a device supports the format. Some devices reproduce it via their own height speakers, while others create the height effect via virtual signal processing. With headphones, this happens binaurally, i.e., via a calculation for two ears.
Any influence is at best indirect. Object-based audio fundamentally offers the possibility of treating speech separately, provided it is available as a separate component and the playback device supports this. However, the mix of the film determines how loud dialogue is in relation to the rest, not the playback format.