Audio Objects vs Bed Channels Audio Objects vs Bed Channels

What beds and objects are

A bed is a channel-based track: its channels correspond to fixed speaker locations, like the left, right, center, surround and overhead feeds of a conventional mix. An audio object is a mono source with positional metadata instead of a speaker assignment. Dolby's Hybrik documentation describes Atmos content as bed channels "which correspond to specific speaker locations" plus audio objects with x, y, z coordinates and size metadata. Trinnov, which builds DTS:X processors, defines an object as an audio element whose metadata "defines how it is rendered in the listening environment."

The split shows up at the mixing desk. Audient's Atmos tutorial says bed parts are routed to a single multichannel bus in up to 7.1.2 format and panned with a conventional surround panner. Object parts are placed by dragging an icon in a three-dimensional panner, and the resulting position is written as metadata that the playback system decodes in real time.

How the numbers work in Dolby Atmos

A Dolby Atmos master carries up to 128 channels. A single 7.1.2 bed uses 10 of them, which leaves 118 for objects, and every additional bed takes channels away from the object pool. The renderer counts each bed channel as one input against the same 128 ceiling. Objects cannot feed the LFE channel; only beds can, so deliberate sub-bass effects live in the bed. Games use a smaller budget: Wikipedia reports that Atmos games use the Intermediate Spatial Format, which allows 32 active objects (a 7.1.4 bed plus 20 dynamic objects).

Home delivery cannot move 128 channels, so Dolby applies spatial coding. Per Hybrik, it reduces the beds and objects to 12 or 16 elements, which is "really 11.1 or 15.1 as the LFE doesn't move." The audio is rendered to a 5.1 or 7.1 core and sent as Dolby Digital Plus JOC at a minimum of 384 kbps for 12 elements or 448 kbps for 16. Because the full set is rarely active at once, the reduction costs less than the numbers suggest. Blu-ray allows a Dolby TrueHD stream up to 18 Mbit/s, which carries the same scene without the lossy carrier (see Dolby TrueHD). At the HDMI hop, Dolby's MAT 2.0 wrapper carries up to 31 objects plus LFE and metadata.

For example, start with a 7.1.2 bed (10 channels) and 118 objects: 10 + 118 = 128 inputs. Spatial coding folds those into 16 elements, one of which is the fixed LFE, so 15 elements can move. At the 448 kbps floor that averages 28 kbps per element (448 / 16); at the 384 kbps floor with 12 elements it averages 32 kbps (384 / 12). Those are averages for scale only, since the real allocation shifts with the content. The decoder then renders the 15 movable elements to your layout, whether that is a 5.1.2 soundbar or a 9.1.6 room. Audient notes Atmos mixes scale across setups from 4.1.2 to 11.1.8, and Wikipedia reports home Atmos supports up to 24.1.10 channels.

Beds, objects and speaker count

Objects adapt to the room; beds are authored for a layout. Audient points out that beds do not automatically adapt to different speaker arrays the way objects do, so mixers often export several beds (2.0, 5.1, 7.1, 7.0.2) for different channel counts. More speakers do not mean more objects. The object count is a property of the mix, while speaker count sets positional resolution: the more speakers you have, the finer the placement. A speaker layout with four overhead speakers resolves a flyover more precisely than two, but both receive the same object stream.

Height speakers alone do not make a mix object-based. Dolby documents a separate Channel Based Immersive form, typically 5.1.4 (five floor channels, LFE and four overhead channels), used for live events. It has no objects and no spatial coding; it is a bed with height. The notation is covered in channel configuration notation.

DTS:X and Auro-3D handle it differently

DTS:X is object-based and rides on a DTS-HD Master Audio carrier. Trinnov reports that the first consumer version supported a maximum of 12 simultaneous discrete channels, with a practical layout ceiling of 7.1.4. DTS:X Pro lifts that cap and upmixes bed channels with the Neural:X engine to as many as 30 speakers; Trinnov says its Altitude 32 plays up to 32 discrete outputs. The same source says beds play through 9 main channels for almost the whole of a typical movie, with objects layered on top. See DTS:X and DTS Neural:X.

Auro-3D starts from beds. Auro Technologies describes a scalable family from Auro 9.1 up to Auro 13.1, and calls the format a hybrid: channel-based in its home layouts, with object technology added only in AuroMax, which combines up to 64 object channels with up to Auro 13.1 beds. That makes Auro the clearest example of the bed-first approach. More in Auro-3D.

The Denon AVR-X3800H lists 9.4 amplified channels at 105 W and supports Dolby Atmos, DTS:X and Auro-3D. Trinnov says it was the first to enable DTS:X in a consumer product, and the first public home Atmos demonstration, at CEDIA Expo 2014, used a Trinnov Altitude 32 processor. On the source side, the Apple TV 4K gained Atmos in tvOS 12 (17 September 2018). Compare the formats in DTS:X vs Dolby Atmos.

Common confusions

The 5.1 core inside a streaming Atmos track is not the authoring bed. Hybrik states that the 5.1 core of a JOC stream contains all the audio present in the Atmos mix, so older decoders can play it, while Atmos decoders use the JOC metadata to recover the objects. A bed is a mixing concept; the core is a delivery concept.

Another mix-up is treating "Atmos" as a codec. Dolby says plainly that Atmos "is not an audio codec, but can be delivered by multiple codecs," including Dolby Digital Plus and Dolby TrueHD. The bed/object structure belongs to the mix and survives whichever carrier is used. See Dolby Atmos for the full format and what Dolby Atmos is for the buying view.