Wrapper around an `AVAudioUnit` of type `kAudioUnitType_Mixer` and sub type `kAudioUnitSubType_MatrixMixer` for convenient usage.

MatrixMixerNodeWrapper.swift

import AVFoundation

// MARK: - MatrixMixerNodeWrapper

/// Wrapper around an audio unit node of type `kAudioUnitSubType_MatrixMixer` for convenience.
public final class MatrixMixerNodeWrapper {

    // MARK: Properties

    /// Matrix mixer to map input channels
    ///
    /// Helpful links about `kAudioUnitSubType_MatrixMixer` usage:
    /// - https://lists.apple.com/archives/coreaudio-api/2008/Apr/msg00169.html
    /// - https://stackoverflow.com/questions/48059405/how-should-an-aumatrixmixer-be-configured-in-an-avaudioengine-graph
    /// - https://stackoverflow.com/questions/53208006/change-audio-volume-of-some-channels-using-avaudioengine
    private let matrixMixerNode: AVAudioUnit

    /// Volume of the node output.
    public var volume: Float = 1 {
        didSet { updateOutputVolume() }
    }

    // MARK: Init

    public init() async throws {
        let matrixUnitDescription = AudioComponentDescription(
            componentType: kAudioUnitType_Mixer,
            componentSubType: kAudioUnitSubType_MatrixMixer,
            componentManufacturer: kAudioUnitManufacturer_Apple,
            componentFlags: 0,
            componentFlagsMask: 0
        )
        matrixMixerNode = try await AVAudioUnit.instantiate(with: matrixUnitDescription)
    }
}

// MARK: - Node

extension MatrixMixerNodeWrapper {

    /// Wrapped node.
    public var node: AVAudioNode { matrixMixerNode }
}

// MARK: - Setup

extension MatrixMixerNodeWrapper {

    /// Setup the matrix mixer to pass audio.
    ///
    /// - important: Has to be called after the `AVAudioEngine` has been started.
    public func setup() {
        AudioUnitSetParameter(matrixMixerNode.audioUnit, kMatrixMixerParam_Volume, kAudioUnitScope_Global, 0xFFFF_FFFF, 1, 0)

        for inputChannelIndex in 0..<matrixMixerNode.inputFormat(forBus: 0).channelCount {
            AudioUnitSetParameter(
                matrixMixerNode.audioUnit,
                kMatrixMixerParam_Volume,
                kAudioUnitScope_Input,
                inputChannelIndex,
                1,
                0
            )
        }

        updateOutputVolume()
    }

    private func updateOutputVolume() {
        for outputChannelIndex in 0..<matrixMixerNode.outputFormat(forBus: 0).channelCount {
            AudioUnitSetParameter(
                matrixMixerNode.audioUnit,
                kMatrixMixerParam_Volume,
                kAudioUnitScope_Output,
                outputChannelIndex,
                volume,
                0
            )
        }
    }
}

// MARK: - Mapping

extension MatrixMixerNodeWrapper {

    /// Set the volume for the input channel to the output channels in the `outputChannelIndexes` set.
    public func setInputVolume(
        _ volume: Float,
        forInputChannel inputChannelIndex: Int,
        toOutputChannels outputChannelIndexes: Set<Int>
    ) {
        var outputChannelIndex = 0

        while outputChannelIndex < matrixMixerNode.outputFormat(forBus: 0).channelCount {
            let volume = outputChannelIndexes.contains(outputChannelIndex) ? volume : 0

            let crossPoint = UInt32((inputChannelIndex << 16) | outputChannelIndex)
            AudioUnitSetParameter(
                matrixMixerNode.audioUnit,
                kMatrixMixerParam_Volume,
                kAudioUnitScope_Global,
                crossPoint,
                volume,
                0
            )

            outputChannelIndex += 1
        }
    }
}

Notes

Usage

Setup

// instantiate the wrapper
let matrixMixerNodeWrapper = try await MatrixMixerNodeWrapper()

// connect the node the engine
engine.attach(matrixMixerNodeWrapper.node)
engine.connect(playerNode, to : matrixMixerNodeWrapper.node, format: nil)

// then each time the engine is started, call:
matrixMixerNodeWrapper.node.setup()

Note

Calling setup every time the engine is started is not mandatory, but I find it easier to manage and couldn't find any drawbacks.

Update volume and channel connexions

Set node output volume:

matrixMixerNodeWrapper.volume = 0.5

Set volume for one connexion:

matrixMixerNodeWrapper.setInputVolume(1, forInputChannel: 0, to: [0, 1])

Important

When no volume is set for a connexion, its default value is 0. Also, it's better to set the volume for each connexion each time the engine is started.

Connexions examples

Make a stereo signal mono

matrixMixerNodeWrapper.setInputVolume(1, forInputChannel: 0, to: [0])
matrixMixerNodeWrapper.setInputVolume(1, forInputChannel: 1, to: [0])

Invert left and right channels

matrixMixerNodeWrapper.setInputVolume(1, forInputChannel: 0, to: [1])
matrixMixerNodeWrapper.setInputVolume(1, forInputChannel: 1, to: [0])

Links

Here are some links that were useful to make that work:

• https://lists.apple.com/archives/coreaudio-api/2008/Apr/msg00169.html
• https://stackoverflow.com/questions/48059405/how-should-an-aumatrixmixer-be-configured-in-an-avaudioengine-graph
• https://stackoverflow.com/questions/53208006/change-audio-volume-of-some-channels-using-avaudioengine