Use NAudio to get Ulaw samples for RTP

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

Here's a few pointers. First of all, as long as you are using NAudio 1.5, no need for the additional WaveFormatConversionStream - Mp3FileReader's Read method returns PCM.

However, you will not be getting 8kHz out, so you need to resample it first. WaveFormatConversionStream can do this, although it uses the built-in Windows ACM sample rate conversion, which doesn't seem to filter the incoming audio well, so there could be aliasing artefacts.

Also, you usually read bigger blocks than just two bytes at a time as the MP3 decoder needs to decode frames one at a time (the resampler also will want to deal with bigger block sizes). I would try reading at least 20ms worth of bytes at a time.

Your use of BitConverter.ToInt16 is correct for getting the 16 bit sample value, but bear in mind that an MP3 is likely stereo, with left, right samples. Are you sure your phone expects stereo.

Finally, I recommend making a mu-law WAV file as a first step, using WaveFileWriter. Then you can easily listen to it in Windows Media Player and check that what you are sending to your softphone is what you intended.