There are three codecs used to deliver Dolby Atmos content to the home.
Dolby TrueHD is a lossless codec for both channel-based and immersive audio. It is used exclusively for Blu-ray disc delivery. When encoding a Dolby Atmos master to TrueHD, the disc author can choose to use spatial coding with 12, 14, or 16 elements. The decision is largely determined by the increased file size that results from using 16 elements and the available “bit budget” for the disc. The object audio renderer (OAR) is used to render a 7.1 presentation from the spatial-coded elements, and both the 5.1 and Stereo bitstream on a Blu-ray disc can be downmixed from the 7.1.
Dolby Digital Plus with Atmos Content (also referred to as Dolby Digital Plus JOC) is a high-efficiency lossy codec used for streaming delivery as well as terrestrial broadcast. Dolby Digital Plus JOC can be used for both channel-based and immersive audio. The number of elements used by spatial coding is determined by the bit rate of the encode. A bit rate of 384kbps uses 12 elements, while bit rates of 448kbps and above use 16 elements.
Dolby Digital Plus is the primary delivery codec for Dolby Atmos and will be discussed in further detail.
After the spatial coding process, the OAR renders the audio to 5.1, or first to 7.1, which is then downmixed to 5.1. All of the audio from the Dolby Atmos master is present in the 5.1 downmix. This is done so that Dolby Digital Plus JOC is backwards compatible with regular channel-based 5.1 Dolby Digital Plus. If a bitstream with Dolby Atmos content is presented to a consumer playback device that is not enabled with Dolby Atmos — it will decode the 5.1.
In addition to the object audio metadata (OAMD) for the elements there is a supplementary type of metadata created by a process called Joint Object Coding (JOC). This is used to calculate the delta between the spatially coded elements.
The graphic below illustrates the process:
The JOC and OAMD metadata is packaged as an Extended Metadata Format (EMF) and inserted into “skip frames” between audio blocks in the Dolby Digital Plus bitstream. Since the EMF occupies what was empty space, devices that are not enabled with Dolby Atmos ignore it. This preserves backwards compatibility as devices enabled and not enabled with Dolby Atmos can both decode the bitstream.
When a consumer playback device enabled with Dolby Atmos receives a Dolby Digital Plus JOC bitstream, the 5.1 is decoded, a JOC decoder recreates the elements, and the OAR uses the OAMD from each element to render audio to the device.
Dolby AC-4 IMS (Immersive Stereo) is a next generation audio Codec. Dolby AC-4 IMS is used to carry Dolby Atmos at extremely low bit rates. Dolby AC-4 IMS uses Binaural metadata created during the Dolby Atmos mastering process to create a bitstream that is compatible with standard stereo playback devices as well as a virtualized immersive Dolby Atmos experience on mobile devices using headphones or speakers.
Spatial coding is not used in the AC-4 IMS encoding process.
As the AC-4 IMS adoption rate increases, it is important for the mixer to also monitor with headphones using the onboard binaural render in the Dolby Atmos Renderer.