Documentation

Unity Engine

User Manual

Script Reference

Unity Engine

Introduction to ambisonic audio decoders

Read time 3 minutesLast updated 10 hours 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.

Handling the number of channels

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:
  • First order has 4 channels
  • Second order has 9 channels
  • Third order has 16 channels
Unity passes the number of input channels to process to your plug-in via the 
UnityAudioEffect_ProcessCallback
.
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:
  • passes in 16 for the in and out channel count
  • sets 
    ambisonicOutChannels
    to 2
  • the plug-in outputs to the first 2 channels and zeroes out the remaining 14.

Formats that Unity ambisonics support

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.

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.

Additional resources