Dolby Atmos
Dolby Atmos uses channel and object information to render an immersive soundtrack for a supported playback arrangement.
An Atmos soundtrack can report “5.1” in a media tool and still supply sound above the listener. The number can describe a compatible channel presentation inside the delivery format. The Atmos information adds what a renderer needs to produce the immersive presentation.
To understand a session, follow that information from the mix to the listening system.
From a scene to a delivery track
An Atmos production uses sound components and spatial instructions. A moving effect can have position updates; channel-based beds can provide other parts of the mix. The consumer delivery encodes a representation of this material and the information required for supported playback.
That delivery is not a census of every individual object used in the studio. Encoders can use spatial coding and component reconstruction to fit the presentation into the delivery system.
| Common delivery or transport | Audio role |
|---|---|
| Dolby Digital Plus with Atmos | Lossy compressed audio with compatible channel playback and additional spatial information |
| Dolby TrueHD with Atmos | Lossless delivery of the encoded audio presentation with Atmos information |
| Dolby MAT with Atmos | A supported transport for audio and spatial information, including PCM-based Atmos delivery |
The rows answer different parts of the chain. The first two identify common encoded source forms. MAT is especially relevant to what is sent over an output connection after the player has received or decoded its source.
Why Dolby Digital Plus can have two useful results
ETSI TS 103 420 describes backwards-compatible object carriage using Enhanced AC-3. The delivery includes a compatible channel presentation together with information used for component recovery and object rendering.
A conventional supported decoder can play the compatible channel mix. An object-aware decoder uses the additional information to prepare the immersive presentation. This allows the same delivered programme to remain audible on conventional systems while enabling more spatial rendering on capable ones.
The difference is not merely adding height speakers to the conventional decode. The renderer must receive the relevant spatial representation. An upmixer can derive height activity from channel audio, but it is working from a different set of information.
Rendering a TrueHD Atmos source for 5.1.4 speakers
Suppose an Emby library contains a TrueHD-with-Atmos soundtrack, and the listening system has five main speakers, one bass output, and four height speakers.
- The selected source supplies the encoded audio and Atmos information.
- Emby's delivery path either retains a suitable source representation or makes another presentation where required.
- The player can pass the supported encoded presentation onward for decoding, or use an object-aware decode and suitable output such as MAT.
- The renderer uses the supplied spatial information and its known speaker configuration to generate outputs for the five main and four height positions.
- The system reproduces those outputs and handles bass according to its configuration.
The 5.1.4 label describes the speaker arrangement, not five audio objects plus four more stored tracks. An effect between available positions can use contributions to several speakers; another moment can use a different combination.
If stage 3 instead outputs an ordinary 7.1 channel render, the receiver can play that presentation, but it no longer has the source's original object metadata to render as Atmos. If the player has already rendered a binaural presentation for headphones, the ear signals are likewise a finished spatial result.
Decoding and compression quality are separate
An object-aware decoder can preserve the metadata needed for later rendering. Decoding therefore does not inevitably remove Atmos. Dolby MAT develops this route through a concrete session.
Likewise, lossless PCM-based transport does not turn a lossy Dolby Digital Plus source into a lossless studio master. It can avoid another lossy encoding while still carrying that source's decoded approximation. TrueHD's lossless property applies to its encoded audio presentation.
Interpret the receiver's displays
A receiver may show both a detected input format and a selected processing mode. “Dolby Surround” upmixing on ordinary channel audio and an accepted Atmos input are different observations. Height-speaker activity alone cannot distinguish them.
A more useful report records the selected source form, delivered audio, player's output, detected receiver input, and speaker/rendering arrangement. That identifies where the scene information was retained, rendered, or replaced by a conventional presentation.