Streaming non-PCM raw audio using NAudio

Question

I'm hell bent on making this work with NAudio, so please tell me if there's a way around this. I have streaming raw audio coming in from a serial device, which I'm trying to play through WaveOut.

Attempt 1:

'Constants 8000, 1, 8000 * 1, 1, 8
Dim CustomWaveOutFormat = WaveFormat.CreateCustomFormat(WaveFormatEncoding.Pcm, SampleRate, Channels, AverageBPS, BlockAlign, BitsPerSample)    
Dim rawStream = New RawSourceWaveStream(VoicePort.BaseStream, CustomWaveOutFormat)

'Run in background
Dim waveOut = New WaveOut(WaveCallbackInfo.FunctionCallback())

'Play stream
waveOut.Init(rawStream)
waveOut.Play()

This code works, but there's a tiny problem - the actual audio stream isn't raw PCM, it's raw MuLaw. It plays out the companding like a Beethoven's 5th on cheese-grater. If I change the WaveFormat to WaveFormatEncoding.MuLaw, I get a bad format exception because it's raw audio and there are no RIFF headers.

So I moved over to converting it to PCM:

Attempt 2:

Dim reader = New MuLawWaveStream(VoicePort.BaseStream, SampleRate, Channels)
Dim pcmStream = WaveFormatConversionStream.CreatePcmStream(reader)
Dim waveOutStream = New BlockAlignReductionStream(pcmStream)

waveOut.Init(waveOutStream)

Here, CreatePcmStream tries to get the length of the stream (even though CanSeek = false) and fails.

Attempt 3

waveOutStream = New BufferedWaveProvider(WaveFormat.CreateMuLawFormat(SampleRate, Channels))
*add samples when OnDataReceived()*

It too seems to suffer from lack of having a header.

I'm hoping there's something minor I missed in all of this. The device only streams audio when in use, and no data is received otherwise - a case which is handled by (1).

Answer 1

To make attempt (1) work, your RawSourceWaveStream should specify the format that the data really is in. Then just use another WaveFormatConversionStream.CreatePcmStream, taking rawStream as the input:

Dim muLawStream = New RawSourceWaveStream(VoicePort.BaseStream, WaveFormat.CreateMuLawFormat(SampleRate, Channels))
Dim pcmStream = WaveFormatConversionStream.CreatePcmStream(muLawStream);

Attempt (2) is actually very close to working. You just need to make MuLawStream.Length return 0. You don't need it for what you are doing. BlockAlignReductionStream is irrelevant to mu-law as well since mu law block align is 1.

Attempt (3) should work. I don't know what you mean by lack of a header?

In NAudio you are building a pipeline of audio data. Each stage in the pipeline can have a different format. Your audio starts off in Mu-law, then gets converted to PCM, then can be played. A buffered WaveProvider is used for you want playback to continue even though your device has stopped providing audio data.

Edit I should add that the IWaveProvider interface in NAudio is a simplified WaveStream. It has only a format and a Read method, and is useful for situations where Length is unknown and repositioning is not possible.