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 SpatializerA plug-in that changes the way audio is transmitted from an audio source into the surrounding space. It takes the source and regulates the gains of the left and right ear contributions based on the distance and angle between the AudioListener and the AudioSource. More info
See in Glossary. However, the following parameters in the AudioPluginInterface.h file are specific to ambisonic audio decoder plug-ins:
The UnityAudioEffectDefinitionFlags_IsAmbisonicDecoder effect definition flag
The UnityAudioEffectDefinitionFlags_UseOutputChannelCount 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).
By default, the process callback of an ambisonic decoder receives the same channel count for its input and output buffers, which is the channel count of the ambisonic audio clip. The UnityAudioEffectDefinitionFlags_UseOutputChannelCount flag notifies Unity that the decoder outputs to the full channel layout of the project instead.
To enable this behavior, set the flag in the description bit-field of the effect, in addition to UnityAudioEffectDefinitionFlags_IsAmbisonicDecoder:
definition.flags |= UnityAudioEffectDefinitionFlags_IsAmbisonicDecoder | UnityAudioEffectDefinitionFlags_UseOutputChannelCount;
With this flag set, the outchannels parameter of the decoder’s process callback equals the channel count of the speaker mode, which can be larger or smaller than inchannels. For more information, refer to Handling the number of channels.
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.
Unity supports up to third order ambisonics, which means the decoder can receive up to 16 input channels. The number of input channels depends on the ambisonic order of the clip:
Unity passes the number of input channels to process to your plug-in via the UnityAudioEffect_ProcessCallback. The number of output channels depends on whether the plug-in sets the UnityAudioEffectDefinitionFlags_UseOutputChannelCount flag.
Without the UnityAudioEffectDefinitionFlags_UseOutputChannelCount flag, the process callback receives the same value for the in and out channel counts, and the plug-in can only use the first ambisonicOutChannels channels of the output buffer. 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 of the buffer and zeroes out the other 2 channels.
For a third order ambisonic clip with 16 channels and stereo output, the process callback:
ambisonicOutChannels to 2
With the UnityAudioEffectDefinitionFlags_UseOutputChannelCount flag, the process callback receives the speaker mode’s channel count for the out channel count, which is also the value of ambisonicOutChannels, and the plug-in must fill length * outchannels samples. The out channel count can be larger or smaller than the in channel count. For example, for a first order ambisonic audio clip that has 4 channels:
In stereo, the process callback passes in 4 for the in channel count and 2 for the out channel count. The plug-in should output to the 2 available channels of the output buffer.
In 7.1 (which has 8 channels), the process callback passes in 4 for the in channel count and 8 for the out channel count, so the decoder should output its spatialized data to the full 8 channels of the output buffer.
Unity supports up to third order ambisonics. The plug-in interface includes information to support any Unity supported outputs, but the plug-in itself determines which outputs are supported.
Ambisonic decoder plug-ins receive up to 16 input channels for third order ambisonic sources. Your decoder plug-in handles all ambisonic orders up to third order (first order: 4 channels, second order: 9 channels, third order: 16 channels).
There’s 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.