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Ambisonic Audio

Introduction

This page explains how to play back ambisonics and the changes to our audio plugin interface to support ambisonic audio decoders.

Ambisonics are stored in a multi-channel format. Instead of each channel being mapped to a specific speaker, ambisonics instead represent the soundfield in a more general way. The soundfield can then be rotated based on the listener’s orientation (i.e. the user’s head rotation in XR). The soundfield can also be decoded into a format that matches the speaker setup. Ambisonics are commonly paired with 360-degree videos and can also be used as an audio skyboxA special type of Material used to represent skies. Usually six-sided. More info
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, for distant ambient sounds.

Selecting an ambisonic audio decoder

Navigate to the project’s Audio settings (main menu: Edit > Project Settings, then select the Audio category). Select an ambisonic decoder from the list of available decoders in the project. There are no built-in decoders that ship with Unity 2017.1, but Google and Oculus each provide one in their audio SDKs for Unity.

Ambisonic options in the Audio settings
Ambisonic options in the Audio settings

Import an ambisonic audio clip

Import a multi-channel WAV file just like normal. In the audio clipA container for audio data in Unity. Unity supports mono, stereo and multichannel audio assets (up to eight channels). Unity can import .aif, .wav, .mp3, and .ogg audio file format, and .xm, .mod, .it, and .s3m tracker module formats. More info
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’s inspectorA Unity window that displays information about the currently selected GameObject, Asset or Project Settings, alowing you to inspect and edit the values. More info
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window, select the new ambisonic check-box. The WAV file should be B-format, with ACN component ordering, and SN3D normalization.

The Ambisonic checkbox in the audio clip inspector
The Ambisonic checkbox in the audio clip inspector

Play the ambisonic audio clip through an audio source

Play the ambisonic audio clip through an audio sourceA component which plays back an Audio Clip in the scene to an audio listener or through an audio mixer. More info
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just like you would any other audio clip. When an ambisonic clip is played, it will first be de-compressed if necessary, then sent through an ambisonic decoder to convert it to the project’s selected speaker mode, then sent through the __audio source__’s effects.

There are a couple of things to note related to audio source properties:

  • Set spatialize to false. When an ambisonic audio clip is played, it is automatically sent through the project’s selected ambisonic audio decoder. The decoder converts the clip from ambisonic format to the project’s selected speaker format. It also already handles spatialization as a part of this decoding operation, based on the orientation of the audio source and audio listenerA component that acts like a microphone, receiving sound from Audio Sources in the scene and outputting to the computer speakers. More info
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    .

  • Reverb zones are disabled for ambisonic audio clips, just like they are for spatialized audio sources.

Audio plugin interface changes

For plugin authors, please start by reading about the native audio plugin SDK and 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
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SDKs in the Unity manual and downloading the audio plugin SDK:

https://docs.unity3d.com/Manual/AudioMixerNativeAudioPlugin.html

https://docs.unity3d.com/Manual/AudioSpatializerSDK.html

https://bitbucket.org/Unity-Technologies/nativeaudioplugins

There are two changes to AudioPluginInterface.h for ambisonic audio decoders. First, there is a new effect definition flag: UnityAudioEffectDefinitionFlags_IsAmbisonicDecoder. Ambisonic decoders should set this flag in the definition bit-field of the effect. During the plugin scanning phase, this flag notifies Unity that this effect is an ambisonic decoder and it will then show up as an option in the Audio settings’s list of ambisonic decoders.

Second, there is a new UnityAudioAmbisonicData struct that is passed into ambisonic decoders, which is very similar to the UnityAudioSpatializerData struct that is passed into spatializers, but there is a new ambisonicOutChannels integer that has been added. This field will be set to the DefaultSpeakerMode’s channel count. Ambisonic decoders are placed very early in the audio pipeline, where we are running at the clip’s channel count, so ambisonicOutChannels tells the plugin how many of the output channels will actually be used.

If we are playing back a first order ambisonic audio clip (4 channels) and our speaker mode is stereo (2 channels), an ambisonic decoder’s process callback will be passed in 4 for the in and out channel count. The ambisonicOutChannels field will be set to 2. In this common scenario, the plugin should output its spatialized data to the first 2 channels and zero out the other 2 channels.

The Unity ambisonic sources framework in 2017.1 can support first and second order ambisonics. The plugin interface includes the information to support binaural stereo, quadA primitive object that resembles a plane but its edges are only one unit long, it uses only 4 vertices, and the surface is oriented in the XY plane of the local coordinate space. More info
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, 5.1, and 7.1 output, but the level of support is really determined by the plugin. Initially, it is only expected that ambisonic decoder plugins will support 1st order ambisonic sources and binaural stereo output.

There is nothing in the current framework that is specific to any of the different ambisonic formats available. If the clip’s format matches the ambisonic decoder plugin’s expected format, then everything should just work. Our current plan, though, is that Unity’s preferred ambisonic format will be B-format, with ACN component ordering, and SN3D normalization.


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