A full-duplex audio player in C# using the waveIn/waveOut APIs

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Introduction

As I already mentioned in my article A low-level audio player in C#, there are no built-in classes in the .NET framework for dealing with sound. This holds true not only for audio playback, but also for audio capture.

It should be noted, though, that the Managed DirectX 9 SDK does include classes for high-level and low-level audio manipulation. However, sometimes you don’t want your application to depend on the full DX 9 runtime, just to do basic sound playback and capture, and there are also some areas where Managed DirectSound doesn’t help at all (for example, multi-channel sound playback and capture).

Nevertheless, I strongly recommend you to use Managed DirectSound for sound playback and capture unless you have a good reason for not doing so.

This article describes a sample application that uses the waveIn and waveOut APIs in C# through P/Invoke to capture an audio signal from the sound card’s input, and play it back (almost) at the same time.

Using the code

The sample code reuses the WaveOutPlayer class from my article A low-level audio player in C#. The new classes in this sample are WaveInRecorder and FifoStream.

The FifoStream class extends System.IO.Stream to implement a FIFO (first-in first-out) of bytes. The overridden Write method adds data to the FIFO’s tail, and the Read method peeks and removes data from the FIFO’s head. The Length property returns the amount of buffered data at any time. Calling Flush will clear all pending data.

The WaveInRecorder class is analogous to the WaveOutPlayer class. In fact, if you look at the source files, you’ll notice that the implementations of these classes are very similar. As with WaveOutPlayer, the interface of this class has been reduced to the strict minimum.

Creating an instance of WaveInRecorder will cause the system to start recording immediately. Here’s the code that creates the WaveOutPlayer and WaveInRecorder instances.

private void Start()
{
    Stop();
    try
    {
        WaveLib.WaveFormat fmt = new WaveLib.WaveFormat(44100, 16, 2);
        m_Player = new WaveLib.WaveOutPlayer(-1, fmt, 16384, 3, 
                        new WaveLib.BufferFillEventHandler(Filler));
        m_Recorder = new WaveLib.WaveInRecorder(-1, fmt, 16384, 3, 
                        new WaveLib.BufferDoneEventHandler(DataArrived));
    }
    catch
    {
        Stop();
        throw;
    }
}

The WaveInRecorder constructor takes five parameters. Except for the last parameter, their meaning is the same as in WaveOutPlayer.

The first parameter is the ID of the wave input device that you want to use. The value -1 represents the default system device, but if your system has more than one sound card, then you can pass any number from 0 to the number of installed sound cards minus one, to select a particular device.

The second parameter is the format of the audio samples.

The third and forth parameters are the size of the internal wave buffers and the number of buffers to allocate. You should set these to reasonable values. Smaller buffers will give you less latency, but the captured audio may have gaps on it if your computer is not fast enough.

The fifth and last parameter is a delegate that will be called periodically as internal audio buffers are full of captured data. In the sample application we just write the captured data to the FIFO, like this:

private void DataArrived(IntPtr data, int size)
{
    if (m_RecBuffer == null || m_RecBuffer.Length < size)
        m_RecBuffer = new byte[size];
    System.Runtime.InteropServices.Marshal.Copy(data, m_RecBuffer, 0, size);
    m_Fifo.Write(m_RecBuffer, 0, m_RecBuffer.Length);
}

Similarly, the Filler method is called every time the player needs more data. Our implementation just reads the data from the FIFO, as shown below:

private void Filler(IntPtr data, int size)
{
    if (m_PlayBuffer == null || m_PlayBuffer.Length < size)
        m_PlayBuffer = new byte[size];
    if (m_Fifo.Length >= size)
        m_Fifo.Read(m_PlayBuffer, 0, size);
    else
        for (int i = 0; i < m_PlayBuffer.Length; i++)
            m_PlayBuffer[i] = 0;
    System.Runtime.InteropServices.Marshal.Copy(m_PlayBuffer, 
                                                 0, data, size);
}

Note that we declared the temporary buffers m_RecBuffer and m_PlayBuffer as member fields in order to improve performance by saving some garbage collections.

To stop streaming, just call Dispose on the player and capture objects. We also need to flush the FIFO so that the next time Start is called there is no residual data to play.

private void Stop()
{
    if (m_Player != null)
        try
        {
            m_Player.Dispose();
        }
        finally
        {
            m_Player = null;
        }
    if (m_Recorder != null)
        try
        {
            m_Recorder.Dispose();
        }
        finally
        {
            m_Recorder = null;
        }
    m_Fifo.Flush(); // clear all pending data

}

Conclusion

This sample demonstrates how to combine the waveIn and waveOut APIs in C#. As an exercise, you may want to combine this code with the audio effect framework in the article Programming Audio Effects in C#, to apply effects to a live audio input in real-time, although latency may be an issue for certain applications.