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Develop an ambisonic audio decoder

Instructions for implementing an ambisonic decoder plug-in and registering it in Project Settings.
Read time 3 minutesLast updated 13 days ago
An ambisonic decoder is an audio component that decodes the ambisonic audio format into a specific output format, such as stereo or surround sound. This format depends on your speaker configuration (menu: Edit >
Project Settings

?

> Audio > Default Speaker Mode), unless your platform overrides this.
Although Unity supports ambisonic audio, it doesn't provide built-in decoders by default. Instead, you must either select a third-party decoder, or use your own decoder plug-in.

Set up an ambisonic audio decoder plug-in

You can set up an ambisonic audio decoder in the same way as you'd set up an Audio Spatializer. However, the following parameters in the 
AudioPluginInterface.h
file are specific to ambisonic audio decoder plug-ins:

UnityAudioEffectDefinitionFlags_IsAmbisonicDecoder

During the plug-in scanning phase, the 
UnityAudioEffectDefinitionFlags_IsAmbisonicDecoder
flag notifies Unity that this is an ambisonic decoder effect. To enable a plug-in to operate as an ambisonic decoder, set a flag in the description bit-field of the effect:
definition.flags |= UnityAudioEffectDefinitionFlags_IsAmbisonicDecoder;
Unity lists your plug-in as an option in the Project Settings window (menu: Edit > Project Settings > Audio > Ambisonic Decoder Plugin).

UnityAudioAmbisonicData struct

The 
UnityAudioAmbisonicData
struct is similar to the 
UnityAudioSpatializerData
 struct that Unity passes into spatializers, and contains an 
ambisonicOutChannels
integer.

The ambisonicOutChannels integer

The Ambisonic decoders run early in the audio pipeline in Unity, and the 
ambisonicOutChannels
variable tells the plug-in how many of the output channels Unity needs to use. 
ambisonicOutChannels
is automatically set to the 
DefaultSpeakerMode
’s channel count.
For example, if you play back a first order ambisonic audio clip that has 4 channels, and your speaker mode is stereo (which has only 2 channels):
  • An ambisonic decoder’s process callback passes in 4 for the in and out channel count.
  • The 
    ambisonicOutChannels
    field is automatically set to 2.
  • The plug-in outputs its spatialized data to the first 2 channels and zero out the other 2 channels.

Steps to develop an ambisonic audio decoder plug-in

Follow these steps to develop your own ambisonic audio decoder plug-in for Unity:
  1. Create a custom audio plug-in using the Native audio plug-in SDK.
  2. Set a flag in the description bit-field of the effect:
    definition.flags |= UnityAudioEffectDefinitionFlags_IsAmbisonicDecoder;
  3. When you are done configuring your plug-in, compile your file. Make sure it is compilable on your preferred platforms.
  4. Optionally, convert your file to a .dll file.
  5. Move your plug-in file into your Unity project’s Asset folder.
For more information on how to work with your ambisonic audio plug-in in Unity, see Ambisonic Audio.

Formats that Unity ambisonics support

The Unity ambisonic sources framework can support first order ambisonics. The plug-in interface includes information to support binaural stereo and quad output, but the plug-in itself determines which outputs are supported.
Initially, ambisonic decoder plug-ins support first order ambisonic sources and binaural stereo output. There is no support for second order ambisonics.
There is nothing in the framework that's specific to any of the different ambisonic formats available. If the clip’s format matches the ambisonic decoder plug-in’s expected format, then ambisonic audio should work without issue. Unity’s preferred ambisonic format is B-format, with ACN component ordering, and SN3D normalization.

Further information

For information on how to develop a plug-in, refer to Native audio plug-in SDK and Audio spatializer SDK. You must also download the Audio plug-in SDK.