Capture audio from WasapiLoopbackCapture, and convert to muLaw

Question

I'm capturing audio with WasapiLoopbackCapture

- format        = IeeeFloat
- SampleRate    = 48000
- BitsPerSample = 32

I need to convert this to muLaw (8Khz, 8 bit, mono) - eventually it'll be sent to a phone via SIP trunking. I've tried 100s of samples (most of them with NAudio) and solutions but still have no clue how to do this ...

Answer 1

The Mu-Law tools in NAudio are limited so you might have to roll your own.

You'll need to set up a chain of IWaveProvider filters to convert to mono, change bit-rate, and change bit-depth.

waveBuffer = new BufferedWaveProvider(waveIn.WaveFormat);
waveBuffer.DiscardOnBufferOverflow = true;
waveBuffer.ReadFully = false;  // leave a buffer?

sampleStream = new WaveToSampleProvider(waveBuffer);

// Stereo to mono
monoStream = new StereoToMonoSampleProvider(sampleStream)
    {
        LeftVolume = 1f,
        RightVolume = 1f
    };

// Downsample to 8000
resamplingProvider = new WdlResamplingSampleProvider(monoStream, 8000);

// Convert to 16-bit in order to use ACM or MuLaw tools.
ieeeToPcm = new SampleToWaveProvider16(resamplingProvider);

Then create a custom IWaveProvider for the next step.

// In MuLawConversionProvider
public int Read(byte[] destinationBuffer, int offset, int readingCount)
{
    // Source buffer has twice as many items as the output array.
    var sizeOfPcmBuffer = readingCount * 2;
    _sourceBuffer = BufferHelpers.Ensure(_sourceBuffer, sizeOfPcmBuffer);
    var sourceBytesRead = _sourceProvider.Read(_sourceBuffer, offset * 2, sizeOfPcmBuffer);
    var samplesRead = sourceBytesRead / 2;

    var outIndex = 0;
    for (var n = 0; n < sizeOfPcmBuffer; n += 2)
    {
        destinationBuffer[outIndex++] = MuLawEncoder.LinearToMuLawSample(BitConverter.ToInt16(_sourceBuffer, offset + n));
    }

    return samplesRead * 2;
}

The new provider can be sent directly to WaveOut

outputStream = new MuLawConversionProvider(ieeeToPcm);
waveOut.Init(outputStream);
waveOut.Play();

These filters remain in place with the BufferedWaveProvider as the "root". Whenever you call BufferedWaveProvider.AddSamples(), the data will go through all these filters.