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WasapiLoopbackCapture.md

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Record Soundcard Output with WasapiLoopbackCapture

Lots of people ask how they can use NAudio to record the audio being played by another program. The answer is that unfortunately Windows does not provide an API that lets you target the output of one specific program to record. However, with WASAPI loopback capture, you can record all the audio that is being played out of a specific output device.

Since NAudio 2.1.0, the audio can be captured at a sample rate of your choosing, although it will make sense to match the sound card's format.

Let's start off by selecting a path to record to, creating an instance of WasapiLoopbackCapture (uses the default system device, but we can pass any rendering MMDevice that we want which we can find with MMDeviceEnumerator). We'll also create a WaveFileWriter using the capture WaveFormat.

var outputFolder = Path.Combine(Environment.GetFolderPath(Environment.SpecialFolder.Desktop), "NAudio");
Directory.CreateDirectory(outputFolder);
var outputFilePath = Path.Combine(outputFolder, "recorded.wav");
var capture = new WasapiLoopbackCapture();
// optionally we can set the capture waveformat here: e.g. capture.WaveFormat = new WaveFormat(44100, 16,2);
var writer = new WaveFileWriter(outputFilePath, capture.WaveFormat);

We need to handle the DataAvailable event, and it's very much the same approach here as recording to a WAV file from a regular WaveIn device. We just write BytesRecorded bytes from the Buffer into the WaveFileWriter. And in this example, I am stopping recording when we've captured 20 seconds worth of audio, by calling StopRecording.

capture.DataAvailable += (s, a) =>
{
    writer.Write(a.Buffer, 0, a.BytesRecorded);
    if (writer.Position > capture.WaveFormat.AverageBytesPerSecond * 20)
    {
        capture.StopRecording();
    }
};

When the RecordingStopped event fires, we Dispose our WaveFileWriter so we create a valid WAV file, and we're done recording so we'll Dispose our capture device as well.

capture.RecordingStopped += (s, a) =>
{
    writer.Dispose();
    writer = null;
    capture.Dispose();
};

All that remains is for us to start recording with StartRecording and wait for recording to finish by monitoring the CaptureState.

capture.StartRecording();
while (capture.CaptureState != NAudio.CoreAudioApi.CaptureState.Stopped)
{
    Thread.Sleep(500);
}

Now there is one gotcha with WasapiLoopbackCapture. If no audio is playing whatsoever, then the DataAvailable event won't fire. So if you want to record "silence", one simple trick is to simply use an NAudio playback device to play silence through that device for the duration of time you're recording. Alternatively, you could insert silence yourself when you detect gaps in the incoming audio.