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.
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:
The UnityAudioEffectDefinitionFlags_IsAmbisonicDecoder effect definition flag
The UnityAudioAmbisonicData data struct
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).
The UnityAudioAmbisonicData struct is similar to the UnityAudioSpatializerData struct that Unity passes into spatializers, and contains an 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.
Follow these steps to develop your own ambisonic audio decoder plug-in for Unity:
Create a custom audio plug-in using the Native audio plug-in SDK.
Set a flag in the description bit-field of the effect:
definition.flags |= UnityAudioEffectDefinitionFlags_IsAmbisonicDecoder;
When you are done configuring your plug-in, compile your file. Make sure it is compilable on your preferred platforms.
Optionally, convert your file to a .dll file.
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.
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.
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.