Custom audio effects

This article describes how to create a Windows Runtime component that implements the IBasicAudioEffect interface to create custom effects for audio streams. Custom effects can be used with several different Windows Runtime APIs including MediaCapture, which provides access to a device's camera, MediaComposition, which allows you to create complex compositions out of media clips, and AudioGraph which allows you to quickly assemble a graph of various audio input, output, and submix nodes.

Add a custom effect to your app

A custom audio effect is defined in a class that implements the IBasicAudioEffect interface. This class can't be included directly in your app's project. Instead, you must use a Windows Runtime component to host your audio effect class.

Add a Windows Runtime component for your audio effect

  1. In Microsoft Visual Studio, with your solution open, go to the File menu and select Add->New Project.
  2. Select the Windows Runtime Component (Universal Windows) project type.
  3. For this example, name the project AudioEffectComponent. This name will be referenced in code later.
  4. Click OK.
  5. The project template creates a class called Class1.cs. In Solution Explorer, right-click the icon for Class1.cs and select Rename.
  6. Rename the file to ExampleAudioEffect.cs. Visual Studio will show a prompt asking if you want to update all references to the new name. Click Yes.
  7. Open ExampleAudioEffect.cs and update the class definition to implement the IBasicAudioEffect interface.
public sealed class ExampleAudioEffect : IBasicAudioEffect

You need to include the following namespaces in your effect class file in order to access all of the types used in the examples in this article.

using Windows.Media.Effects;
using Windows.Media.MediaProperties;
using Windows.Foundation.Collections;
using System.Runtime.InteropServices;
using Windows.Media;
using Windows.Foundation;

Implement the IBasicAudioEffect interface

Your audio effect must implement all of the methods and properties of the IBasicAudioEffect interface. This section walks you through a simple implementation of this interface to create a basic echo effect.

SupportedEncodingProperties property

The system checks the SupportedEncodingProperties property to determine which encoding properties are supported by your effect. Note that if the consumer of your effect can't encode audio using the properties you specify, the system will call Close on your effect and will remove your effect from the audio pipeline. In this example, AudioEncodingProperties objects are created and added to the returned list to support 44.1 kHz and 48 kHz, 32-bit float, mono encoding.

public IReadOnlyList<AudioEncodingProperties> SupportedEncodingProperties
{
    get
    {
        var supportedEncodingProperties = new List<AudioEncodingProperties>();
        AudioEncodingProperties encodingProps1 = AudioEncodingProperties.CreatePcm(44100, 1, 32);
        encodingProps1.Subtype = MediaEncodingSubtypes.Float;
        AudioEncodingProperties encodingProps2 = AudioEncodingProperties.CreatePcm(48000, 1, 32);
        encodingProps2.Subtype = MediaEncodingSubtypes.Float;

        supportedEncodingProperties.Add(encodingProps1);
        supportedEncodingProperties.Add(encodingProps2);

        return supportedEncodingProperties;
        
    }
}

SetEncodingProperties method

The system calls SetEncodingProperties on your effect to let you know the encoding properties for the audio stream upon which the effect is operating. In order to implement an echo effect, this example uses a buffer to store one second of audio data. This method provides the opportunity to initialize the size of the buffer to the number of samples in one second of audio, based on the sample rate in which the audio is encoded. The delay effect also uses an integer counter to keep track of the current position in the delay buffer. Since SetEncodingProperties is called whenever the effect is added to the audio pipeline, this is a good time to initialize that value to 0. You may also want to capture the AudioEncodingProperties object passed into this method to use elsewhere in your effect.

private float[] echoBuffer;
private int currentActiveSampleIndex;
private AudioEncodingProperties currentEncodingProperties;
public void SetEncodingProperties(AudioEncodingProperties encodingProperties)
{
    currentEncodingProperties = encodingProperties;
    echoBuffer = new float[encodingProperties.SampleRate]; // exactly one second delay
    currentActiveSampleIndex = 0;
}

SetProperties method

The SetProperties method allows the app that is using your effect to adjust effect parameters. Properties are passed as an IPropertySet map of property names and values.

IPropertySet configuration;
public void SetProperties(IPropertySet configuration)
{
    this.configuration = configuration;
}

This simple example will mix the current audio sample with a value from the delay buffer according the value of the Mix property. A property is declared and TryGetValue is used to get the value set by the calling app. If no value was set, a default value of .5 is used. Note that this property is read-only. The property value must be set using SetProperties.

public float Mix
{
    get
    {
        object val;
        if (configuration != null && configuration.TryGetValue("Mix", out val))
        {
            return (float)val;
        }
        return .5f;
    }
}

ProcessFrame method

The ProcessFrame method is where your effect modifies the audio data of the stream. The method is called once per frame and is passed a ProcessAudioFrameContext object. This object contains an input AudioFrame object that contains the incoming frame to be processed and an output AudioFrame object to which you write audio data that will be passed on to rest of the audio pipeline. An audio frame is a buffer of audio samples representing a short slice of audio data.

Accessing the data buffer of a AudioFrame requires COM interop, so you should include the System.Runtime.InteropServices namespace in your effect class file and then add the following code inside the namespace for your effect to import the interface for accessing the audio buffer.

[ComImport]
[Guid("5B0D3235-4DBA-4D44-865E-8F1D0E4FD04D")]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
unsafe interface IMemoryBufferByteAccess
{
    void GetBuffer(out byte* buffer, out uint capacity);
}

Note

Because this technique accesses a native, unmanaged image buffer, you will need to configure your project to allow unsafe code.

  1. In Solution Explorer, right-click the AudioEffectComponent project and select Properties.
  2. Select the Build tab.
  3. Select the Allow unsafe code check box.

 

Now you can add the ProcessFrame method implementation to your effect. First, this method obtains a AudioBuffer object from both the input and output audio frames. Note that the output frame is opened for writing and the input for reading. Next, an IMemoryBufferReference is obtained for each buffer by calling CreateReference. Then, the actual data buffer is obtained by casting the IMemoryBufferReference objects as the COM interop interface defined above, IMemoryByteAccess, and then calling GetBuffer.

Now that the data buffers have been obtained, you can read from the input buffer and write to the output buffer. For each sample in the inputbuffer, the value is obtained and multiplied by 1 - Mix to set the dry signal value of the effect. Next, a sample is retrieved from the current position in the echo buffer and multiplied by Mix to set the wet value of the effect. The output sample is set to the sum of the dry and wet values. Finally, each input sample is stored in the echo buffer and the current sample index is incremented.

unsafe public void ProcessFrame(ProcessAudioFrameContext context)
{
    AudioFrame inputFrame = context.InputFrame;
    AudioFrame outputFrame = context.OutputFrame;

    using (AudioBuffer inputBuffer = inputFrame.LockBuffer(AudioBufferAccessMode.Read),
                        outputBuffer = outputFrame.LockBuffer(AudioBufferAccessMode.Write))
    using (IMemoryBufferReference inputReference = inputBuffer.CreateReference(),
                                    outputReference = outputBuffer.CreateReference())
    {
        byte* inputDataInBytes;
        byte* outputDataInBytes;
        uint inputCapacity;
        uint outputCapacity;

        ((IMemoryBufferByteAccess)inputReference).GetBuffer(out inputDataInBytes, out inputCapacity);
        ((IMemoryBufferByteAccess)outputReference).GetBuffer(out outputDataInBytes, out outputCapacity);

        float* inputDataInFloat = (float*)inputDataInBytes;
        float* outputDataInFloat = (float*)outputDataInBytes;

        float inputData;
        float echoData;

        // Process audio data
        int dataInFloatLength = (int)inputBuffer.Length / sizeof(float);

        for (int i = 0; i < dataInFloatLength; i++)
        {
            inputData = inputDataInFloat[i] * (1.0f - this.Mix);
            echoData = echoBuffer[currentActiveSampleIndex] * this.Mix;
            outputDataInFloat[i] = inputData + echoData;
            echoBuffer[currentActiveSampleIndex] = inputDataInFloat[i];
            currentActiveSampleIndex++;

            if (currentActiveSampleIndex == echoBuffer.Length)
            {
                // Wrap around (after one second of samples)
                currentActiveSampleIndex = 0;
            }
        }
    }
}

Close method

The system will call the Close Close method on your class when the effect should shut down. You should use this method to dispose of any resources you have created. The argument to the method is a MediaEffectClosedReason that lets you know whether the effect was closed normally, if an error occurred, or if the effect does not support the required encoding format.

public void Close(MediaEffectClosedReason reason)
{
    // Dispose of effect resources
    echoBuffer = null;
}

DiscardQueuedFrames method

The DiscardQueuedFrames method is called when your effect should reset. A typical scenario for this is if your effect stores previously processed frames to use in processing the current frame. When this method is called, you should dispose of the set of previous frames you saved. This method can be used to reset any state related to previous frames, not only accumulated audio frames.

public void DiscardQueuedFrames()
{
    // Reset contents of the samples buffer
    Array.Clear(echoBuffer, 0, echoBuffer.Length - 1);
    currentActiveSampleIndex = 0;
}

TimeIndependent property

The TimeIndependent TimeIndependent property lets the system know if your effect does not require uniform timing. When set to true, the system can use optimizations that enhance effect performance.

public bool TimeIndependent { get { return true; } }

UseInputFrameForOutput property

Set the UseInputFrameForOutput property to true to tell the system that your effect will write its output to the audio buffer of the InputFrame of the ProcessAudioFrameContext passed into ProcessFrame instead of writing to the OutputFrame.

public bool UseInputFrameForOutput { get { return false; } }

Adding your custom effect to your app

To use your audio effect from your app, you must add a reference to the effect project to your app.

  1. In Solution Explorer, under your app project, right-click References and select Add reference.
  2. Expand the Projects tab, select Solution, and then select the check box for your effect project name. For this example, the name is AudioEffectComponent.
  3. Click OK

If your audio effect class is declared is a different namespace, be sure to include that namespace in your code file.

using AudioEffectComponent;

Add your custom effect to an AudioGraph node

For general information about using audio graphs, see Audio garphs. The following code snippet shows you how to add the example echoe effect shown in this article to an audio graph node. First, a PropertySet is created and a value for the Mix property, defined by the effect, is set. Next, the AudioEffectDefinition constructor is called, passing in the full class name of the custom effect type and the property set. Finally, the effect definition is added to the EffectDefinitions property of an existing FileInputNode, causing the audio emitted to be processed by the custom effect.

// Create a property set and add a property/value pair
PropertySet echoProperties = new PropertySet();
echoProperties.Add("Mix", 0.5f);

// Instantiate the custom effect defined in the 'AudioEffectComponent' project
AudioEffectDefinition echoEffectDefinition = new AudioEffectDefinition(typeof(ExampleAudioEffect).FullName, echoProperties);
fileInputNode.EffectDefinitions.Add(echoEffectDefinition);

After it has been added to a node, the custom effect can be disabled by calling DisableEffectsByDefinition and passing in the AudioEffectDefinition object. For more information about using audio graphs in your app, see AudioGraph.

Add your custom effect to a clip in a MediaComposition

The following code snippet demonstrates adding the custom audio effect to a video clip and a background audio track in a media composition. For general guidance for creating media compositions from video clips and adding background audio tracks, see Media compositions and editing.

// Create a property set and add a property/value pair
PropertySet echoProperties = new PropertySet();
echoProperties.Add("Mix", 0.5f);

// Instantiate the custom effect defined in the 'AudioEffectComponent' project
AudioEffectDefinition echoEffectDefinition = new AudioEffectDefinition(typeof(ExampleAudioEffect).FullName, echoProperties);

// Add custom audio effect to the current clip in the timeline
var currentClip = composition.Clips.FirstOrDefault(
    mc => mc.StartTimeInComposition <= mediaPlayerElement.MediaPlayer.PlaybackSession.Position &&
    mc.EndTimeInComposition >= mediaPlayerElement.MediaPlayer.PlaybackSession.Position);
currentClip.AudioEffectDefinitions.Add(echoEffectDefinition);

// Add custom audio effect to the first background audio track
if (composition.BackgroundAudioTracks.Count > 0)
{
    composition.BackgroundAudioTracks[0].AudioEffectDefinitions.Add(echoEffectDefinition);
}