# Introduction to ambisonic audio decoders


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** > Editor > **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](/engine/6000.6/manual/audio/mixer-native-audio-plugin/spatializer-sdk.md). However, the following parameters in the `AudioPluginInterface.h` file are specific to ambisonic audio decoder plug-ins:

* The [`UnityAudioEffectDefinitionFlags_IsAmbisonicDecoder`](#unityaudioeffectdefinitionflags_isambisonicdecoder) effect definition flag

* The [`UnityAudioAmbisonicData`](#unityaudioambisonicdata-struct) data struct

### 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:

```lang-cs
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`](/engine/6000.6/manual/audio/mixer-native-audio-plugin/spatializer-sdk.md#SpatializerMetaData) 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](/engine/6000.6/manual/audio/mixer-native-audio-plugin.md) and [Audio spatializer SDK](/engine/6000.6/manual/audio/mixer-native-audio-plugin/spatializer-sdk.md). You must also download the [Audio plug-in SDK](https://github.com/Unity-Technologies/NativeAudioPlugins).

## Additional resources

* [Introduction to ambisonic audio](/engine/6000.6/manual/audio/ambisonic/ambisonic.md)
* [Use ambisonic audio in your project](/engine/6000.6/manual/audio/ambisonic/instructions.md)
* [Develop an ambisonic audio decoder](/engine/6000.6/manual/audio/ambisonic/develop-ambisonic-decoder.md)