Question

I’ve been looking over the NAudio examples trying to work out how I can get ulaw samples suitable for packaging up as an RTP payload. I’m attempting to generate the samples from an mp3 file using the code below. Not surprisingly, since I don’t really have a clue what I’m doing with NAudio, when I transmit the samples across the network to a softphone all I get is static.

Can anyone provide any direction on how I should be getting 160 bytes (8Khz @ 20ms) ULAW samples from an MP3 file using NAudio?

private void GetAudioSamples()
{
    var pcmStream = WaveFormatConversionStream.CreatePcmStream(new Mp3FileReader("whitelight.mp3"));
    byte[] buffer = new byte[2];
    byte[] sampleBuffer = new byte[160];
    int sampleIndex = 0;
    int bytesRead = pcmStream.Read(buffer, 0, 2);

    while (bytesRead > 0)
    {
        var ulawByte = MuLawEncoder.LinearToMuLawSample(BitConverter.ToInt16(buffer, 0));
        sampleBuffer[sampleIndex++] = ulawByte;

        if (sampleIndex == 160)
        {
            m_rtpChannel.AddSample(sampleBuffer);
            sampleBuffer = new byte[160];
            sampleIndex = 0;
        }

        bytesRead = pcmStream.Read(buffer, 0, 2);
    }

    logger.Debug("Finished adding audio samples.");
}

Answer 1

Below is the way I eventually got it working. I do lose one of the channels from the mp3, and I guess there’s some way to combine the channels as part of a conversion, but that doesn’t matter for my situation.

The 160 byte buffer size gives me 20ms ulaw samples which work perfectly with the SIP softphone I’m testing with.

var pcmFormat = new WaveFormat(8000, 16, 1);
var ulawFormat = WaveFormat.CreateMuLawFormat(8000, 1);

using (WaveFormatConversionStream pcmStm = new WaveFormatConversionStream(pcmFormat, new Mp3FileReader("whitelight.mp3")))
{
    using (WaveFormatConversionStream ulawStm = new WaveFormatConversionStream(ulawFormat, pcmStm))
    {
        byte[] buffer = new byte[160];
        int bytesRead = ulawStm.Read(buffer, 0, 160);

        while (bytesRead > 0)
        {
            byte[] sample = new byte[bytesRead];
            Array.Copy(buffer, sample, bytesRead);
            m_rtpChannel.AddSample(sample);

            bytesRead = ulawStm.Read(buffer, 0, 160);
        }
    }
}